ssh with fake host support (wip)

Makefile

@@ -39,3 +39,18 @@ generate:
 install_protobuf:
 	@go install google.golang.org/protobuf/cmd/protoc-gen-go@v1.28
 	@go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@v1.2
+
+update_sshlib:
+	@rm -rf cryptolib && \
+		git clone https://go.googlesource.com/crypto cryptolib && \
+		rm -rf pkg/proto/ssh && \
+		mv cryptolib/ssh pkg/proto/ && \
+		mv cryptolib/internal/poly1305 pkg/proto/ssh/internal/ && \
+		find pkg/proto/ssh -type f -name '*.go' -exec sed -i 's?golang.org/x/crypto/ssh?github.com/Neur0toxine/sshpoke/pkg/proto/ssh?g' {} \; && \
+		find pkg/proto/ssh -type f -name '*.go' -exec sed -i 's?golang.org/x/crypto/internal/poly1305?github.com/Neur0toxine/sshpoke/pkg/proto/ssh/internal/poly1305?g' {} \; && \
+		find pkg/proto/ssh -type f -name '*_test.go' -delete && \
+		patch -p0 < patch/ssh_fakehost.patch && \
+		rm -rf pkg/proto/ssh/test && \
+		rm -rf pkg/proto/ssh/testdata && \
+		rm -rf cryptolib
+

go.mod

@@ -15,6 +15,8 @@ require (
 	go.uber.org/zap v1.26.0
 	golang.design/x/lockfree v0.0.1
 	golang.org/x/crypto v0.14.0
+	golang.org/x/sys v0.13.0
+	golang.org/x/term v0.13.0
 	google.golang.org/grpc v1.58.2
 	google.golang.org/protobuf v1.31.0
 )
@@ -52,7 +54,6 @@ require (
 	golang.org/x/mod v0.12.0 // indirect
 	golang.org/x/net v0.17.0 // indirect
 	golang.org/x/sync v0.3.0 // indirect
-	golang.org/x/sys v0.13.0 // indirect
 	golang.org/x/text v0.13.0 // indirect
 	golang.org/x/tools v0.13.0 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20230920204549-e6e6cdab5c13 // indirect

internal/server/driver/ssh/driver.go

@@ -13,7 +13,7 @@ import (
 	"github.com/Neur0toxine/sshpoke/internal/server/driver/ssh/types"
 	"github.com/Neur0toxine/sshpoke/internal/server/driver/util"
 	"github.com/Neur0toxine/sshpoke/pkg/dto"
-	"golang.org/x/crypto/ssh"
+	"github.com/Neur0toxine/sshpoke/pkg/proto/ssh"
 )
 
 type SSH struct {

internal/server/driver/ssh/sshtun/auth.go

@@ -8,7 +8,7 @@ import (
 	"strings"
 
 	"github.com/Neur0toxine/sshpoke/internal/server/driver/ssh/types"
-	"golang.org/x/crypto/ssh"
+	"github.com/Neur0toxine/sshpoke/pkg/proto/ssh"
 )
 
 func AuthKeyFile(keyFile types.SmartPath) (ssh.AuthMethod, error) {

internal/server/driver/ssh/sshtun/printer.go

@@ -3,8 +3,8 @@ package sshtun
 import (
 	"bufio"
 
+	"github.com/Neur0toxine/sshpoke/pkg/proto/ssh"
 	"go.uber.org/zap"
-	"golang.org/x/crypto/ssh"
 )
 
 func StdoutPrinterSessionCallback(log *zap.SugaredLogger) SessionCallback {

internal/server/driver/ssh/sshtun/sish_compat.go

@@ -1,76 +0,0 @@
-package sshtun
-
-import (
-	"errors"
-	"net"
-	"reflect"
-	"sync"
-	"unsafe"
-
-	"golang.org/x/crypto/ssh"
-)
-
-type sishHostListener struct {
-	parent *ssh.Client
-}
-
-func newSishHostListener(parent *ssh.Client) *sishHostListener {
-	return &sishHostListener{parent: parent}
-}
-
-func (c *sishHostListener) ListenFakeRemoteHost(ep Endpoint) (net.Listener, error) {
-	c.doHandleForwardsOnce()
-	m := channelForwardMsg{
-		ep.Host,
-		uint32(ep.Port),
-	}
-	// send message
-	ok, resp, err := c.parent.SendRequest("tcpip-forward", true, ssh.Marshal(&m))
-	if err != nil {
-		return nil, err
-	}
-	if !ok {
-		return nil, errors.New("ssh: tcpip-forward request denied by peer")
-	}
-
-	laddr := &net.TCPAddr{
-		IP:   net.ParseIP("127.0.0.1"),
-		Port: ep.Port,
-	}
-	if ep.Port == 0 {
-		var p struct {
-			Port uint32
-		}
-		if err := ssh.Unmarshal(resp, &p); err != nil {
-			return nil, err
-		}
-
-		laddr.Port = int(p.Port)
-	}
-	c.registerForward(laddr)
-	return nil, nil
-}
-
-func (c *sishHostListener) registerForward(addr *net.TCPAddr) {
-	cl := reflect.ValueOf(c.parent).Elem()
-	forwardsUn := cl.FieldByName("forwards")
-	forwards := reflect.NewAt(forwardsUn.Type(), unsafe.Pointer(forwardsUn.UnsafeAddr())).Elem()
-	forwardVal := forwards.MethodByName("add").Call([]reflect.Value{reflect.ValueOf(addr)})[0]
-	_ = forwardVal
-}
-
-func (c *sishHostListener) doHandleForwardsOnce() {
-	cl := reflect.ValueOf(c.parent)
-	clVal := cl.Elem()
-	onceField := clVal.FieldByName("handleForwardsOnce")
-	once := reflect.NewAt(onceField.Type(), unsafe.Pointer(onceField.UnsafeAddr())).Interface().(*sync.Once)
-	handleForwards := clVal.MethodByName("handleForwards")
-	once.Do(func() {
-		handleForwards.Call(nil)
-	})
-}
-
-type channelForwardMsg struct {
-	addr  string
-	rport uint32
-}

internal/server/driver/ssh/sshtun/ssh.go

@@ -4,15 +4,14 @@ import (
 	"context"
 	"fmt"
 	"net"
-	"os"
 	"sync"
 	"sync/atomic"
 	"time"
 
+	"github.com/Neur0toxine/sshpoke/pkg/proto/ssh"
 	"github.com/function61/gokit/app/backoff"
 	"github.com/function61/gokit/io/bidipipe"
 	"go.uber.org/zap"
-	"golang.org/x/crypto/ssh"
 )
 
 type SessionCallback func(*ssh.Session)
@@ -132,12 +131,18 @@ func (c *Tunnel) connect(ctx context.Context, sessionCb SessionCallback) error {
 //
 // nonblocking flow: if Accept() call fails, stops goroutine and returns error on ch listenerStopped
 func (c *Tunnel) reverseForwardOnePort(sshClient *ssh.Client, listenerStopped chan<- error) error {
+	var (
+		listener net.Listener
+		err      error
+	)
 	if c.fakeRemoteHost {
-		newSishHostListener(sshClient).ListenFakeRemoteHost(c.forward.Remote)
-		time.Sleep(time.Second)
-		os.Exit(0)
+		listener, err = sshClient.ListenTCP(&net.TCPAddr{
+			IP:   c.ipFromAddr(sshClient.Conn.RemoteAddr()),
+			Port: c.forward.Remote.Port,
+		}, c.forward.Remote.Host)
+	} else {
+		listener, err = sshClient.Listen("tcp", c.forward.Remote.String())
 	}
-	listener, err := sshClient.Listen("tcp", c.forward.Remote.String())
 	if err != nil {
 		return err
 	}
@@ -160,7 +165,9 @@ func (c *Tunnel) reverseForwardOnePort(sshClient *ssh.Client, listenerStopped ch
 	return nil
 }
 
-func (c *Tunnel) listenTCPWithoutResolving() {
+func (c *Tunnel) ipFromAddr(addr net.Addr) net.IP {
+	host, _, _ := net.SplitHostPort(addr.String())
+	return net.ParseIP(host)
 }
 
 func handleReverseForwardConn(client net.Conn, forward Forward, log *zap.SugaredLogger) {

patch/ssh_fakehost.patch

@@ -0,0 +1,23 @@
+--- pkg/proto/ssh/tcpip.go	2023-11-18 21:39:15.394837005 +0300
++++ pkg/proto/ssh/tcpip.go	2023-11-18 21:38:25.706173351 +0300
+@@ -101,14 +101,18 @@
+ // ListenTCP requests the remote peer open a listening socket
+ // on laddr. Incoming connections will be available by calling
+ // Accept on the returned net.Listener.
+-func (c *Client) ListenTCP(laddr *net.TCPAddr) (net.Listener, error) {
++func (c *Client) ListenTCP(laddr *net.TCPAddr, fakeHost ...string) (net.Listener, error) {
+ 	c.handleForwardsOnce.Do(c.handleForwards)
+ 	if laddr.Port == 0 && isBrokenOpenSSHVersion(string(c.ServerVersion())) {
+ 		return c.autoPortListenWorkaround(laddr)
+ 	}
+
++	host := laddr.IP.String()
++	if len(fakeHost) > 0 {
++		host = fakeHost[0]
++	}
+ 	m := channelForwardMsg{
+-		laddr.IP.String(),
++		host,
+ 		uint32(laddr.Port),
+ 	}
+ 	// send message

pkg/proto/ssh/agent/client.go

@@ -0,0 +1,854 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package agent implements the ssh-agent protocol, and provides both
+// a client and a server. The client can talk to a standard ssh-agent
+// that uses UNIX sockets, and one could implement an alternative
+// ssh-agent process using the sample server.
+//
+// References:
+//
+//	[PROTOCOL.agent]: https://tools.ietf.org/html/draft-miller-ssh-agent-00
+package agent // import "github.com/Neur0toxine/sshpoke/pkg/proto/ssh/agent"
+
+import (
+	"bytes"
+	"crypto/dsa"
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/elliptic"
+	"crypto/rsa"
+	"encoding/base64"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"math/big"
+	"sync"
+
+	"github.com/Neur0toxine/sshpoke/pkg/proto/ssh"
+)
+
+// SignatureFlags represent additional flags that can be passed to the signature
+// requests an defined in [PROTOCOL.agent] section 4.5.1.
+type SignatureFlags uint32
+
+// SignatureFlag values as defined in [PROTOCOL.agent] section 5.3.
+const (
+	SignatureFlagReserved SignatureFlags = 1 << iota
+	SignatureFlagRsaSha256
+	SignatureFlagRsaSha512
+)
+
+// Agent represents the capabilities of an ssh-agent.
+type Agent interface {
+	// List returns the identities known to the agent.
+	List() ([]*Key, error)
+
+	// Sign has the agent sign the data using a protocol 2 key as defined
+	// in [PROTOCOL.agent] section 2.6.2.
+	Sign(key ssh.PublicKey, data []byte) (*ssh.Signature, error)
+
+	// Add adds a private key to the agent.
+	Add(key AddedKey) error
+
+	// Remove removes all identities with the given public key.
+	Remove(key ssh.PublicKey) error
+
+	// RemoveAll removes all identities.
+	RemoveAll() error
+
+	// Lock locks the agent. Sign and Remove will fail, and List will empty an empty list.
+	Lock(passphrase []byte) error
+
+	// Unlock undoes the effect of Lock
+	Unlock(passphrase []byte) error
+
+	// Signers returns signers for all the known keys.
+	Signers() ([]ssh.Signer, error)
+}
+
+type ExtendedAgent interface {
+	Agent
+
+	// SignWithFlags signs like Sign, but allows for additional flags to be sent/received
+	SignWithFlags(key ssh.PublicKey, data []byte, flags SignatureFlags) (*ssh.Signature, error)
+
+	// Extension processes a custom extension request. Standard-compliant agents are not
+	// required to support any extensions, but this method allows agents to implement
+	// vendor-specific methods or add experimental features. See [PROTOCOL.agent] section 4.7.
+	// If agent extensions are unsupported entirely this method MUST return an
+	// ErrExtensionUnsupported error. Similarly, if just the specific extensionType in
+	// the request is unsupported by the agent then ErrExtensionUnsupported MUST be
+	// returned.
+	//
+	// In the case of success, since [PROTOCOL.agent] section 4.7 specifies that the contents
+	// of the response are unspecified (including the type of the message), the complete
+	// response will be returned as a []byte slice, including the "type" byte of the message.
+	Extension(extensionType string, contents []byte) ([]byte, error)
+}
+
+// ConstraintExtension describes an optional constraint defined by users.
+type ConstraintExtension struct {
+	// ExtensionName consist of a UTF-8 string suffixed by the
+	// implementation domain following the naming scheme defined
+	// in Section 4.2 of RFC 4251, e.g.  "foo@example.com".
+	ExtensionName string
+	// ExtensionDetails contains the actual content of the extended
+	// constraint.
+	ExtensionDetails []byte
+}
+
+// AddedKey describes an SSH key to be added to an Agent.
+type AddedKey struct {
+	// PrivateKey must be a *rsa.PrivateKey, *dsa.PrivateKey,
+	// ed25519.PrivateKey or *ecdsa.PrivateKey, which will be inserted into the
+	// agent.
+	PrivateKey interface{}
+	// Certificate, if not nil, is communicated to the agent and will be
+	// stored with the key.
+	Certificate *ssh.Certificate
+	// Comment is an optional, free-form string.
+	Comment string
+	// LifetimeSecs, if not zero, is the number of seconds that the
+	// agent will store the key for.
+	LifetimeSecs uint32
+	// ConfirmBeforeUse, if true, requests that the agent confirm with the
+	// user before each use of this key.
+	ConfirmBeforeUse bool
+	// ConstraintExtensions are the experimental or private-use constraints
+	// defined by users.
+	ConstraintExtensions []ConstraintExtension
+}
+
+// See [PROTOCOL.agent], section 3.
+const (
+	agentRequestV1Identities   = 1
+	agentRemoveAllV1Identities = 9
+
+	// 3.2 Requests from client to agent for protocol 2 key operations
+	agentAddIdentity         = 17
+	agentRemoveIdentity      = 18
+	agentRemoveAllIdentities = 19
+	agentAddIDConstrained    = 25
+
+	// 3.3 Key-type independent requests from client to agent
+	agentAddSmartcardKey            = 20
+	agentRemoveSmartcardKey         = 21
+	agentLock                       = 22
+	agentUnlock                     = 23
+	agentAddSmartcardKeyConstrained = 26
+
+	// 3.7 Key constraint identifiers
+	agentConstrainLifetime = 1
+	agentConstrainConfirm  = 2
+	// Constraint extension identifier up to version 2 of the protocol. A
+	// backward incompatible change will be required if we want to add support
+	// for SSH_AGENT_CONSTRAIN_MAXSIGN which uses the same ID.
+	agentConstrainExtensionV00 = 3
+	// Constraint extension identifier in version 3 and later of the protocol.
+	agentConstrainExtension = 255
+)
+
+// maxAgentResponseBytes is the maximum agent reply size that is accepted. This
+// is a sanity check, not a limit in the spec.
+const maxAgentResponseBytes = 16 << 20
+
+// Agent messages:
+// These structures mirror the wire format of the corresponding ssh agent
+// messages found in [PROTOCOL.agent].
+
+// 3.4 Generic replies from agent to client
+const agentFailure = 5
+
+type failureAgentMsg struct{}
+
+const agentSuccess = 6
+
+type successAgentMsg struct{}
+
+// See [PROTOCOL.agent], section 2.5.2.
+const agentRequestIdentities = 11
+
+type requestIdentitiesAgentMsg struct{}
+
+// See [PROTOCOL.agent], section 2.5.2.
+const agentIdentitiesAnswer = 12
+
+type identitiesAnswerAgentMsg struct {
+	NumKeys uint32 `sshtype:"12"`
+	Keys    []byte `ssh:"rest"`
+}
+
+// See [PROTOCOL.agent], section 2.6.2.
+const agentSignRequest = 13
+
+type signRequestAgentMsg struct {
+	KeyBlob []byte `sshtype:"13"`
+	Data    []byte
+	Flags   uint32
+}
+
+// See [PROTOCOL.agent], section 2.6.2.
+
+// 3.6 Replies from agent to client for protocol 2 key operations
+const agentSignResponse = 14
+
+type signResponseAgentMsg struct {
+	SigBlob []byte `sshtype:"14"`
+}
+
+type publicKey struct {
+	Format string
+	Rest   []byte `ssh:"rest"`
+}
+
+// 3.7 Key constraint identifiers
+type constrainLifetimeAgentMsg struct {
+	LifetimeSecs uint32 `sshtype:"1"`
+}
+
+type constrainExtensionAgentMsg struct {
+	ExtensionName    string `sshtype:"255|3"`
+	ExtensionDetails []byte
+
+	// Rest is a field used for parsing, not part of message
+	Rest []byte `ssh:"rest"`
+}
+
+// See [PROTOCOL.agent], section 4.7
+const agentExtension = 27
+const agentExtensionFailure = 28
+
+// ErrExtensionUnsupported indicates that an extension defined in
+// [PROTOCOL.agent] section 4.7 is unsupported by the agent. Specifically this
+// error indicates that the agent returned a standard SSH_AGENT_FAILURE message
+// as the result of a SSH_AGENTC_EXTENSION request. Note that the protocol
+// specification (and therefore this error) does not distinguish between a
+// specific extension being unsupported and extensions being unsupported entirely.
+var ErrExtensionUnsupported = errors.New("agent: extension unsupported")
+
+type extensionAgentMsg struct {
+	ExtensionType string `sshtype:"27"`
+	// NOTE: this matches OpenSSH's PROTOCOL.agent, not the IETF draft [PROTOCOL.agent],
+	// so that it matches what OpenSSH actually implements in the wild.
+	Contents []byte `ssh:"rest"`
+}
+
+// Key represents a protocol 2 public key as defined in
+// [PROTOCOL.agent], section 2.5.2.
+type Key struct {
+	Format  string
+	Blob    []byte
+	Comment string
+}
+
+func clientErr(err error) error {
+	return fmt.Errorf("agent: client error: %v", err)
+}
+
+// String returns the storage form of an agent key with the format, base64
+// encoded serialized key, and the comment if it is not empty.
+func (k *Key) String() string {
+	s := string(k.Format) + " " + base64.StdEncoding.EncodeToString(k.Blob)
+
+	if k.Comment != "" {
+		s += " " + k.Comment
+	}
+
+	return s
+}
+
+// Type returns the public key type.
+func (k *Key) Type() string {
+	return k.Format
+}
+
+// Marshal returns key blob to satisfy the ssh.PublicKey interface.
+func (k *Key) Marshal() []byte {
+	return k.Blob
+}
+
+// Verify satisfies the ssh.PublicKey interface.
+func (k *Key) Verify(data []byte, sig *ssh.Signature) error {
+	pubKey, err := ssh.ParsePublicKey(k.Blob)
+	if err != nil {
+		return fmt.Errorf("agent: bad public key: %v", err)
+	}
+	return pubKey.Verify(data, sig)
+}
+
+type wireKey struct {
+	Format string
+	Rest   []byte `ssh:"rest"`
+}
+
+func parseKey(in []byte) (out *Key, rest []byte, err error) {
+	var record struct {
+		Blob    []byte
+		Comment string
+		Rest    []byte `ssh:"rest"`
+	}
+
+	if err := ssh.Unmarshal(in, &record); err != nil {
+		return nil, nil, err
+	}
+
+	var wk wireKey
+	if err := ssh.Unmarshal(record.Blob, &wk); err != nil {
+		return nil, nil, err
+	}
+
+	return &Key{
+		Format:  wk.Format,
+		Blob:    record.Blob,
+		Comment: record.Comment,
+	}, record.Rest, nil
+}
+
+// client is a client for an ssh-agent process.
+type client struct {
+	// conn is typically a *net.UnixConn
+	conn io.ReadWriter
+	// mu is used to prevent concurrent access to the agent
+	mu sync.Mutex
+}
+
+// NewClient returns an Agent that talks to an ssh-agent process over
+// the given connection.
+func NewClient(rw io.ReadWriter) ExtendedAgent {
+	return &client{conn: rw}
+}
+
+// call sends an RPC to the agent. On success, the reply is
+// unmarshaled into reply and replyType is set to the first byte of
+// the reply, which contains the type of the message.
+func (c *client) call(req []byte) (reply interface{}, err error) {
+	buf, err := c.callRaw(req)
+	if err != nil {
+		return nil, err
+	}
+	reply, err = unmarshal(buf)
+	if err != nil {
+		return nil, clientErr(err)
+	}
+	return reply, nil
+}
+
+// callRaw sends an RPC to the agent. On success, the raw
+// bytes of the response are returned; no unmarshalling is
+// performed on the response.
+func (c *client) callRaw(req []byte) (reply []byte, err error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	msg := make([]byte, 4+len(req))
+	binary.BigEndian.PutUint32(msg, uint32(len(req)))
+	copy(msg[4:], req)
+	if _, err = c.conn.Write(msg); err != nil {
+		return nil, clientErr(err)
+	}
+
+	var respSizeBuf [4]byte
+	if _, err = io.ReadFull(c.conn, respSizeBuf[:]); err != nil {
+		return nil, clientErr(err)
+	}
+	respSize := binary.BigEndian.Uint32(respSizeBuf[:])
+	if respSize > maxAgentResponseBytes {
+		return nil, clientErr(errors.New("response too large"))
+	}
+
+	buf := make([]byte, respSize)
+	if _, err = io.ReadFull(c.conn, buf); err != nil {
+		return nil, clientErr(err)
+	}
+	return buf, nil
+}
+
+func (c *client) simpleCall(req []byte) error {
+	resp, err := c.call(req)
+	if err != nil {
+		return err
+	}
+	if _, ok := resp.(*successAgentMsg); ok {
+		return nil
+	}
+	return errors.New("agent: failure")
+}
+
+func (c *client) RemoveAll() error {
+	return c.simpleCall([]byte{agentRemoveAllIdentities})
+}
+
+func (c *client) Remove(key ssh.PublicKey) error {
+	req := ssh.Marshal(&agentRemoveIdentityMsg{
+		KeyBlob: key.Marshal(),
+	})
+	return c.simpleCall(req)
+}
+
+func (c *client) Lock(passphrase []byte) error {
+	req := ssh.Marshal(&agentLockMsg{
+		Passphrase: passphrase,
+	})
+	return c.simpleCall(req)
+}
+
+func (c *client) Unlock(passphrase []byte) error {
+	req := ssh.Marshal(&agentUnlockMsg{
+		Passphrase: passphrase,
+	})
+	return c.simpleCall(req)
+}
+
+// List returns the identities known to the agent.
+func (c *client) List() ([]*Key, error) {
+	// see [PROTOCOL.agent] section 2.5.2.
+	req := []byte{agentRequestIdentities}
+
+	msg, err := c.call(req)
+	if err != nil {
+		return nil, err
+	}
+
+	switch msg := msg.(type) {
+	case *identitiesAnswerAgentMsg:
+		if msg.NumKeys > maxAgentResponseBytes/8 {
+			return nil, errors.New("agent: too many keys in agent reply")
+		}
+		keys := make([]*Key, msg.NumKeys)
+		data := msg.Keys
+		for i := uint32(0); i < msg.NumKeys; i++ {
+			var key *Key
+			var err error
+			if key, data, err = parseKey(data); err != nil {
+				return nil, err
+			}
+			keys[i] = key
+		}
+		return keys, nil
+	case *failureAgentMsg:
+		return nil, errors.New("agent: failed to list keys")
+	}
+	panic("unreachable")
+}
+
+// Sign has the agent sign the data using a protocol 2 key as defined
+// in [PROTOCOL.agent] section 2.6.2.
+func (c *client) Sign(key ssh.PublicKey, data []byte) (*ssh.Signature, error) {
+	return c.SignWithFlags(key, data, 0)
+}
+
+func (c *client) SignWithFlags(key ssh.PublicKey, data []byte, flags SignatureFlags) (*ssh.Signature, error) {
+	req := ssh.Marshal(signRequestAgentMsg{
+		KeyBlob: key.Marshal(),
+		Data:    data,
+		Flags:   uint32(flags),
+	})
+
+	msg, err := c.call(req)
+	if err != nil {
+		return nil, err
+	}
+
+	switch msg := msg.(type) {
+	case *signResponseAgentMsg:
+		var sig ssh.Signature
+		if err := ssh.Unmarshal(msg.SigBlob, &sig); err != nil {
+			return nil, err
+		}
+
+		return &sig, nil
+	case *failureAgentMsg:
+		return nil, errors.New("agent: failed to sign challenge")
+	}
+	panic("unreachable")
+}
+
+// unmarshal parses an agent message in packet, returning the parsed
+// form and the message type of packet.
+func unmarshal(packet []byte) (interface{}, error) {
+	if len(packet) < 1 {
+		return nil, errors.New("agent: empty packet")
+	}
+	var msg interface{}
+	switch packet[0] {
+	case agentFailure:
+		return new(failureAgentMsg), nil
+	case agentSuccess:
+		return new(successAgentMsg), nil
+	case agentIdentitiesAnswer:
+		msg = new(identitiesAnswerAgentMsg)
+	case agentSignResponse:
+		msg = new(signResponseAgentMsg)
+	case agentV1IdentitiesAnswer:
+		msg = new(agentV1IdentityMsg)
+	default:
+		return nil, fmt.Errorf("agent: unknown type tag %d", packet[0])
+	}
+	if err := ssh.Unmarshal(packet, msg); err != nil {
+		return nil, err
+	}
+	return msg, nil
+}
+
+type rsaKeyMsg struct {
+	Type        string `sshtype:"17|25"`
+	N           *big.Int
+	E           *big.Int
+	D           *big.Int
+	Iqmp        *big.Int // IQMP = Inverse Q Mod P
+	P           *big.Int
+	Q           *big.Int
+	Comments    string
+	Constraints []byte `ssh:"rest"`
+}
+
+type dsaKeyMsg struct {
+	Type        string `sshtype:"17|25"`
+	P           *big.Int
+	Q           *big.Int
+	G           *big.Int
+	Y           *big.Int
+	X           *big.Int
+	Comments    string
+	Constraints []byte `ssh:"rest"`
+}
+
+type ecdsaKeyMsg struct {
+	Type        string `sshtype:"17|25"`
+	Curve       string
+	KeyBytes    []byte
+	D           *big.Int
+	Comments    string
+	Constraints []byte `ssh:"rest"`
+}
+
+type ed25519KeyMsg struct {
+	Type        string `sshtype:"17|25"`
+	Pub         []byte
+	Priv        []byte
+	Comments    string
+	Constraints []byte `ssh:"rest"`
+}
+
+// Insert adds a private key to the agent.
+func (c *client) insertKey(s interface{}, comment string, constraints []byte) error {
+	var req []byte
+	switch k := s.(type) {
+	case *rsa.PrivateKey:
+		if len(k.Primes) != 2 {
+			return fmt.Errorf("agent: unsupported RSA key with %d primes", len(k.Primes))
+		}
+		k.Precompute()
+		req = ssh.Marshal(rsaKeyMsg{
+			Type:        ssh.KeyAlgoRSA,
+			N:           k.N,
+			E:           big.NewInt(int64(k.E)),
+			D:           k.D,
+			Iqmp:        k.Precomputed.Qinv,
+			P:           k.Primes[0],
+			Q:           k.Primes[1],
+			Comments:    comment,
+			Constraints: constraints,
+		})
+	case *dsa.PrivateKey:
+		req = ssh.Marshal(dsaKeyMsg{
+			Type:        ssh.KeyAlgoDSA,
+			P:           k.P,
+			Q:           k.Q,
+			G:           k.G,
+			Y:           k.Y,
+			X:           k.X,
+			Comments:    comment,
+			Constraints: constraints,
+		})
+	case *ecdsa.PrivateKey:
+		nistID := fmt.Sprintf("nistp%d", k.Params().BitSize)
+		req = ssh.Marshal(ecdsaKeyMsg{
+			Type:        "ecdsa-sha2-" + nistID,
+			Curve:       nistID,
+			KeyBytes:    elliptic.Marshal(k.Curve, k.X, k.Y),
+			D:           k.D,
+			Comments:    comment,
+			Constraints: constraints,
+		})
+	case ed25519.PrivateKey:
+		req = ssh.Marshal(ed25519KeyMsg{
+			Type:        ssh.KeyAlgoED25519,
+			Pub:         []byte(k)[32:],
+			Priv:        []byte(k),
+			Comments:    comment,
+			Constraints: constraints,
+		})
+	// This function originally supported only *ed25519.PrivateKey, however the
+	// general idiom is to pass ed25519.PrivateKey by value, not by pointer.
+	// We still support the pointer variant for backwards compatibility.
+	case *ed25519.PrivateKey:
+		req = ssh.Marshal(ed25519KeyMsg{
+			Type:        ssh.KeyAlgoED25519,
+			Pub:         []byte(*k)[32:],
+			Priv:        []byte(*k),
+			Comments:    comment,
+			Constraints: constraints,
+		})
+	default:
+		return fmt.Errorf("agent: unsupported key type %T", s)
+	}
+
+	// if constraints are present then the message type needs to be changed.
+	if len(constraints) != 0 {
+		req[0] = agentAddIDConstrained
+	}
+
+	resp, err := c.call(req)
+	if err != nil {
+		return err
+	}
+	if _, ok := resp.(*successAgentMsg); ok {
+		return nil
+	}
+	return errors.New("agent: failure")
+}
+
+type rsaCertMsg struct {
+	Type        string `sshtype:"17|25"`
+	CertBytes   []byte
+	D           *big.Int
+	Iqmp        *big.Int // IQMP = Inverse Q Mod P
+	P           *big.Int
+	Q           *big.Int
+	Comments    string
+	Constraints []byte `ssh:"rest"`
+}
+
+type dsaCertMsg struct {
+	Type        string `sshtype:"17|25"`
+	CertBytes   []byte
+	X           *big.Int
+	Comments    string
+	Constraints []byte `ssh:"rest"`
+}
+
+type ecdsaCertMsg struct {
+	Type        string `sshtype:"17|25"`
+	CertBytes   []byte
+	D           *big.Int
+	Comments    string
+	Constraints []byte `ssh:"rest"`
+}
+
+type ed25519CertMsg struct {
+	Type        string `sshtype:"17|25"`
+	CertBytes   []byte
+	Pub         []byte
+	Priv        []byte
+	Comments    string
+	Constraints []byte `ssh:"rest"`
+}
+
+// Add adds a private key to the agent. If a certificate is given,
+// that certificate is added instead as public key.
+func (c *client) Add(key AddedKey) error {
+	var constraints []byte
+
+	if secs := key.LifetimeSecs; secs != 0 {
+		constraints = append(constraints, ssh.Marshal(constrainLifetimeAgentMsg{secs})...)
+	}
+
+	if key.ConfirmBeforeUse {
+		constraints = append(constraints, agentConstrainConfirm)
+	}
+
+	cert := key.Certificate
+	if cert == nil {
+		return c.insertKey(key.PrivateKey, key.Comment, constraints)
+	}
+	return c.insertCert(key.PrivateKey, cert, key.Comment, constraints)
+}
+
+func (c *client) insertCert(s interface{}, cert *ssh.Certificate, comment string, constraints []byte) error {
+	var req []byte
+	switch k := s.(type) {
+	case *rsa.PrivateKey:
+		if len(k.Primes) != 2 {
+			return fmt.Errorf("agent: unsupported RSA key with %d primes", len(k.Primes))
+		}
+		k.Precompute()
+		req = ssh.Marshal(rsaCertMsg{
+			Type:        cert.Type(),
+			CertBytes:   cert.Marshal(),
+			D:           k.D,
+			Iqmp:        k.Precomputed.Qinv,
+			P:           k.Primes[0],
+			Q:           k.Primes[1],
+			Comments:    comment,
+			Constraints: constraints,
+		})
+	case *dsa.PrivateKey:
+		req = ssh.Marshal(dsaCertMsg{
+			Type:        cert.Type(),
+			CertBytes:   cert.Marshal(),
+			X:           k.X,
+			Comments:    comment,
+			Constraints: constraints,
+		})
+	case *ecdsa.PrivateKey:
+		req = ssh.Marshal(ecdsaCertMsg{
+			Type:        cert.Type(),
+			CertBytes:   cert.Marshal(),
+			D:           k.D,
+			Comments:    comment,
+			Constraints: constraints,
+		})
+	case ed25519.PrivateKey:
+		req = ssh.Marshal(ed25519CertMsg{
+			Type:        cert.Type(),
+			CertBytes:   cert.Marshal(),
+			Pub:         []byte(k)[32:],
+			Priv:        []byte(k),
+			Comments:    comment,
+			Constraints: constraints,
+		})
+	// This function originally supported only *ed25519.PrivateKey, however the
+	// general idiom is to pass ed25519.PrivateKey by value, not by pointer.
+	// We still support the pointer variant for backwards compatibility.
+	case *ed25519.PrivateKey:
+		req = ssh.Marshal(ed25519CertMsg{
+			Type:        cert.Type(),
+			CertBytes:   cert.Marshal(),
+			Pub:         []byte(*k)[32:],
+			Priv:        []byte(*k),
+			Comments:    comment,
+			Constraints: constraints,
+		})
+	default:
+		return fmt.Errorf("agent: unsupported key type %T", s)
+	}
+
+	// if constraints are present then the message type needs to be changed.
+	if len(constraints) != 0 {
+		req[0] = agentAddIDConstrained
+	}
+
+	signer, err := ssh.NewSignerFromKey(s)
+	if err != nil {
+		return err
+	}
+	if !bytes.Equal(cert.Key.Marshal(), signer.PublicKey().Marshal()) {
+		return errors.New("agent: signer and cert have different public key")
+	}
+
+	resp, err := c.call(req)
+	if err != nil {
+		return err
+	}
+	if _, ok := resp.(*successAgentMsg); ok {
+		return nil
+	}
+	return errors.New("agent: failure")
+}
+
+// Signers provides a callback for client authentication.
+func (c *client) Signers() ([]ssh.Signer, error) {
+	keys, err := c.List()
+	if err != nil {
+		return nil, err
+	}
+
+	var result []ssh.Signer
+	for _, k := range keys {
+		result = append(result, &agentKeyringSigner{c, k})
+	}
+	return result, nil
+}
+
+type agentKeyringSigner struct {
+	agent *client
+	pub   ssh.PublicKey
+}
+
+func (s *agentKeyringSigner) PublicKey() ssh.PublicKey {
+	return s.pub
+}
+
+func (s *agentKeyringSigner) Sign(rand io.Reader, data []byte) (*ssh.Signature, error) {
+	// The agent has its own entropy source, so the rand argument is ignored.
+	return s.agent.Sign(s.pub, data)
+}
+
+func (s *agentKeyringSigner) SignWithAlgorithm(rand io.Reader, data []byte, algorithm string) (*ssh.Signature, error) {
+	if algorithm == "" || algorithm == underlyingAlgo(s.pub.Type()) {
+		return s.Sign(rand, data)
+	}
+
+	var flags SignatureFlags
+	switch algorithm {
+	case ssh.KeyAlgoRSASHA256:
+		flags = SignatureFlagRsaSha256
+	case ssh.KeyAlgoRSASHA512:
+		flags = SignatureFlagRsaSha512
+	default:
+		return nil, fmt.Errorf("agent: unsupported algorithm %q", algorithm)
+	}
+
+	return s.agent.SignWithFlags(s.pub, data, flags)
+}
+
+var _ ssh.AlgorithmSigner = &agentKeyringSigner{}
+
+// certKeyAlgoNames is a mapping from known certificate algorithm names to the
+// corresponding public key signature algorithm.
+//
+// This map must be kept in sync with the one in certs.go.
+var certKeyAlgoNames = map[string]string{
+	ssh.CertAlgoRSAv01:        ssh.KeyAlgoRSA,
+	ssh.CertAlgoRSASHA256v01:  ssh.KeyAlgoRSASHA256,
+	ssh.CertAlgoRSASHA512v01:  ssh.KeyAlgoRSASHA512,
+	ssh.CertAlgoDSAv01:        ssh.KeyAlgoDSA,
+	ssh.CertAlgoECDSA256v01:   ssh.KeyAlgoECDSA256,
+	ssh.CertAlgoECDSA384v01:   ssh.KeyAlgoECDSA384,
+	ssh.CertAlgoECDSA521v01:   ssh.KeyAlgoECDSA521,
+	ssh.CertAlgoSKECDSA256v01: ssh.KeyAlgoSKECDSA256,
+	ssh.CertAlgoED25519v01:    ssh.KeyAlgoED25519,
+	ssh.CertAlgoSKED25519v01:  ssh.KeyAlgoSKED25519,
+}
+
+// underlyingAlgo returns the signature algorithm associated with algo (which is
+// an advertised or negotiated public key or host key algorithm). These are
+// usually the same, except for certificate algorithms.
+func underlyingAlgo(algo string) string {
+	if a, ok := certKeyAlgoNames[algo]; ok {
+		return a
+	}
+	return algo
+}
+
+// Calls an extension method. It is up to the agent implementation as to whether or not
+// any particular extension is supported and may always return an error. Because the
+// type of the response is up to the implementation, this returns the bytes of the
+// response and does not attempt any type of unmarshalling.
+func (c *client) Extension(extensionType string, contents []byte) ([]byte, error) {
+	req := ssh.Marshal(extensionAgentMsg{
+		ExtensionType: extensionType,
+		Contents:      contents,
+	})
+	buf, err := c.callRaw(req)
+	if err != nil {
+		return nil, err
+	}
+	if len(buf) == 0 {
+		return nil, errors.New("agent: failure; empty response")
+	}
+	// [PROTOCOL.agent] section 4.7 indicates that an SSH_AGENT_FAILURE message
+	// represents an agent that does not support the extension
+	if buf[0] == agentFailure {
+		return nil, ErrExtensionUnsupported
+	}
+	if buf[0] == agentExtensionFailure {
+		return nil, errors.New("agent: generic extension failure")
+	}
+
+	return buf, nil
+}

pkg/proto/ssh/agent/forward.go

@@ -0,0 +1,103 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package agent
+
+import (
+	"errors"
+	"io"
+	"net"
+	"sync"
+
+	"github.com/Neur0toxine/sshpoke/pkg/proto/ssh"
+)
+
+// RequestAgentForwarding sets up agent forwarding for the session.
+// ForwardToAgent or ForwardToRemote should be called to route
+// the authentication requests.
+func RequestAgentForwarding(session *ssh.Session) error {
+	ok, err := session.SendRequest("auth-agent-req@openssh.com", true, nil)
+	if err != nil {
+		return err
+	}
+	if !ok {
+		return errors.New("forwarding request denied")
+	}
+	return nil
+}
+
+// ForwardToAgent routes authentication requests to the given keyring.
+func ForwardToAgent(client *ssh.Client, keyring Agent) error {
+	channels := client.HandleChannelOpen(channelType)
+	if channels == nil {
+		return errors.New("agent: already have handler for " + channelType)
+	}
+
+	go func() {
+		for ch := range channels {
+			channel, reqs, err := ch.Accept()
+			if err != nil {
+				continue
+			}
+			go ssh.DiscardRequests(reqs)
+			go func() {
+				ServeAgent(keyring, channel)
+				channel.Close()
+			}()
+		}
+	}()
+	return nil
+}
+
+const channelType = "auth-agent@openssh.com"
+
+// ForwardToRemote routes authentication requests to the ssh-agent
+// process serving on the given unix socket.
+func ForwardToRemote(client *ssh.Client, addr string) error {
+	channels := client.HandleChannelOpen(channelType)
+	if channels == nil {
+		return errors.New("agent: already have handler for " + channelType)
+	}
+	conn, err := net.Dial("unix", addr)
+	if err != nil {
+		return err
+	}
+	conn.Close()
+
+	go func() {
+		for ch := range channels {
+			channel, reqs, err := ch.Accept()
+			if err != nil {
+				continue
+			}
+			go ssh.DiscardRequests(reqs)
+			go forwardUnixSocket(channel, addr)
+		}
+	}()
+	return nil
+}
+
+func forwardUnixSocket(channel ssh.Channel, addr string) {
+	conn, err := net.Dial("unix", addr)
+	if err != nil {
+		return
+	}
+
+	var wg sync.WaitGroup
+	wg.Add(2)
+	go func() {
+		io.Copy(conn, channel)
+		conn.(*net.UnixConn).CloseWrite()
+		wg.Done()
+	}()
+	go func() {
+		io.Copy(channel, conn)
+		channel.CloseWrite()
+		wg.Done()
+	}()
+
+	wg.Wait()
+	conn.Close()
+	channel.Close()
+}

pkg/proto/ssh/agent/keyring.go

@@ -0,0 +1,241 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package agent
+
+import (
+	"bytes"
+	"crypto/rand"
+	"crypto/subtle"
+	"errors"
+	"fmt"
+	"sync"
+	"time"
+
+	"github.com/Neur0toxine/sshpoke/pkg/proto/ssh"
+)
+
+type privKey struct {
+	signer  ssh.Signer
+	comment string
+	expire  *time.Time
+}
+
+type keyring struct {
+	mu   sync.Mutex
+	keys []privKey
+
+	locked     bool
+	passphrase []byte
+}
+
+var errLocked = errors.New("agent: locked")
+
+// NewKeyring returns an Agent that holds keys in memory.  It is safe
+// for concurrent use by multiple goroutines.
+func NewKeyring() Agent {
+	return &keyring{}
+}
+
+// RemoveAll removes all identities.
+func (r *keyring) RemoveAll() error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if r.locked {
+		return errLocked
+	}
+
+	r.keys = nil
+	return nil
+}
+
+// removeLocked does the actual key removal. The caller must already be holding the
+// keyring mutex.
+func (r *keyring) removeLocked(want []byte) error {
+	found := false
+	for i := 0; i < len(r.keys); {
+		if bytes.Equal(r.keys[i].signer.PublicKey().Marshal(), want) {
+			found = true
+			r.keys[i] = r.keys[len(r.keys)-1]
+			r.keys = r.keys[:len(r.keys)-1]
+			continue
+		} else {
+			i++
+		}
+	}
+
+	if !found {
+		return errors.New("agent: key not found")
+	}
+	return nil
+}
+
+// Remove removes all identities with the given public key.
+func (r *keyring) Remove(key ssh.PublicKey) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if r.locked {
+		return errLocked
+	}
+
+	return r.removeLocked(key.Marshal())
+}
+
+// Lock locks the agent. Sign and Remove will fail, and List will return an empty list.
+func (r *keyring) Lock(passphrase []byte) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if r.locked {
+		return errLocked
+	}
+
+	r.locked = true
+	r.passphrase = passphrase
+	return nil
+}
+
+// Unlock undoes the effect of Lock
+func (r *keyring) Unlock(passphrase []byte) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if !r.locked {
+		return errors.New("agent: not locked")
+	}
+	if 1 != subtle.ConstantTimeCompare(passphrase, r.passphrase) {
+		return fmt.Errorf("agent: incorrect passphrase")
+	}
+
+	r.locked = false
+	r.passphrase = nil
+	return nil
+}
+
+// expireKeysLocked removes expired keys from the keyring. If a key was added
+// with a lifetimesecs contraint and seconds >= lifetimesecs seconds have
+// elapsed, it is removed. The caller *must* be holding the keyring mutex.
+func (r *keyring) expireKeysLocked() {
+	for _, k := range r.keys {
+		if k.expire != nil && time.Now().After(*k.expire) {
+			r.removeLocked(k.signer.PublicKey().Marshal())
+		}
+	}
+}
+
+// List returns the identities known to the agent.
+func (r *keyring) List() ([]*Key, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if r.locked {
+		// section 2.7: locked agents return empty.
+		return nil, nil
+	}
+
+	r.expireKeysLocked()
+	var ids []*Key
+	for _, k := range r.keys {
+		pub := k.signer.PublicKey()
+		ids = append(ids, &Key{
+			Format:  pub.Type(),
+			Blob:    pub.Marshal(),
+			Comment: k.comment})
+	}
+	return ids, nil
+}
+
+// Insert adds a private key to the keyring. If a certificate
+// is given, that certificate is added as public key. Note that
+// any constraints given are ignored.
+func (r *keyring) Add(key AddedKey) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if r.locked {
+		return errLocked
+	}
+	signer, err := ssh.NewSignerFromKey(key.PrivateKey)
+
+	if err != nil {
+		return err
+	}
+
+	if cert := key.Certificate; cert != nil {
+		signer, err = ssh.NewCertSigner(cert, signer)
+		if err != nil {
+			return err
+		}
+	}
+
+	p := privKey{
+		signer:  signer,
+		comment: key.Comment,
+	}
+
+	if key.LifetimeSecs > 0 {
+		t := time.Now().Add(time.Duration(key.LifetimeSecs) * time.Second)
+		p.expire = &t
+	}
+
+	r.keys = append(r.keys, p)
+
+	return nil
+}
+
+// Sign returns a signature for the data.
+func (r *keyring) Sign(key ssh.PublicKey, data []byte) (*ssh.Signature, error) {
+	return r.SignWithFlags(key, data, 0)
+}
+
+func (r *keyring) SignWithFlags(key ssh.PublicKey, data []byte, flags SignatureFlags) (*ssh.Signature, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if r.locked {
+		return nil, errLocked
+	}
+
+	r.expireKeysLocked()
+	wanted := key.Marshal()
+	for _, k := range r.keys {
+		if bytes.Equal(k.signer.PublicKey().Marshal(), wanted) {
+			if flags == 0 {
+				return k.signer.Sign(rand.Reader, data)
+			} else {
+				if algorithmSigner, ok := k.signer.(ssh.AlgorithmSigner); !ok {
+					return nil, fmt.Errorf("agent: signature does not support non-default signature algorithm: %T", k.signer)
+				} else {
+					var algorithm string
+					switch flags {
+					case SignatureFlagRsaSha256:
+						algorithm = ssh.KeyAlgoRSASHA256
+					case SignatureFlagRsaSha512:
+						algorithm = ssh.KeyAlgoRSASHA512
+					default:
+						return nil, fmt.Errorf("agent: unsupported signature flags: %d", flags)
+					}
+					return algorithmSigner.SignWithAlgorithm(rand.Reader, data, algorithm)
+				}
+			}
+		}
+	}
+	return nil, errors.New("not found")
+}
+
+// Signers returns signers for all the known keys.
+func (r *keyring) Signers() ([]ssh.Signer, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if r.locked {
+		return nil, errLocked
+	}
+
+	r.expireKeysLocked()
+	s := make([]ssh.Signer, 0, len(r.keys))
+	for _, k := range r.keys {
+		s = append(s, k.signer)
+	}
+	return s, nil
+}
+
+// The keyring does not support any extensions
+func (r *keyring) Extension(extensionType string, contents []byte) ([]byte, error) {
+	return nil, ErrExtensionUnsupported
+}

pkg/proto/ssh/agent/server.go

@@ -0,0 +1,570 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package agent
+
+import (
+	"crypto/dsa"
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/elliptic"
+	"crypto/rsa"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"math/big"
+
+	"github.com/Neur0toxine/sshpoke/pkg/proto/ssh"
+)
+
+// server wraps an Agent and uses it to implement the agent side of
+// the SSH-agent, wire protocol.
+type server struct {
+	agent Agent
+}
+
+func (s *server) processRequestBytes(reqData []byte) []byte {
+	rep, err := s.processRequest(reqData)
+	if err != nil {
+		if err != errLocked {
+			// TODO(hanwen): provide better logging interface?
+			log.Printf("agent %d: %v", reqData[0], err)
+		}
+		return []byte{agentFailure}
+	}
+
+	if err == nil && rep == nil {
+		return []byte{agentSuccess}
+	}
+
+	return ssh.Marshal(rep)
+}
+
+func marshalKey(k *Key) []byte {
+	var record struct {
+		Blob    []byte
+		Comment string
+	}
+	record.Blob = k.Marshal()
+	record.Comment = k.Comment
+
+	return ssh.Marshal(&record)
+}
+
+// See [PROTOCOL.agent], section 2.5.1.
+const agentV1IdentitiesAnswer = 2
+
+type agentV1IdentityMsg struct {
+	Numkeys uint32 `sshtype:"2"`
+}
+
+type agentRemoveIdentityMsg struct {
+	KeyBlob []byte `sshtype:"18"`
+}
+
+type agentLockMsg struct {
+	Passphrase []byte `sshtype:"22"`
+}
+
+type agentUnlockMsg struct {
+	Passphrase []byte `sshtype:"23"`
+}
+
+func (s *server) processRequest(data []byte) (interface{}, error) {
+	switch data[0] {
+	case agentRequestV1Identities:
+		return &agentV1IdentityMsg{0}, nil
+
+	case agentRemoveAllV1Identities:
+		return nil, nil
+
+	case agentRemoveIdentity:
+		var req agentRemoveIdentityMsg
+		if err := ssh.Unmarshal(data, &req); err != nil {
+			return nil, err
+		}
+
+		var wk wireKey
+		if err := ssh.Unmarshal(req.KeyBlob, &wk); err != nil {
+			return nil, err
+		}
+
+		return nil, s.agent.Remove(&Key{Format: wk.Format, Blob: req.KeyBlob})
+
+	case agentRemoveAllIdentities:
+		return nil, s.agent.RemoveAll()
+
+	case agentLock:
+		var req agentLockMsg
+		if err := ssh.Unmarshal(data, &req); err != nil {
+			return nil, err
+		}
+
+		return nil, s.agent.Lock(req.Passphrase)
+
+	case agentUnlock:
+		var req agentUnlockMsg
+		if err := ssh.Unmarshal(data, &req); err != nil {
+			return nil, err
+		}
+		return nil, s.agent.Unlock(req.Passphrase)
+
+	case agentSignRequest:
+		var req signRequestAgentMsg
+		if err := ssh.Unmarshal(data, &req); err != nil {
+			return nil, err
+		}
+
+		var wk wireKey
+		if err := ssh.Unmarshal(req.KeyBlob, &wk); err != nil {
+			return nil, err
+		}
+
+		k := &Key{
+			Format: wk.Format,
+			Blob:   req.KeyBlob,
+		}
+
+		var sig *ssh.Signature
+		var err error
+		if extendedAgent, ok := s.agent.(ExtendedAgent); ok {
+			sig, err = extendedAgent.SignWithFlags(k, req.Data, SignatureFlags(req.Flags))
+		} else {
+			sig, err = s.agent.Sign(k, req.Data)
+		}
+
+		if err != nil {
+			return nil, err
+		}
+		return &signResponseAgentMsg{SigBlob: ssh.Marshal(sig)}, nil
+
+	case agentRequestIdentities:
+		keys, err := s.agent.List()
+		if err != nil {
+			return nil, err
+		}
+
+		rep := identitiesAnswerAgentMsg{
+			NumKeys: uint32(len(keys)),
+		}
+		for _, k := range keys {
+			rep.Keys = append(rep.Keys, marshalKey(k)...)
+		}
+		return rep, nil
+
+	case agentAddIDConstrained, agentAddIdentity:
+		return nil, s.insertIdentity(data)
+
+	case agentExtension:
+		// Return a stub object where the whole contents of the response gets marshaled.
+		var responseStub struct {
+			Rest []byte `ssh:"rest"`
+		}
+
+		if extendedAgent, ok := s.agent.(ExtendedAgent); !ok {
+			// If this agent doesn't implement extensions, [PROTOCOL.agent] section 4.7
+			// requires that we return a standard SSH_AGENT_FAILURE message.
+			responseStub.Rest = []byte{agentFailure}
+		} else {
+			var req extensionAgentMsg
+			if err := ssh.Unmarshal(data, &req); err != nil {
+				return nil, err
+			}
+			res, err := extendedAgent.Extension(req.ExtensionType, req.Contents)
+			if err != nil {
+				// If agent extensions are unsupported, return a standard SSH_AGENT_FAILURE
+				// message as required by [PROTOCOL.agent] section 4.7.
+				if err == ErrExtensionUnsupported {
+					responseStub.Rest = []byte{agentFailure}
+				} else {
+					// As the result of any other error processing an extension request,
+					// [PROTOCOL.agent] section 4.7 requires that we return a
+					// SSH_AGENT_EXTENSION_FAILURE code.
+					responseStub.Rest = []byte{agentExtensionFailure}
+				}
+			} else {
+				if len(res) == 0 {
+					return nil, nil
+				}
+				responseStub.Rest = res
+			}
+		}
+
+		return responseStub, nil
+	}
+
+	return nil, fmt.Errorf("unknown opcode %d", data[0])
+}
+
+func parseConstraints(constraints []byte) (lifetimeSecs uint32, confirmBeforeUse bool, extensions []ConstraintExtension, err error) {
+	for len(constraints) != 0 {
+		switch constraints[0] {
+		case agentConstrainLifetime:
+			lifetimeSecs = binary.BigEndian.Uint32(constraints[1:5])
+			constraints = constraints[5:]
+		case agentConstrainConfirm:
+			confirmBeforeUse = true
+			constraints = constraints[1:]
+		case agentConstrainExtension, agentConstrainExtensionV00:
+			var msg constrainExtensionAgentMsg
+			if err = ssh.Unmarshal(constraints, &msg); err != nil {
+				return 0, false, nil, err
+			}
+			extensions = append(extensions, ConstraintExtension{
+				ExtensionName:    msg.ExtensionName,
+				ExtensionDetails: msg.ExtensionDetails,
+			})
+			constraints = msg.Rest
+		default:
+			return 0, false, nil, fmt.Errorf("unknown constraint type: %d", constraints[0])
+		}
+	}
+	return
+}
+
+func setConstraints(key *AddedKey, constraintBytes []byte) error {
+	lifetimeSecs, confirmBeforeUse, constraintExtensions, err := parseConstraints(constraintBytes)
+	if err != nil {
+		return err
+	}
+
+	key.LifetimeSecs = lifetimeSecs
+	key.ConfirmBeforeUse = confirmBeforeUse
+	key.ConstraintExtensions = constraintExtensions
+	return nil
+}
+
+func parseRSAKey(req []byte) (*AddedKey, error) {
+	var k rsaKeyMsg
+	if err := ssh.Unmarshal(req, &k); err != nil {
+		return nil, err
+	}
+	if k.E.BitLen() > 30 {
+		return nil, errors.New("agent: RSA public exponent too large")
+	}
+	priv := &rsa.PrivateKey{
+		PublicKey: rsa.PublicKey{
+			E: int(k.E.Int64()),
+			N: k.N,
+		},
+		D:      k.D,
+		Primes: []*big.Int{k.P, k.Q},
+	}
+	priv.Precompute()
+
+	addedKey := &AddedKey{PrivateKey: priv, Comment: k.Comments}
+	if err := setConstraints(addedKey, k.Constraints); err != nil {
+		return nil, err
+	}
+	return addedKey, nil
+}
+
+func parseEd25519Key(req []byte) (*AddedKey, error) {
+	var k ed25519KeyMsg
+	if err := ssh.Unmarshal(req, &k); err != nil {
+		return nil, err
+	}
+	priv := ed25519.PrivateKey(k.Priv)
+
+	addedKey := &AddedKey{PrivateKey: &priv, Comment: k.Comments}
+	if err := setConstraints(addedKey, k.Constraints); err != nil {
+		return nil, err
+	}
+	return addedKey, nil
+}
+
+func parseDSAKey(req []byte) (*AddedKey, error) {
+	var k dsaKeyMsg
+	if err := ssh.Unmarshal(req, &k); err != nil {
+		return nil, err
+	}
+	priv := &dsa.PrivateKey{
+		PublicKey: dsa.PublicKey{
+			Parameters: dsa.Parameters{
+				P: k.P,
+				Q: k.Q,
+				G: k.G,
+			},
+			Y: k.Y,
+		},
+		X: k.X,
+	}
+
+	addedKey := &AddedKey{PrivateKey: priv, Comment: k.Comments}
+	if err := setConstraints(addedKey, k.Constraints); err != nil {
+		return nil, err
+	}
+	return addedKey, nil
+}
+
+func unmarshalECDSA(curveName string, keyBytes []byte, privScalar *big.Int) (priv *ecdsa.PrivateKey, err error) {
+	priv = &ecdsa.PrivateKey{
+		D: privScalar,
+	}
+
+	switch curveName {
+	case "nistp256":
+		priv.Curve = elliptic.P256()
+	case "nistp384":
+		priv.Curve = elliptic.P384()
+	case "nistp521":
+		priv.Curve = elliptic.P521()
+	default:
+		return nil, fmt.Errorf("agent: unknown curve %q", curveName)
+	}
+
+	priv.X, priv.Y = elliptic.Unmarshal(priv.Curve, keyBytes)
+	if priv.X == nil || priv.Y == nil {
+		return nil, errors.New("agent: point not on curve")
+	}
+
+	return priv, nil
+}
+
+func parseEd25519Cert(req []byte) (*AddedKey, error) {
+	var k ed25519CertMsg
+	if err := ssh.Unmarshal(req, &k); err != nil {
+		return nil, err
+	}
+	pubKey, err := ssh.ParsePublicKey(k.CertBytes)
+	if err != nil {
+		return nil, err
+	}
+	priv := ed25519.PrivateKey(k.Priv)
+	cert, ok := pubKey.(*ssh.Certificate)
+	if !ok {
+		return nil, errors.New("agent: bad ED25519 certificate")
+	}
+
+	addedKey := &AddedKey{PrivateKey: &priv, Certificate: cert, Comment: k.Comments}
+	if err := setConstraints(addedKey, k.Constraints); err != nil {
+		return nil, err
+	}
+	return addedKey, nil
+}
+
+func parseECDSAKey(req []byte) (*AddedKey, error) {
+	var k ecdsaKeyMsg
+	if err := ssh.Unmarshal(req, &k); err != nil {
+		return nil, err
+	}
+
+	priv, err := unmarshalECDSA(k.Curve, k.KeyBytes, k.D)
+	if err != nil {
+		return nil, err
+	}
+
+	addedKey := &AddedKey{PrivateKey: priv, Comment: k.Comments}
+	if err := setConstraints(addedKey, k.Constraints); err != nil {
+		return nil, err
+	}
+	return addedKey, nil
+}
+
+func parseRSACert(req []byte) (*AddedKey, error) {
+	var k rsaCertMsg
+	if err := ssh.Unmarshal(req, &k); err != nil {
+		return nil, err
+	}
+
+	pubKey, err := ssh.ParsePublicKey(k.CertBytes)
+	if err != nil {
+		return nil, err
+	}
+
+	cert, ok := pubKey.(*ssh.Certificate)
+	if !ok {
+		return nil, errors.New("agent: bad RSA certificate")
+	}
+
+	// An RSA publickey as marshaled by rsaPublicKey.Marshal() in keys.go
+	var rsaPub struct {
+		Name string
+		E    *big.Int
+		N    *big.Int
+	}
+	if err := ssh.Unmarshal(cert.Key.Marshal(), &rsaPub); err != nil {
+		return nil, fmt.Errorf("agent: Unmarshal failed to parse public key: %v", err)
+	}
+
+	if rsaPub.E.BitLen() > 30 {
+		return nil, errors.New("agent: RSA public exponent too large")
+	}
+
+	priv := rsa.PrivateKey{
+		PublicKey: rsa.PublicKey{
+			E: int(rsaPub.E.Int64()),
+			N: rsaPub.N,
+		},
+		D:      k.D,
+		Primes: []*big.Int{k.Q, k.P},
+	}
+	priv.Precompute()
+
+	addedKey := &AddedKey{PrivateKey: &priv, Certificate: cert, Comment: k.Comments}
+	if err := setConstraints(addedKey, k.Constraints); err != nil {
+		return nil, err
+	}
+	return addedKey, nil
+}
+
+func parseDSACert(req []byte) (*AddedKey, error) {
+	var k dsaCertMsg
+	if err := ssh.Unmarshal(req, &k); err != nil {
+		return nil, err
+	}
+	pubKey, err := ssh.ParsePublicKey(k.CertBytes)
+	if err != nil {
+		return nil, err
+	}
+	cert, ok := pubKey.(*ssh.Certificate)
+	if !ok {
+		return nil, errors.New("agent: bad DSA certificate")
+	}
+
+	// A DSA publickey as marshaled by dsaPublicKey.Marshal() in keys.go
+	var w struct {
+		Name       string
+		P, Q, G, Y *big.Int
+	}
+	if err := ssh.Unmarshal(cert.Key.Marshal(), &w); err != nil {
+		return nil, fmt.Errorf("agent: Unmarshal failed to parse public key: %v", err)
+	}
+
+	priv := &dsa.PrivateKey{
+		PublicKey: dsa.PublicKey{
+			Parameters: dsa.Parameters{
+				P: w.P,
+				Q: w.Q,
+				G: w.G,
+			},
+			Y: w.Y,
+		},
+		X: k.X,
+	}
+
+	addedKey := &AddedKey{PrivateKey: priv, Certificate: cert, Comment: k.Comments}
+	if err := setConstraints(addedKey, k.Constraints); err != nil {
+		return nil, err
+	}
+	return addedKey, nil
+}
+
+func parseECDSACert(req []byte) (*AddedKey, error) {
+	var k ecdsaCertMsg
+	if err := ssh.Unmarshal(req, &k); err != nil {
+		return nil, err
+	}
+
+	pubKey, err := ssh.ParsePublicKey(k.CertBytes)
+	if err != nil {
+		return nil, err
+	}
+	cert, ok := pubKey.(*ssh.Certificate)
+	if !ok {
+		return nil, errors.New("agent: bad ECDSA certificate")
+	}
+
+	// An ECDSA publickey as marshaled by ecdsaPublicKey.Marshal() in keys.go
+	var ecdsaPub struct {
+		Name string
+		ID   string
+		Key  []byte
+	}
+	if err := ssh.Unmarshal(cert.Key.Marshal(), &ecdsaPub); err != nil {
+		return nil, err
+	}
+
+	priv, err := unmarshalECDSA(ecdsaPub.ID, ecdsaPub.Key, k.D)
+	if err != nil {
+		return nil, err
+	}
+
+	addedKey := &AddedKey{PrivateKey: priv, Certificate: cert, Comment: k.Comments}
+	if err := setConstraints(addedKey, k.Constraints); err != nil {
+		return nil, err
+	}
+	return addedKey, nil
+}
+
+func (s *server) insertIdentity(req []byte) error {
+	var record struct {
+		Type string `sshtype:"17|25"`
+		Rest []byte `ssh:"rest"`
+	}
+
+	if err := ssh.Unmarshal(req, &record); err != nil {
+		return err
+	}
+
+	var addedKey *AddedKey
+	var err error
+
+	switch record.Type {
+	case ssh.KeyAlgoRSA:
+		addedKey, err = parseRSAKey(req)
+	case ssh.KeyAlgoDSA:
+		addedKey, err = parseDSAKey(req)
+	case ssh.KeyAlgoECDSA256, ssh.KeyAlgoECDSA384, ssh.KeyAlgoECDSA521:
+		addedKey, err = parseECDSAKey(req)
+	case ssh.KeyAlgoED25519:
+		addedKey, err = parseEd25519Key(req)
+	case ssh.CertAlgoRSAv01:
+		addedKey, err = parseRSACert(req)
+	case ssh.CertAlgoDSAv01:
+		addedKey, err = parseDSACert(req)
+	case ssh.CertAlgoECDSA256v01, ssh.CertAlgoECDSA384v01, ssh.CertAlgoECDSA521v01:
+		addedKey, err = parseECDSACert(req)
+	case ssh.CertAlgoED25519v01:
+		addedKey, err = parseEd25519Cert(req)
+	default:
+		return fmt.Errorf("agent: not implemented: %q", record.Type)
+	}
+
+	if err != nil {
+		return err
+	}
+	return s.agent.Add(*addedKey)
+}
+
+// ServeAgent serves the agent protocol on the given connection. It
+// returns when an I/O error occurs.
+func ServeAgent(agent Agent, c io.ReadWriter) error {
+	s := &server{agent}
+
+	var length [4]byte
+	for {
+		if _, err := io.ReadFull(c, length[:]); err != nil {
+			return err
+		}
+		l := binary.BigEndian.Uint32(length[:])
+		if l == 0 {
+			return fmt.Errorf("agent: request size is 0")
+		}
+		if l > maxAgentResponseBytes {
+			// We also cap requests.
+			return fmt.Errorf("agent: request too large: %d", l)
+		}
+
+		req := make([]byte, l)
+		if _, err := io.ReadFull(c, req); err != nil {
+			return err
+		}
+
+		repData := s.processRequestBytes(req)
+		if len(repData) > maxAgentResponseBytes {
+			return fmt.Errorf("agent: reply too large: %d bytes", len(repData))
+		}
+
+		binary.BigEndian.PutUint32(length[:], uint32(len(repData)))
+		if _, err := c.Write(length[:]); err != nil {
+			return err
+		}
+		if _, err := c.Write(repData); err != nil {
+			return err
+		}
+	}
+}

pkg/proto/ssh/buffer.go

@@ -0,0 +1,97 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"io"
+	"sync"
+)
+
+// buffer provides a linked list buffer for data exchange
+// between producer and consumer. Theoretically the buffer is
+// of unlimited capacity as it does no allocation of its own.
+type buffer struct {
+	// protects concurrent access to head, tail and closed
+	*sync.Cond
+
+	head *element // the buffer that will be read first
+	tail *element // the buffer that will be read last
+
+	closed bool
+}
+
+// An element represents a single link in a linked list.
+type element struct {
+	buf  []byte
+	next *element
+}
+
+// newBuffer returns an empty buffer that is not closed.
+func newBuffer() *buffer {
+	e := new(element)
+	b := &buffer{
+		Cond: newCond(),
+		head: e,
+		tail: e,
+	}
+	return b
+}
+
+// write makes buf available for Read to receive.
+// buf must not be modified after the call to write.
+func (b *buffer) write(buf []byte) {
+	b.Cond.L.Lock()
+	e := &element{buf: buf}
+	b.tail.next = e
+	b.tail = e
+	b.Cond.Signal()
+	b.Cond.L.Unlock()
+}
+
+// eof closes the buffer. Reads from the buffer once all
+// the data has been consumed will receive io.EOF.
+func (b *buffer) eof() {
+	b.Cond.L.Lock()
+	b.closed = true
+	b.Cond.Signal()
+	b.Cond.L.Unlock()
+}
+
+// Read reads data from the internal buffer in buf.  Reads will block
+// if no data is available, or until the buffer is closed.
+func (b *buffer) Read(buf []byte) (n int, err error) {
+	b.Cond.L.Lock()
+	defer b.Cond.L.Unlock()
+
+	for len(buf) > 0 {
+		// if there is data in b.head, copy it
+		if len(b.head.buf) > 0 {
+			r := copy(buf, b.head.buf)
+			buf, b.head.buf = buf[r:], b.head.buf[r:]
+			n += r
+			continue
+		}
+		// if there is a next buffer, make it the head
+		if len(b.head.buf) == 0 && b.head != b.tail {
+			b.head = b.head.next
+			continue
+		}
+
+		// if at least one byte has been copied, return
+		if n > 0 {
+			break
+		}
+
+		// if nothing was read, and there is nothing outstanding
+		// check to see if the buffer is closed.
+		if b.closed {
+			err = io.EOF
+			break
+		}
+		// out of buffers, wait for producer
+		b.Cond.Wait()
+	}
+	return
+}

pkg/proto/ssh/certs.go

@@ -0,0 +1,611 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"sort"
+	"time"
+)
+
+// Certificate algorithm names from [PROTOCOL.certkeys]. These values can appear
+// in Certificate.Type, PublicKey.Type, and ClientConfig.HostKeyAlgorithms.
+// Unlike key algorithm names, these are not passed to AlgorithmSigner nor
+// returned by MultiAlgorithmSigner and don't appear in the Signature.Format
+// field.
+const (
+	CertAlgoRSAv01        = "ssh-rsa-cert-v01@openssh.com"
+	CertAlgoDSAv01        = "ssh-dss-cert-v01@openssh.com"
+	CertAlgoECDSA256v01   = "ecdsa-sha2-nistp256-cert-v01@openssh.com"
+	CertAlgoECDSA384v01   = "ecdsa-sha2-nistp384-cert-v01@openssh.com"
+	CertAlgoECDSA521v01   = "ecdsa-sha2-nistp521-cert-v01@openssh.com"
+	CertAlgoSKECDSA256v01 = "sk-ecdsa-sha2-nistp256-cert-v01@openssh.com"
+	CertAlgoED25519v01    = "ssh-ed25519-cert-v01@openssh.com"
+	CertAlgoSKED25519v01  = "sk-ssh-ed25519-cert-v01@openssh.com"
+
+	// CertAlgoRSASHA256v01 and CertAlgoRSASHA512v01 can't appear as a
+	// Certificate.Type (or PublicKey.Type), but only in
+	// ClientConfig.HostKeyAlgorithms.
+	CertAlgoRSASHA256v01 = "rsa-sha2-256-cert-v01@openssh.com"
+	CertAlgoRSASHA512v01 = "rsa-sha2-512-cert-v01@openssh.com"
+)
+
+const (
+	// Deprecated: use CertAlgoRSAv01.
+	CertSigAlgoRSAv01 = CertAlgoRSAv01
+	// Deprecated: use CertAlgoRSASHA256v01.
+	CertSigAlgoRSASHA2256v01 = CertAlgoRSASHA256v01
+	// Deprecated: use CertAlgoRSASHA512v01.
+	CertSigAlgoRSASHA2512v01 = CertAlgoRSASHA512v01
+)
+
+// Certificate types distinguish between host and user
+// certificates. The values can be set in the CertType field of
+// Certificate.
+const (
+	UserCert = 1
+	HostCert = 2
+)
+
+// Signature represents a cryptographic signature.
+type Signature struct {
+	Format string
+	Blob   []byte
+	Rest   []byte `ssh:"rest"`
+}
+
+// CertTimeInfinity can be used for OpenSSHCertV01.ValidBefore to indicate that
+// a certificate does not expire.
+const CertTimeInfinity = 1<<64 - 1
+
+// An Certificate represents an OpenSSH certificate as defined in
+// [PROTOCOL.certkeys]?rev=1.8. The Certificate type implements the
+// PublicKey interface, so it can be unmarshaled using
+// ParsePublicKey.
+type Certificate struct {
+	Nonce           []byte
+	Key             PublicKey
+	Serial          uint64
+	CertType        uint32
+	KeyId           string
+	ValidPrincipals []string
+	ValidAfter      uint64
+	ValidBefore     uint64
+	Permissions
+	Reserved     []byte
+	SignatureKey PublicKey
+	Signature    *Signature
+}
+
+// genericCertData holds the key-independent part of the certificate data.
+// Overall, certificates contain an nonce, public key fields and
+// key-independent fields.
+type genericCertData struct {
+	Serial          uint64
+	CertType        uint32
+	KeyId           string
+	ValidPrincipals []byte
+	ValidAfter      uint64
+	ValidBefore     uint64
+	CriticalOptions []byte
+	Extensions      []byte
+	Reserved        []byte
+	SignatureKey    []byte
+	Signature       []byte
+}
+
+func marshalStringList(namelist []string) []byte {
+	var to []byte
+	for _, name := range namelist {
+		s := struct{ N string }{name}
+		to = append(to, Marshal(&s)...)
+	}
+	return to
+}
+
+type optionsTuple struct {
+	Key   string
+	Value []byte
+}
+
+type optionsTupleValue struct {
+	Value string
+}
+
+// serialize a map of critical options or extensions
+// issue #10569 - per [PROTOCOL.certkeys] and SSH implementation,
+// we need two length prefixes for a non-empty string value
+func marshalTuples(tups map[string]string) []byte {
+	keys := make([]string, 0, len(tups))
+	for key := range tups {
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+
+	var ret []byte
+	for _, key := range keys {
+		s := optionsTuple{Key: key}
+		if value := tups[key]; len(value) > 0 {
+			s.Value = Marshal(&optionsTupleValue{value})
+		}
+		ret = append(ret, Marshal(&s)...)
+	}
+	return ret
+}
+
+// issue #10569 - per [PROTOCOL.certkeys] and SSH implementation,
+// we need two length prefixes for a non-empty option value
+func parseTuples(in []byte) (map[string]string, error) {
+	tups := map[string]string{}
+	var lastKey string
+	var haveLastKey bool
+
+	for len(in) > 0 {
+		var key, val, extra []byte
+		var ok bool
+
+		if key, in, ok = parseString(in); !ok {
+			return nil, errShortRead
+		}
+		keyStr := string(key)
+		// according to [PROTOCOL.certkeys], the names must be in
+		// lexical order.
+		if haveLastKey && keyStr <= lastKey {
+			return nil, fmt.Errorf("ssh: certificate options are not in lexical order")
+		}
+		lastKey, haveLastKey = keyStr, true
+		// the next field is a data field, which if non-empty has a string embedded
+		if val, in, ok = parseString(in); !ok {
+			return nil, errShortRead
+		}
+		if len(val) > 0 {
+			val, extra, ok = parseString(val)
+			if !ok {
+				return nil, errShortRead
+			}
+			if len(extra) > 0 {
+				return nil, fmt.Errorf("ssh: unexpected trailing data after certificate option value")
+			}
+			tups[keyStr] = string(val)
+		} else {
+			tups[keyStr] = ""
+		}
+	}
+	return tups, nil
+}
+
+func parseCert(in []byte, privAlgo string) (*Certificate, error) {
+	nonce, rest, ok := parseString(in)
+	if !ok {
+		return nil, errShortRead
+	}
+
+	key, rest, err := parsePubKey(rest, privAlgo)
+	if err != nil {
+		return nil, err
+	}
+
+	var g genericCertData
+	if err := Unmarshal(rest, &g); err != nil {
+		return nil, err
+	}
+
+	c := &Certificate{
+		Nonce:       nonce,
+		Key:         key,
+		Serial:      g.Serial,
+		CertType:    g.CertType,
+		KeyId:       g.KeyId,
+		ValidAfter:  g.ValidAfter,
+		ValidBefore: g.ValidBefore,
+	}
+
+	for principals := g.ValidPrincipals; len(principals) > 0; {
+		principal, rest, ok := parseString(principals)
+		if !ok {
+			return nil, errShortRead
+		}
+		c.ValidPrincipals = append(c.ValidPrincipals, string(principal))
+		principals = rest
+	}
+
+	c.CriticalOptions, err = parseTuples(g.CriticalOptions)
+	if err != nil {
+		return nil, err
+	}
+	c.Extensions, err = parseTuples(g.Extensions)
+	if err != nil {
+		return nil, err
+	}
+	c.Reserved = g.Reserved
+	k, err := ParsePublicKey(g.SignatureKey)
+	if err != nil {
+		return nil, err
+	}
+
+	c.SignatureKey = k
+	c.Signature, rest, ok = parseSignatureBody(g.Signature)
+	if !ok || len(rest) > 0 {
+		return nil, errors.New("ssh: signature parse error")
+	}
+
+	return c, nil
+}
+
+type openSSHCertSigner struct {
+	pub    *Certificate
+	signer Signer
+}
+
+type algorithmOpenSSHCertSigner struct {
+	*openSSHCertSigner
+	algorithmSigner AlgorithmSigner
+}
+
+// NewCertSigner returns a Signer that signs with the given Certificate, whose
+// private key is held by signer. It returns an error if the public key in cert
+// doesn't match the key used by signer.
+func NewCertSigner(cert *Certificate, signer Signer) (Signer, error) {
+	if !bytes.Equal(cert.Key.Marshal(), signer.PublicKey().Marshal()) {
+		return nil, errors.New("ssh: signer and cert have different public key")
+	}
+
+	switch s := signer.(type) {
+	case MultiAlgorithmSigner:
+		return &multiAlgorithmSigner{
+			AlgorithmSigner: &algorithmOpenSSHCertSigner{
+				&openSSHCertSigner{cert, signer}, s},
+			supportedAlgorithms: s.Algorithms(),
+		}, nil
+	case AlgorithmSigner:
+		return &algorithmOpenSSHCertSigner{
+			&openSSHCertSigner{cert, signer}, s}, nil
+	default:
+		return &openSSHCertSigner{cert, signer}, nil
+	}
+}
+
+func (s *openSSHCertSigner) Sign(rand io.Reader, data []byte) (*Signature, error) {
+	return s.signer.Sign(rand, data)
+}
+
+func (s *openSSHCertSigner) PublicKey() PublicKey {
+	return s.pub
+}
+
+func (s *algorithmOpenSSHCertSigner) SignWithAlgorithm(rand io.Reader, data []byte, algorithm string) (*Signature, error) {
+	return s.algorithmSigner.SignWithAlgorithm(rand, data, algorithm)
+}
+
+const sourceAddressCriticalOption = "source-address"
+
+// CertChecker does the work of verifying a certificate. Its methods
+// can be plugged into ClientConfig.HostKeyCallback and
+// ServerConfig.PublicKeyCallback. For the CertChecker to work,
+// minimally, the IsAuthority callback should be set.
+type CertChecker struct {
+	// SupportedCriticalOptions lists the CriticalOptions that the
+	// server application layer understands. These are only used
+	// for user certificates.
+	SupportedCriticalOptions []string
+
+	// IsUserAuthority should return true if the key is recognized as an
+	// authority for the given user certificate. This allows for
+	// certificates to be signed by other certificates. This must be set
+	// if this CertChecker will be checking user certificates.
+	IsUserAuthority func(auth PublicKey) bool
+
+	// IsHostAuthority should report whether the key is recognized as
+	// an authority for this host. This allows for certificates to be
+	// signed by other keys, and for those other keys to only be valid
+	// signers for particular hostnames. This must be set if this
+	// CertChecker will be checking host certificates.
+	IsHostAuthority func(auth PublicKey, address string) bool
+
+	// Clock is used for verifying time stamps. If nil, time.Now
+	// is used.
+	Clock func() time.Time
+
+	// UserKeyFallback is called when CertChecker.Authenticate encounters a
+	// public key that is not a certificate. It must implement validation
+	// of user keys or else, if nil, all such keys are rejected.
+	UserKeyFallback func(conn ConnMetadata, key PublicKey) (*Permissions, error)
+
+	// HostKeyFallback is called when CertChecker.CheckHostKey encounters a
+	// public key that is not a certificate. It must implement host key
+	// validation or else, if nil, all such keys are rejected.
+	HostKeyFallback HostKeyCallback
+
+	// IsRevoked is called for each certificate so that revocation checking
+	// can be implemented. It should return true if the given certificate
+	// is revoked and false otherwise. If nil, no certificates are
+	// considered to have been revoked.
+	IsRevoked func(cert *Certificate) bool
+}
+
+// CheckHostKey checks a host key certificate. This method can be
+// plugged into ClientConfig.HostKeyCallback.
+func (c *CertChecker) CheckHostKey(addr string, remote net.Addr, key PublicKey) error {
+	cert, ok := key.(*Certificate)
+	if !ok {
+		if c.HostKeyFallback != nil {
+			return c.HostKeyFallback(addr, remote, key)
+		}
+		return errors.New("ssh: non-certificate host key")
+	}
+	if cert.CertType != HostCert {
+		return fmt.Errorf("ssh: certificate presented as a host key has type %d", cert.CertType)
+	}
+	if !c.IsHostAuthority(cert.SignatureKey, addr) {
+		return fmt.Errorf("ssh: no authorities for hostname: %v", addr)
+	}
+
+	hostname, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		return err
+	}
+
+	// Pass hostname only as principal for host certificates (consistent with OpenSSH)
+	return c.CheckCert(hostname, cert)
+}
+
+// Authenticate checks a user certificate. Authenticate can be used as
+// a value for ServerConfig.PublicKeyCallback.
+func (c *CertChecker) Authenticate(conn ConnMetadata, pubKey PublicKey) (*Permissions, error) {
+	cert, ok := pubKey.(*Certificate)
+	if !ok {
+		if c.UserKeyFallback != nil {
+			return c.UserKeyFallback(conn, pubKey)
+		}
+		return nil, errors.New("ssh: normal key pairs not accepted")
+	}
+
+	if cert.CertType != UserCert {
+		return nil, fmt.Errorf("ssh: cert has type %d", cert.CertType)
+	}
+	if !c.IsUserAuthority(cert.SignatureKey) {
+		return nil, fmt.Errorf("ssh: certificate signed by unrecognized authority")
+	}
+
+	if err := c.CheckCert(conn.User(), cert); err != nil {
+		return nil, err
+	}
+
+	return &cert.Permissions, nil
+}
+
+// CheckCert checks CriticalOptions, ValidPrincipals, revocation, timestamp and
+// the signature of the certificate.
+func (c *CertChecker) CheckCert(principal string, cert *Certificate) error {
+	if c.IsRevoked != nil && c.IsRevoked(cert) {
+		return fmt.Errorf("ssh: certificate serial %d revoked", cert.Serial)
+	}
+
+	for opt := range cert.CriticalOptions {
+		// sourceAddressCriticalOption will be enforced by
+		// serverAuthenticate
+		if opt == sourceAddressCriticalOption {
+			continue
+		}
+
+		found := false
+		for _, supp := range c.SupportedCriticalOptions {
+			if supp == opt {
+				found = true
+				break
+			}
+		}
+		if !found {
+			return fmt.Errorf("ssh: unsupported critical option %q in certificate", opt)
+		}
+	}
+
+	if len(cert.ValidPrincipals) > 0 {
+		// By default, certs are valid for all users/hosts.
+		found := false
+		for _, p := range cert.ValidPrincipals {
+			if p == principal {
+				found = true
+				break
+			}
+		}
+		if !found {
+			return fmt.Errorf("ssh: principal %q not in the set of valid principals for given certificate: %q", principal, cert.ValidPrincipals)
+		}
+	}
+
+	clock := c.Clock
+	if clock == nil {
+		clock = time.Now
+	}
+
+	unixNow := clock().Unix()
+	if after := int64(cert.ValidAfter); after < 0 || unixNow < int64(cert.ValidAfter) {
+		return fmt.Errorf("ssh: cert is not yet valid")
+	}
+	if before := int64(cert.ValidBefore); cert.ValidBefore != uint64(CertTimeInfinity) && (unixNow >= before || before < 0) {
+		return fmt.Errorf("ssh: cert has expired")
+	}
+	if err := cert.SignatureKey.Verify(cert.bytesForSigning(), cert.Signature); err != nil {
+		return fmt.Errorf("ssh: certificate signature does not verify")
+	}
+
+	return nil
+}
+
+// SignCert signs the certificate with an authority, setting the Nonce,
+// SignatureKey, and Signature fields. If the authority implements the
+// MultiAlgorithmSigner interface the first algorithm in the list is used. This
+// is useful if you want to sign with a specific algorithm.
+func (c *Certificate) SignCert(rand io.Reader, authority Signer) error {
+	c.Nonce = make([]byte, 32)
+	if _, err := io.ReadFull(rand, c.Nonce); err != nil {
+		return err
+	}
+	c.SignatureKey = authority.PublicKey()
+
+	if v, ok := authority.(MultiAlgorithmSigner); ok {
+		if len(v.Algorithms()) == 0 {
+			return errors.New("the provided authority has no signature algorithm")
+		}
+		// Use the first algorithm in the list.
+		sig, err := v.SignWithAlgorithm(rand, c.bytesForSigning(), v.Algorithms()[0])
+		if err != nil {
+			return err
+		}
+		c.Signature = sig
+		return nil
+	} else if v, ok := authority.(AlgorithmSigner); ok && v.PublicKey().Type() == KeyAlgoRSA {
+		// Default to KeyAlgoRSASHA512 for ssh-rsa signers.
+		// TODO: consider using KeyAlgoRSASHA256 as default.
+		sig, err := v.SignWithAlgorithm(rand, c.bytesForSigning(), KeyAlgoRSASHA512)
+		if err != nil {
+			return err
+		}
+		c.Signature = sig
+		return nil
+	}
+
+	sig, err := authority.Sign(rand, c.bytesForSigning())
+	if err != nil {
+		return err
+	}
+	c.Signature = sig
+	return nil
+}
+
+// certKeyAlgoNames is a mapping from known certificate algorithm names to the
+// corresponding public key signature algorithm.
+//
+// This map must be kept in sync with the one in agent/client.go.
+var certKeyAlgoNames = map[string]string{
+	CertAlgoRSAv01:        KeyAlgoRSA,
+	CertAlgoRSASHA256v01:  KeyAlgoRSASHA256,
+	CertAlgoRSASHA512v01:  KeyAlgoRSASHA512,
+	CertAlgoDSAv01:        KeyAlgoDSA,
+	CertAlgoECDSA256v01:   KeyAlgoECDSA256,
+	CertAlgoECDSA384v01:   KeyAlgoECDSA384,
+	CertAlgoECDSA521v01:   KeyAlgoECDSA521,
+	CertAlgoSKECDSA256v01: KeyAlgoSKECDSA256,
+	CertAlgoED25519v01:    KeyAlgoED25519,
+	CertAlgoSKED25519v01:  KeyAlgoSKED25519,
+}
+
+// underlyingAlgo returns the signature algorithm associated with algo (which is
+// an advertised or negotiated public key or host key algorithm). These are
+// usually the same, except for certificate algorithms.
+func underlyingAlgo(algo string) string {
+	if a, ok := certKeyAlgoNames[algo]; ok {
+		return a
+	}
+	return algo
+}
+
+// certificateAlgo returns the certificate algorithms that uses the provided
+// underlying signature algorithm.
+func certificateAlgo(algo string) (certAlgo string, ok bool) {
+	for certName, algoName := range certKeyAlgoNames {
+		if algoName == algo {
+			return certName, true
+		}
+	}
+	return "", false
+}
+
+func (cert *Certificate) bytesForSigning() []byte {
+	c2 := *cert
+	c2.Signature = nil
+	out := c2.Marshal()
+	// Drop trailing signature length.
+	return out[:len(out)-4]
+}
+
+// Marshal serializes c into OpenSSH's wire format. It is part of the
+// PublicKey interface.
+func (c *Certificate) Marshal() []byte {
+	generic := genericCertData{
+		Serial:          c.Serial,
+		CertType:        c.CertType,
+		KeyId:           c.KeyId,
+		ValidPrincipals: marshalStringList(c.ValidPrincipals),
+		ValidAfter:      uint64(c.ValidAfter),
+		ValidBefore:     uint64(c.ValidBefore),
+		CriticalOptions: marshalTuples(c.CriticalOptions),
+		Extensions:      marshalTuples(c.Extensions),
+		Reserved:        c.Reserved,
+		SignatureKey:    c.SignatureKey.Marshal(),
+	}
+	if c.Signature != nil {
+		generic.Signature = Marshal(c.Signature)
+	}
+	genericBytes := Marshal(&generic)
+	keyBytes := c.Key.Marshal()
+	_, keyBytes, _ = parseString(keyBytes)
+	prefix := Marshal(&struct {
+		Name  string
+		Nonce []byte
+		Key   []byte `ssh:"rest"`
+	}{c.Type(), c.Nonce, keyBytes})
+
+	result := make([]byte, 0, len(prefix)+len(genericBytes))
+	result = append(result, prefix...)
+	result = append(result, genericBytes...)
+	return result
+}
+
+// Type returns the certificate algorithm name. It is part of the PublicKey interface.
+func (c *Certificate) Type() string {
+	certName, ok := certificateAlgo(c.Key.Type())
+	if !ok {
+		panic("unknown certificate type for key type " + c.Key.Type())
+	}
+	return certName
+}
+
+// Verify verifies a signature against the certificate's public
+// key. It is part of the PublicKey interface.
+func (c *Certificate) Verify(data []byte, sig *Signature) error {
+	return c.Key.Verify(data, sig)
+}
+
+func parseSignatureBody(in []byte) (out *Signature, rest []byte, ok bool) {
+	format, in, ok := parseString(in)
+	if !ok {
+		return
+	}
+
+	out = &Signature{
+		Format: string(format),
+	}
+
+	if out.Blob, in, ok = parseString(in); !ok {
+		return
+	}
+
+	switch out.Format {
+	case KeyAlgoSKECDSA256, CertAlgoSKECDSA256v01, KeyAlgoSKED25519, CertAlgoSKED25519v01:
+		out.Rest = in
+		return out, nil, ok
+	}
+
+	return out, in, ok
+}
+
+func parseSignature(in []byte) (out *Signature, rest []byte, ok bool) {
+	sigBytes, rest, ok := parseString(in)
+	if !ok {
+		return
+	}
+
+	out, trailing, ok := parseSignatureBody(sigBytes)
+	if !ok || len(trailing) > 0 {
+		return nil, nil, false
+	}
+	return
+}

pkg/proto/ssh/channel.go

@@ -0,0 +1,633 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"sync"
+)
+
+const (
+	minPacketLength = 9
+	// channelMaxPacket contains the maximum number of bytes that will be
+	// sent in a single packet. As per RFC 4253, section 6.1, 32k is also
+	// the minimum.
+	channelMaxPacket = 1 << 15
+	// We follow OpenSSH here.
+	channelWindowSize = 64 * channelMaxPacket
+)
+
+// NewChannel represents an incoming request to a channel. It must either be
+// accepted for use by calling Accept, or rejected by calling Reject.
+type NewChannel interface {
+	// Accept accepts the channel creation request. It returns the Channel
+	// and a Go channel containing SSH requests. The Go channel must be
+	// serviced otherwise the Channel will hang.
+	Accept() (Channel, <-chan *Request, error)
+
+	// Reject rejects the channel creation request. After calling
+	// this, no other methods on the Channel may be called.
+	Reject(reason RejectionReason, message string) error
+
+	// ChannelType returns the type of the channel, as supplied by the
+	// client.
+	ChannelType() string
+
+	// ExtraData returns the arbitrary payload for this channel, as supplied
+	// by the client. This data is specific to the channel type.
+	ExtraData() []byte
+}
+
+// A Channel is an ordered, reliable, flow-controlled, duplex stream
+// that is multiplexed over an SSH connection.
+type Channel interface {
+	// Read reads up to len(data) bytes from the channel.
+	Read(data []byte) (int, error)
+
+	// Write writes len(data) bytes to the channel.
+	Write(data []byte) (int, error)
+
+	// Close signals end of channel use. No data may be sent after this
+	// call.
+	Close() error
+
+	// CloseWrite signals the end of sending in-band
+	// data. Requests may still be sent, and the other side may
+	// still send data
+	CloseWrite() error
+
+	// SendRequest sends a channel request.  If wantReply is true,
+	// it will wait for a reply and return the result as a
+	// boolean, otherwise the return value will be false. Channel
+	// requests are out-of-band messages so they may be sent even
+	// if the data stream is closed or blocked by flow control.
+	// If the channel is closed before a reply is returned, io.EOF
+	// is returned.
+	SendRequest(name string, wantReply bool, payload []byte) (bool, error)
+
+	// Stderr returns an io.ReadWriter that writes to this channel
+	// with the extended data type set to stderr. Stderr may
+	// safely be read and written from a different goroutine than
+	// Read and Write respectively.
+	Stderr() io.ReadWriter
+}
+
+// Request is a request sent outside of the normal stream of
+// data. Requests can either be specific to an SSH channel, or they
+// can be global.
+type Request struct {
+	Type      string
+	WantReply bool
+	Payload   []byte
+
+	ch  *channel
+	mux *mux
+}
+
+// Reply sends a response to a request. It must be called for all requests
+// where WantReply is true and is a no-op otherwise. The payload argument is
+// ignored for replies to channel-specific requests.
+func (r *Request) Reply(ok bool, payload []byte) error {
+	if !r.WantReply {
+		return nil
+	}
+
+	if r.ch == nil {
+		return r.mux.ackRequest(ok, payload)
+	}
+
+	return r.ch.ackRequest(ok)
+}
+
+// RejectionReason is an enumeration used when rejecting channel creation
+// requests. See RFC 4254, section 5.1.
+type RejectionReason uint32
+
+const (
+	Prohibited RejectionReason = iota + 1
+	ConnectionFailed
+	UnknownChannelType
+	ResourceShortage
+)
+
+// String converts the rejection reason to human readable form.
+func (r RejectionReason) String() string {
+	switch r {
+	case Prohibited:
+		return "administratively prohibited"
+	case ConnectionFailed:
+		return "connect failed"
+	case UnknownChannelType:
+		return "unknown channel type"
+	case ResourceShortage:
+		return "resource shortage"
+	}
+	return fmt.Sprintf("unknown reason %d", int(r))
+}
+
+func min(a uint32, b int) uint32 {
+	if a < uint32(b) {
+		return a
+	}
+	return uint32(b)
+}
+
+type channelDirection uint8
+
+const (
+	channelInbound channelDirection = iota
+	channelOutbound
+)
+
+// channel is an implementation of the Channel interface that works
+// with the mux class.
+type channel struct {
+	// R/O after creation
+	chanType          string
+	extraData         []byte
+	localId, remoteId uint32
+
+	// maxIncomingPayload and maxRemotePayload are the maximum
+	// payload sizes of normal and extended data packets for
+	// receiving and sending, respectively. The wire packet will
+	// be 9 or 13 bytes larger (excluding encryption overhead).
+	maxIncomingPayload uint32
+	maxRemotePayload   uint32
+
+	mux *mux
+
+	// decided is set to true if an accept or reject message has been sent
+	// (for outbound channels) or received (for inbound channels).
+	decided bool
+
+	// direction contains either channelOutbound, for channels created
+	// locally, or channelInbound, for channels created by the peer.
+	direction channelDirection
+
+	// Pending internal channel messages.
+	msg chan interface{}
+
+	// Since requests have no ID, there can be only one request
+	// with WantReply=true outstanding.  This lock is held by a
+	// goroutine that has such an outgoing request pending.
+	sentRequestMu sync.Mutex
+
+	incomingRequests chan *Request
+
+	sentEOF bool
+
+	// thread-safe data
+	remoteWin  window
+	pending    *buffer
+	extPending *buffer
+
+	// windowMu protects myWindow, the flow-control window.
+	windowMu sync.Mutex
+	myWindow uint32
+
+	// writeMu serializes calls to mux.conn.writePacket() and
+	// protects sentClose and packetPool. This mutex must be
+	// different from windowMu, as writePacket can block if there
+	// is a key exchange pending.
+	writeMu   sync.Mutex
+	sentClose bool
+
+	// packetPool has a buffer for each extended channel ID to
+	// save allocations during writes.
+	packetPool map[uint32][]byte
+}
+
+// writePacket sends a packet. If the packet is a channel close, it updates
+// sentClose. This method takes the lock c.writeMu.
+func (ch *channel) writePacket(packet []byte) error {
+	ch.writeMu.Lock()
+	if ch.sentClose {
+		ch.writeMu.Unlock()
+		return io.EOF
+	}
+	ch.sentClose = (packet[0] == msgChannelClose)
+	err := ch.mux.conn.writePacket(packet)
+	ch.writeMu.Unlock()
+	return err
+}
+
+func (ch *channel) sendMessage(msg interface{}) error {
+	if debugMux {
+		log.Printf("send(%d): %#v", ch.mux.chanList.offset, msg)
+	}
+
+	p := Marshal(msg)
+	binary.BigEndian.PutUint32(p[1:], ch.remoteId)
+	return ch.writePacket(p)
+}
+
+// WriteExtended writes data to a specific extended stream. These streams are
+// used, for example, for stderr.
+func (ch *channel) WriteExtended(data []byte, extendedCode uint32) (n int, err error) {
+	if ch.sentEOF {
+		return 0, io.EOF
+	}
+	// 1 byte message type, 4 bytes remoteId, 4 bytes data length
+	opCode := byte(msgChannelData)
+	headerLength := uint32(9)
+	if extendedCode > 0 {
+		headerLength += 4
+		opCode = msgChannelExtendedData
+	}
+
+	ch.writeMu.Lock()
+	packet := ch.packetPool[extendedCode]
+	// We don't remove the buffer from packetPool, so
+	// WriteExtended calls from different goroutines will be
+	// flagged as errors by the race detector.
+	ch.writeMu.Unlock()
+
+	for len(data) > 0 {
+		space := min(ch.maxRemotePayload, len(data))
+		if space, err = ch.remoteWin.reserve(space); err != nil {
+			return n, err
+		}
+		if want := headerLength + space; uint32(cap(packet)) < want {
+			packet = make([]byte, want)
+		} else {
+			packet = packet[:want]
+		}
+
+		todo := data[:space]
+
+		packet[0] = opCode
+		binary.BigEndian.PutUint32(packet[1:], ch.remoteId)
+		if extendedCode > 0 {
+			binary.BigEndian.PutUint32(packet[5:], uint32(extendedCode))
+		}
+		binary.BigEndian.PutUint32(packet[headerLength-4:], uint32(len(todo)))
+		copy(packet[headerLength:], todo)
+		if err = ch.writePacket(packet); err != nil {
+			return n, err
+		}
+
+		n += len(todo)
+		data = data[len(todo):]
+	}
+
+	ch.writeMu.Lock()
+	ch.packetPool[extendedCode] = packet
+	ch.writeMu.Unlock()
+
+	return n, err
+}
+
+func (ch *channel) handleData(packet []byte) error {
+	headerLen := 9
+	isExtendedData := packet[0] == msgChannelExtendedData
+	if isExtendedData {
+		headerLen = 13
+	}
+	if len(packet) < headerLen {
+		// malformed data packet
+		return parseError(packet[0])
+	}
+
+	var extended uint32
+	if isExtendedData {
+		extended = binary.BigEndian.Uint32(packet[5:])
+	}
+
+	length := binary.BigEndian.Uint32(packet[headerLen-4 : headerLen])
+	if length == 0 {
+		return nil
+	}
+	if length > ch.maxIncomingPayload {
+		// TODO(hanwen): should send Disconnect?
+		return errors.New("ssh: incoming packet exceeds maximum payload size")
+	}
+
+	data := packet[headerLen:]
+	if length != uint32(len(data)) {
+		return errors.New("ssh: wrong packet length")
+	}
+
+	ch.windowMu.Lock()
+	if ch.myWindow < length {
+		ch.windowMu.Unlock()
+		// TODO(hanwen): should send Disconnect with reason?
+		return errors.New("ssh: remote side wrote too much")
+	}
+	ch.myWindow -= length
+	ch.windowMu.Unlock()
+
+	if extended == 1 {
+		ch.extPending.write(data)
+	} else if extended > 0 {
+		// discard other extended data.
+	} else {
+		ch.pending.write(data)
+	}
+	return nil
+}
+
+func (c *channel) adjustWindow(n uint32) error {
+	c.windowMu.Lock()
+	// Since myWindow is managed on our side, and can never exceed
+	// the initial window setting, we don't worry about overflow.
+	c.myWindow += uint32(n)
+	c.windowMu.Unlock()
+	return c.sendMessage(windowAdjustMsg{
+		AdditionalBytes: uint32(n),
+	})
+}
+
+func (c *channel) ReadExtended(data []byte, extended uint32) (n int, err error) {
+	switch extended {
+	case 1:
+		n, err = c.extPending.Read(data)
+	case 0:
+		n, err = c.pending.Read(data)
+	default:
+		return 0, fmt.Errorf("ssh: extended code %d unimplemented", extended)
+	}
+
+	if n > 0 {
+		err = c.adjustWindow(uint32(n))
+		// sendWindowAdjust can return io.EOF if the remote
+		// peer has closed the connection, however we want to
+		// defer forwarding io.EOF to the caller of Read until
+		// the buffer has been drained.
+		if n > 0 && err == io.EOF {
+			err = nil
+		}
+	}
+
+	return n, err
+}
+
+func (c *channel) close() {
+	c.pending.eof()
+	c.extPending.eof()
+	close(c.msg)
+	close(c.incomingRequests)
+	c.writeMu.Lock()
+	// This is not necessary for a normal channel teardown, but if
+	// there was another error, it is.
+	c.sentClose = true
+	c.writeMu.Unlock()
+	// Unblock writers.
+	c.remoteWin.close()
+}
+
+// responseMessageReceived is called when a success or failure message is
+// received on a channel to check that such a message is reasonable for the
+// given channel.
+func (ch *channel) responseMessageReceived() error {
+	if ch.direction == channelInbound {
+		return errors.New("ssh: channel response message received on inbound channel")
+	}
+	if ch.decided {
+		return errors.New("ssh: duplicate response received for channel")
+	}
+	ch.decided = true
+	return nil
+}
+
+func (ch *channel) handlePacket(packet []byte) error {
+	switch packet[0] {
+	case msgChannelData, msgChannelExtendedData:
+		return ch.handleData(packet)
+	case msgChannelClose:
+		ch.sendMessage(channelCloseMsg{PeersID: ch.remoteId})
+		ch.mux.chanList.remove(ch.localId)
+		ch.close()
+		return nil
+	case msgChannelEOF:
+		// RFC 4254 is mute on how EOF affects dataExt messages but
+		// it is logical to signal EOF at the same time.
+		ch.extPending.eof()
+		ch.pending.eof()
+		return nil
+	}
+
+	decoded, err := decode(packet)
+	if err != nil {
+		return err
+	}
+
+	switch msg := decoded.(type) {
+	case *channelOpenFailureMsg:
+		if err := ch.responseMessageReceived(); err != nil {
+			return err
+		}
+		ch.mux.chanList.remove(msg.PeersID)
+		ch.msg <- msg
+	case *channelOpenConfirmMsg:
+		if err := ch.responseMessageReceived(); err != nil {
+			return err
+		}
+		if msg.MaxPacketSize < minPacketLength || msg.MaxPacketSize > 1<<31 {
+			return fmt.Errorf("ssh: invalid MaxPacketSize %d from peer", msg.MaxPacketSize)
+		}
+		ch.remoteId = msg.MyID
+		ch.maxRemotePayload = msg.MaxPacketSize
+		ch.remoteWin.add(msg.MyWindow)
+		ch.msg <- msg
+	case *windowAdjustMsg:
+		if !ch.remoteWin.add(msg.AdditionalBytes) {
+			return fmt.Errorf("ssh: invalid window update for %d bytes", msg.AdditionalBytes)
+		}
+	case *channelRequestMsg:
+		req := Request{
+			Type:      msg.Request,
+			WantReply: msg.WantReply,
+			Payload:   msg.RequestSpecificData,
+			ch:        ch,
+		}
+
+		ch.incomingRequests <- &req
+	default:
+		ch.msg <- msg
+	}
+	return nil
+}
+
+func (m *mux) newChannel(chanType string, direction channelDirection, extraData []byte) *channel {
+	ch := &channel{
+		remoteWin:        window{Cond: newCond()},
+		myWindow:         channelWindowSize,
+		pending:          newBuffer(),
+		extPending:       newBuffer(),
+		direction:        direction,
+		incomingRequests: make(chan *Request, chanSize),
+		msg:              make(chan interface{}, chanSize),
+		chanType:         chanType,
+		extraData:        extraData,
+		mux:              m,
+		packetPool:       make(map[uint32][]byte),
+	}
+	ch.localId = m.chanList.add(ch)
+	return ch
+}
+
+var errUndecided = errors.New("ssh: must Accept or Reject channel")
+var errDecidedAlready = errors.New("ssh: can call Accept or Reject only once")
+
+type extChannel struct {
+	code uint32
+	ch   *channel
+}
+
+func (e *extChannel) Write(data []byte) (n int, err error) {
+	return e.ch.WriteExtended(data, e.code)
+}
+
+func (e *extChannel) Read(data []byte) (n int, err error) {
+	return e.ch.ReadExtended(data, e.code)
+}
+
+func (ch *channel) Accept() (Channel, <-chan *Request, error) {
+	if ch.decided {
+		return nil, nil, errDecidedAlready
+	}
+	ch.maxIncomingPayload = channelMaxPacket
+	confirm := channelOpenConfirmMsg{
+		PeersID:       ch.remoteId,
+		MyID:          ch.localId,
+		MyWindow:      ch.myWindow,
+		MaxPacketSize: ch.maxIncomingPayload,
+	}
+	ch.decided = true
+	if err := ch.sendMessage(confirm); err != nil {
+		return nil, nil, err
+	}
+
+	return ch, ch.incomingRequests, nil
+}
+
+func (ch *channel) Reject(reason RejectionReason, message string) error {
+	if ch.decided {
+		return errDecidedAlready
+	}
+	reject := channelOpenFailureMsg{
+		PeersID:  ch.remoteId,
+		Reason:   reason,
+		Message:  message,
+		Language: "en",
+	}
+	ch.decided = true
+	return ch.sendMessage(reject)
+}
+
+func (ch *channel) Read(data []byte) (int, error) {
+	if !ch.decided {
+		return 0, errUndecided
+	}
+	return ch.ReadExtended(data, 0)
+}
+
+func (ch *channel) Write(data []byte) (int, error) {
+	if !ch.decided {
+		return 0, errUndecided
+	}
+	return ch.WriteExtended(data, 0)
+}
+
+func (ch *channel) CloseWrite() error {
+	if !ch.decided {
+		return errUndecided
+	}
+	ch.sentEOF = true
+	return ch.sendMessage(channelEOFMsg{
+		PeersID: ch.remoteId})
+}
+
+func (ch *channel) Close() error {
+	if !ch.decided {
+		return errUndecided
+	}
+
+	return ch.sendMessage(channelCloseMsg{
+		PeersID: ch.remoteId})
+}
+
+// Extended returns an io.ReadWriter that sends and receives data on the given,
+// SSH extended stream. Such streams are used, for example, for stderr.
+func (ch *channel) Extended(code uint32) io.ReadWriter {
+	if !ch.decided {
+		return nil
+	}
+	return &extChannel{code, ch}
+}
+
+func (ch *channel) Stderr() io.ReadWriter {
+	return ch.Extended(1)
+}
+
+func (ch *channel) SendRequest(name string, wantReply bool, payload []byte) (bool, error) {
+	if !ch.decided {
+		return false, errUndecided
+	}
+
+	if wantReply {
+		ch.sentRequestMu.Lock()
+		defer ch.sentRequestMu.Unlock()
+	}
+
+	msg := channelRequestMsg{
+		PeersID:             ch.remoteId,
+		Request:             name,
+		WantReply:           wantReply,
+		RequestSpecificData: payload,
+	}
+
+	if err := ch.sendMessage(msg); err != nil {
+		return false, err
+	}
+
+	if wantReply {
+		m, ok := (<-ch.msg)
+		if !ok {
+			return false, io.EOF
+		}
+		switch m.(type) {
+		case *channelRequestFailureMsg:
+			return false, nil
+		case *channelRequestSuccessMsg:
+			return true, nil
+		default:
+			return false, fmt.Errorf("ssh: unexpected response to channel request: %#v", m)
+		}
+	}
+
+	return false, nil
+}
+
+// ackRequest either sends an ack or nack to the channel request.
+func (ch *channel) ackRequest(ok bool) error {
+	if !ch.decided {
+		return errUndecided
+	}
+
+	var msg interface{}
+	if !ok {
+		msg = channelRequestFailureMsg{
+			PeersID: ch.remoteId,
+		}
+	} else {
+		msg = channelRequestSuccessMsg{
+			PeersID: ch.remoteId,
+		}
+	}
+	return ch.sendMessage(msg)
+}
+
+func (ch *channel) ChannelType() string {
+	return ch.chanType
+}
+
+func (ch *channel) ExtraData() []byte {
+	return ch.extraData
+}

pkg/proto/ssh/cipher.go

@@ -0,0 +1,789 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/des"
+	"crypto/rc4"
+	"crypto/subtle"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"hash"
+	"io"
+
+	"github.com/Neur0toxine/sshpoke/pkg/proto/ssh/internal/poly1305"
+	"golang.org/x/crypto/chacha20"
+)
+
+const (
+	packetSizeMultiple = 16 // TODO(huin) this should be determined by the cipher.
+
+	// RFC 4253 section 6.1 defines a minimum packet size of 32768 that implementations
+	// MUST be able to process (plus a few more kilobytes for padding and mac). The RFC
+	// indicates implementations SHOULD be able to handle larger packet sizes, but then
+	// waffles on about reasonable limits.
+	//
+	// OpenSSH caps their maxPacket at 256kB so we choose to do
+	// the same. maxPacket is also used to ensure that uint32
+	// length fields do not overflow, so it should remain well
+	// below 4G.
+	maxPacket = 256 * 1024
+)
+
+// noneCipher implements cipher.Stream and provides no encryption. It is used
+// by the transport before the first key-exchange.
+type noneCipher struct{}
+
+func (c noneCipher) XORKeyStream(dst, src []byte) {
+	copy(dst, src)
+}
+
+func newAESCTR(key, iv []byte) (cipher.Stream, error) {
+	c, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+	return cipher.NewCTR(c, iv), nil
+}
+
+func newRC4(key, iv []byte) (cipher.Stream, error) {
+	return rc4.NewCipher(key)
+}
+
+type cipherMode struct {
+	keySize int
+	ivSize  int
+	create  func(key, iv []byte, macKey []byte, algs directionAlgorithms) (packetCipher, error)
+}
+
+func streamCipherMode(skip int, createFunc func(key, iv []byte) (cipher.Stream, error)) func(key, iv []byte, macKey []byte, algs directionAlgorithms) (packetCipher, error) {
+	return func(key, iv, macKey []byte, algs directionAlgorithms) (packetCipher, error) {
+		stream, err := createFunc(key, iv)
+		if err != nil {
+			return nil, err
+		}
+
+		var streamDump []byte
+		if skip > 0 {
+			streamDump = make([]byte, 512)
+		}
+
+		for remainingToDump := skip; remainingToDump > 0; {
+			dumpThisTime := remainingToDump
+			if dumpThisTime > len(streamDump) {
+				dumpThisTime = len(streamDump)
+			}
+			stream.XORKeyStream(streamDump[:dumpThisTime], streamDump[:dumpThisTime])
+			remainingToDump -= dumpThisTime
+		}
+
+		mac := macModes[algs.MAC].new(macKey)
+		return &streamPacketCipher{
+			mac:       mac,
+			etm:       macModes[algs.MAC].etm,
+			macResult: make([]byte, mac.Size()),
+			cipher:    stream,
+		}, nil
+	}
+}
+
+// cipherModes documents properties of supported ciphers. Ciphers not included
+// are not supported and will not be negotiated, even if explicitly requested in
+// ClientConfig.Crypto.Ciphers.
+var cipherModes = map[string]*cipherMode{
+	// Ciphers from RFC 4344, which introduced many CTR-based ciphers. Algorithms
+	// are defined in the order specified in the RFC.
+	"aes128-ctr": {16, aes.BlockSize, streamCipherMode(0, newAESCTR)},
+	"aes192-ctr": {24, aes.BlockSize, streamCipherMode(0, newAESCTR)},
+	"aes256-ctr": {32, aes.BlockSize, streamCipherMode(0, newAESCTR)},
+
+	// Ciphers from RFC 4345, which introduces security-improved arcfour ciphers.
+	// They are defined in the order specified in the RFC.
+	"arcfour128": {16, 0, streamCipherMode(1536, newRC4)},
+	"arcfour256": {32, 0, streamCipherMode(1536, newRC4)},
+
+	// Cipher defined in RFC 4253, which describes SSH Transport Layer Protocol.
+	// Note that this cipher is not safe, as stated in RFC 4253: "Arcfour (and
+	// RC4) has problems with weak keys, and should be used with caution."
+	// RFC 4345 introduces improved versions of Arcfour.
+	"arcfour": {16, 0, streamCipherMode(0, newRC4)},
+
+	// AEAD ciphers
+	gcm128CipherID:     {16, 12, newGCMCipher},
+	gcm256CipherID:     {32, 12, newGCMCipher},
+	chacha20Poly1305ID: {64, 0, newChaCha20Cipher},
+
+	// CBC mode is insecure and so is not included in the default config.
+	// (See https://www.ieee-security.org/TC/SP2013/papers/4977a526.pdf). If absolutely
+	// needed, it's possible to specify a custom Config to enable it.
+	// You should expect that an active attacker can recover plaintext if
+	// you do.
+	aes128cbcID: {16, aes.BlockSize, newAESCBCCipher},
+
+	// 3des-cbc is insecure and is not included in the default
+	// config.
+	tripledescbcID: {24, des.BlockSize, newTripleDESCBCCipher},
+}
+
+// prefixLen is the length of the packet prefix that contains the packet length
+// and number of padding bytes.
+const prefixLen = 5
+
+// streamPacketCipher is a packetCipher using a stream cipher.
+type streamPacketCipher struct {
+	mac    hash.Hash
+	cipher cipher.Stream
+	etm    bool
+
+	// The following members are to avoid per-packet allocations.
+	prefix      [prefixLen]byte
+	seqNumBytes [4]byte
+	padding     [2 * packetSizeMultiple]byte
+	packetData  []byte
+	macResult   []byte
+}
+
+// readCipherPacket reads and decrypt a single packet from the reader argument.
+func (s *streamPacketCipher) readCipherPacket(seqNum uint32, r io.Reader) ([]byte, error) {
+	if _, err := io.ReadFull(r, s.prefix[:]); err != nil {
+		return nil, err
+	}
+
+	var encryptedPaddingLength [1]byte
+	if s.mac != nil && s.etm {
+		copy(encryptedPaddingLength[:], s.prefix[4:5])
+		s.cipher.XORKeyStream(s.prefix[4:5], s.prefix[4:5])
+	} else {
+		s.cipher.XORKeyStream(s.prefix[:], s.prefix[:])
+	}
+
+	length := binary.BigEndian.Uint32(s.prefix[0:4])
+	paddingLength := uint32(s.prefix[4])
+
+	var macSize uint32
+	if s.mac != nil {
+		s.mac.Reset()
+		binary.BigEndian.PutUint32(s.seqNumBytes[:], seqNum)
+		s.mac.Write(s.seqNumBytes[:])
+		if s.etm {
+			s.mac.Write(s.prefix[:4])
+			s.mac.Write(encryptedPaddingLength[:])
+		} else {
+			s.mac.Write(s.prefix[:])
+		}
+		macSize = uint32(s.mac.Size())
+	}
+
+	if length <= paddingLength+1 {
+		return nil, errors.New("ssh: invalid packet length, packet too small")
+	}
+
+	if length > maxPacket {
+		return nil, errors.New("ssh: invalid packet length, packet too large")
+	}
+
+	// the maxPacket check above ensures that length-1+macSize
+	// does not overflow.
+	if uint32(cap(s.packetData)) < length-1+macSize {
+		s.packetData = make([]byte, length-1+macSize)
+	} else {
+		s.packetData = s.packetData[:length-1+macSize]
+	}
+
+	if _, err := io.ReadFull(r, s.packetData); err != nil {
+		return nil, err
+	}
+	mac := s.packetData[length-1:]
+	data := s.packetData[:length-1]
+
+	if s.mac != nil && s.etm {
+		s.mac.Write(data)
+	}
+
+	s.cipher.XORKeyStream(data, data)
+
+	if s.mac != nil {
+		if !s.etm {
+			s.mac.Write(data)
+		}
+		s.macResult = s.mac.Sum(s.macResult[:0])
+		if subtle.ConstantTimeCompare(s.macResult, mac) != 1 {
+			return nil, errors.New("ssh: MAC failure")
+		}
+	}
+
+	return s.packetData[:length-paddingLength-1], nil
+}
+
+// writeCipherPacket encrypts and sends a packet of data to the writer argument
+func (s *streamPacketCipher) writeCipherPacket(seqNum uint32, w io.Writer, rand io.Reader, packet []byte) error {
+	if len(packet) > maxPacket {
+		return errors.New("ssh: packet too large")
+	}
+
+	aadlen := 0
+	if s.mac != nil && s.etm {
+		// packet length is not encrypted for EtM modes
+		aadlen = 4
+	}
+
+	paddingLength := packetSizeMultiple - (prefixLen+len(packet)-aadlen)%packetSizeMultiple
+	if paddingLength < 4 {
+		paddingLength += packetSizeMultiple
+	}
+
+	length := len(packet) + 1 + paddingLength
+	binary.BigEndian.PutUint32(s.prefix[:], uint32(length))
+	s.prefix[4] = byte(paddingLength)
+	padding := s.padding[:paddingLength]
+	if _, err := io.ReadFull(rand, padding); err != nil {
+		return err
+	}
+
+	if s.mac != nil {
+		s.mac.Reset()
+		binary.BigEndian.PutUint32(s.seqNumBytes[:], seqNum)
+		s.mac.Write(s.seqNumBytes[:])
+
+		if s.etm {
+			// For EtM algorithms, the packet length must stay unencrypted,
+			// but the following data (padding length) must be encrypted
+			s.cipher.XORKeyStream(s.prefix[4:5], s.prefix[4:5])
+		}
+
+		s.mac.Write(s.prefix[:])
+
+		if !s.etm {
+			// For non-EtM algorithms, the algorithm is applied on unencrypted data
+			s.mac.Write(packet)
+			s.mac.Write(padding)
+		}
+	}
+
+	if !(s.mac != nil && s.etm) {
+		// For EtM algorithms, the padding length has already been encrypted
+		// and the packet length must remain unencrypted
+		s.cipher.XORKeyStream(s.prefix[:], s.prefix[:])
+	}
+
+	s.cipher.XORKeyStream(packet, packet)
+	s.cipher.XORKeyStream(padding, padding)
+
+	if s.mac != nil && s.etm {
+		// For EtM algorithms, packet and padding must be encrypted
+		s.mac.Write(packet)
+		s.mac.Write(padding)
+	}
+
+	if _, err := w.Write(s.prefix[:]); err != nil {
+		return err
+	}
+	if _, err := w.Write(packet); err != nil {
+		return err
+	}
+	if _, err := w.Write(padding); err != nil {
+		return err
+	}
+
+	if s.mac != nil {
+		s.macResult = s.mac.Sum(s.macResult[:0])
+		if _, err := w.Write(s.macResult); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+type gcmCipher struct {
+	aead   cipher.AEAD
+	prefix [4]byte
+	iv     []byte
+	buf    []byte
+}
+
+func newGCMCipher(key, iv, unusedMacKey []byte, unusedAlgs directionAlgorithms) (packetCipher, error) {
+	c, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+
+	aead, err := cipher.NewGCM(c)
+	if err != nil {
+		return nil, err
+	}
+
+	return &gcmCipher{
+		aead: aead,
+		iv:   iv,
+	}, nil
+}
+
+const gcmTagSize = 16
+
+func (c *gcmCipher) writeCipherPacket(seqNum uint32, w io.Writer, rand io.Reader, packet []byte) error {
+	// Pad out to multiple of 16 bytes. This is different from the
+	// stream cipher because that encrypts the length too.
+	padding := byte(packetSizeMultiple - (1+len(packet))%packetSizeMultiple)
+	if padding < 4 {
+		padding += packetSizeMultiple
+	}
+
+	length := uint32(len(packet) + int(padding) + 1)
+	binary.BigEndian.PutUint32(c.prefix[:], length)
+	if _, err := w.Write(c.prefix[:]); err != nil {
+		return err
+	}
+
+	if cap(c.buf) < int(length) {
+		c.buf = make([]byte, length)
+	} else {
+		c.buf = c.buf[:length]
+	}
+
+	c.buf[0] = padding
+	copy(c.buf[1:], packet)
+	if _, err := io.ReadFull(rand, c.buf[1+len(packet):]); err != nil {
+		return err
+	}
+	c.buf = c.aead.Seal(c.buf[:0], c.iv, c.buf, c.prefix[:])
+	if _, err := w.Write(c.buf); err != nil {
+		return err
+	}
+	c.incIV()
+
+	return nil
+}
+
+func (c *gcmCipher) incIV() {
+	for i := 4 + 7; i >= 4; i-- {
+		c.iv[i]++
+		if c.iv[i] != 0 {
+			break
+		}
+	}
+}
+
+func (c *gcmCipher) readCipherPacket(seqNum uint32, r io.Reader) ([]byte, error) {
+	if _, err := io.ReadFull(r, c.prefix[:]); err != nil {
+		return nil, err
+	}
+	length := binary.BigEndian.Uint32(c.prefix[:])
+	if length > maxPacket {
+		return nil, errors.New("ssh: max packet length exceeded")
+	}
+
+	if cap(c.buf) < int(length+gcmTagSize) {
+		c.buf = make([]byte, length+gcmTagSize)
+	} else {
+		c.buf = c.buf[:length+gcmTagSize]
+	}
+
+	if _, err := io.ReadFull(r, c.buf); err != nil {
+		return nil, err
+	}
+
+	plain, err := c.aead.Open(c.buf[:0], c.iv, c.buf, c.prefix[:])
+	if err != nil {
+		return nil, err
+	}
+	c.incIV()
+
+	if len(plain) == 0 {
+		return nil, errors.New("ssh: empty packet")
+	}
+
+	padding := plain[0]
+	if padding < 4 {
+		// padding is a byte, so it automatically satisfies
+		// the maximum size, which is 255.
+		return nil, fmt.Errorf("ssh: illegal padding %d", padding)
+	}
+
+	if int(padding+1) >= len(plain) {
+		return nil, fmt.Errorf("ssh: padding %d too large", padding)
+	}
+	plain = plain[1 : length-uint32(padding)]
+	return plain, nil
+}
+
+// cbcCipher implements aes128-cbc cipher defined in RFC 4253 section 6.1
+type cbcCipher struct {
+	mac       hash.Hash
+	macSize   uint32
+	decrypter cipher.BlockMode
+	encrypter cipher.BlockMode
+
+	// The following members are to avoid per-packet allocations.
+	seqNumBytes [4]byte
+	packetData  []byte
+	macResult   []byte
+
+	// Amount of data we should still read to hide which
+	// verification error triggered.
+	oracleCamouflage uint32
+}
+
+func newCBCCipher(c cipher.Block, key, iv, macKey []byte, algs directionAlgorithms) (packetCipher, error) {
+	cbc := &cbcCipher{
+		mac:        macModes[algs.MAC].new(macKey),
+		decrypter:  cipher.NewCBCDecrypter(c, iv),
+		encrypter:  cipher.NewCBCEncrypter(c, iv),
+		packetData: make([]byte, 1024),
+	}
+	if cbc.mac != nil {
+		cbc.macSize = uint32(cbc.mac.Size())
+	}
+
+	return cbc, nil
+}
+
+func newAESCBCCipher(key, iv, macKey []byte, algs directionAlgorithms) (packetCipher, error) {
+	c, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+
+	cbc, err := newCBCCipher(c, key, iv, macKey, algs)
+	if err != nil {
+		return nil, err
+	}
+
+	return cbc, nil
+}
+
+func newTripleDESCBCCipher(key, iv, macKey []byte, algs directionAlgorithms) (packetCipher, error) {
+	c, err := des.NewTripleDESCipher(key)
+	if err != nil {
+		return nil, err
+	}
+
+	cbc, err := newCBCCipher(c, key, iv, macKey, algs)
+	if err != nil {
+		return nil, err
+	}
+
+	return cbc, nil
+}
+
+func maxUInt32(a, b int) uint32 {
+	if a > b {
+		return uint32(a)
+	}
+	return uint32(b)
+}
+
+const (
+	cbcMinPacketSizeMultiple = 8
+	cbcMinPacketSize         = 16
+	cbcMinPaddingSize        = 4
+)
+
+// cbcError represents a verification error that may leak information.
+type cbcError string
+
+func (e cbcError) Error() string { return string(e) }
+
+func (c *cbcCipher) readCipherPacket(seqNum uint32, r io.Reader) ([]byte, error) {
+	p, err := c.readCipherPacketLeaky(seqNum, r)
+	if err != nil {
+		if _, ok := err.(cbcError); ok {
+			// Verification error: read a fixed amount of
+			// data, to make distinguishing between
+			// failing MAC and failing length check more
+			// difficult.
+			io.CopyN(io.Discard, r, int64(c.oracleCamouflage))
+		}
+	}
+	return p, err
+}
+
+func (c *cbcCipher) readCipherPacketLeaky(seqNum uint32, r io.Reader) ([]byte, error) {
+	blockSize := c.decrypter.BlockSize()
+
+	// Read the header, which will include some of the subsequent data in the
+	// case of block ciphers - this is copied back to the payload later.
+	// How many bytes of payload/padding will be read with this first read.
+	firstBlockLength := uint32((prefixLen + blockSize - 1) / blockSize * blockSize)
+	firstBlock := c.packetData[:firstBlockLength]
+	if _, err := io.ReadFull(r, firstBlock); err != nil {
+		return nil, err
+	}
+
+	c.oracleCamouflage = maxPacket + 4 + c.macSize - firstBlockLength
+
+	c.decrypter.CryptBlocks(firstBlock, firstBlock)
+	length := binary.BigEndian.Uint32(firstBlock[:4])
+	if length > maxPacket {
+		return nil, cbcError("ssh: packet too large")
+	}
+	if length+4 < maxUInt32(cbcMinPacketSize, blockSize) {
+		// The minimum size of a packet is 16 (or the cipher block size, whichever
+		// is larger) bytes.
+		return nil, cbcError("ssh: packet too small")
+	}
+	// The length of the packet (including the length field but not the MAC) must
+	// be a multiple of the block size or 8, whichever is larger.
+	if (length+4)%maxUInt32(cbcMinPacketSizeMultiple, blockSize) != 0 {
+		return nil, cbcError("ssh: invalid packet length multiple")
+	}
+
+	paddingLength := uint32(firstBlock[4])
+	if paddingLength < cbcMinPaddingSize || length <= paddingLength+1 {
+		return nil, cbcError("ssh: invalid packet length")
+	}
+
+	// Positions within the c.packetData buffer:
+	macStart := 4 + length
+	paddingStart := macStart - paddingLength
+
+	// Entire packet size, starting before length, ending at end of mac.
+	entirePacketSize := macStart + c.macSize
+
+	// Ensure c.packetData is large enough for the entire packet data.
+	if uint32(cap(c.packetData)) < entirePacketSize {
+		// Still need to upsize and copy, but this should be rare at runtime, only
+		// on upsizing the packetData buffer.
+		c.packetData = make([]byte, entirePacketSize)
+		copy(c.packetData, firstBlock)
+	} else {
+		c.packetData = c.packetData[:entirePacketSize]
+	}
+
+	n, err := io.ReadFull(r, c.packetData[firstBlockLength:])
+	if err != nil {
+		return nil, err
+	}
+	c.oracleCamouflage -= uint32(n)
+
+	remainingCrypted := c.packetData[firstBlockLength:macStart]
+	c.decrypter.CryptBlocks(remainingCrypted, remainingCrypted)
+
+	mac := c.packetData[macStart:]
+	if c.mac != nil {
+		c.mac.Reset()
+		binary.BigEndian.PutUint32(c.seqNumBytes[:], seqNum)
+		c.mac.Write(c.seqNumBytes[:])
+		c.mac.Write(c.packetData[:macStart])
+		c.macResult = c.mac.Sum(c.macResult[:0])
+		if subtle.ConstantTimeCompare(c.macResult, mac) != 1 {
+			return nil, cbcError("ssh: MAC failure")
+		}
+	}
+
+	return c.packetData[prefixLen:paddingStart], nil
+}
+
+func (c *cbcCipher) writeCipherPacket(seqNum uint32, w io.Writer, rand io.Reader, packet []byte) error {
+	effectiveBlockSize := maxUInt32(cbcMinPacketSizeMultiple, c.encrypter.BlockSize())
+
+	// Length of encrypted portion of the packet (header, payload, padding).
+	// Enforce minimum padding and packet size.
+	encLength := maxUInt32(prefixLen+len(packet)+cbcMinPaddingSize, cbcMinPaddingSize)
+	// Enforce block size.
+	encLength = (encLength + effectiveBlockSize - 1) / effectiveBlockSize * effectiveBlockSize
+
+	length := encLength - 4
+	paddingLength := int(length) - (1 + len(packet))
+
+	// Overall buffer contains: header, payload, padding, mac.
+	// Space for the MAC is reserved in the capacity but not the slice length.
+	bufferSize := encLength + c.macSize
+	if uint32(cap(c.packetData)) < bufferSize {
+		c.packetData = make([]byte, encLength, bufferSize)
+	} else {
+		c.packetData = c.packetData[:encLength]
+	}
+
+	p := c.packetData
+
+	// Packet header.
+	binary.BigEndian.PutUint32(p, length)
+	p = p[4:]
+	p[0] = byte(paddingLength)
+
+	// Payload.
+	p = p[1:]
+	copy(p, packet)
+
+	// Padding.
+	p = p[len(packet):]
+	if _, err := io.ReadFull(rand, p); err != nil {
+		return err
+	}
+
+	if c.mac != nil {
+		c.mac.Reset()
+		binary.BigEndian.PutUint32(c.seqNumBytes[:], seqNum)
+		c.mac.Write(c.seqNumBytes[:])
+		c.mac.Write(c.packetData)
+		// The MAC is now appended into the capacity reserved for it earlier.
+		c.packetData = c.mac.Sum(c.packetData)
+	}
+
+	c.encrypter.CryptBlocks(c.packetData[:encLength], c.packetData[:encLength])
+
+	if _, err := w.Write(c.packetData); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+const chacha20Poly1305ID = "chacha20-poly1305@openssh.com"
+
+// chacha20Poly1305Cipher implements the chacha20-poly1305@openssh.com
+// AEAD, which is described here:
+//
+//	https://tools.ietf.org/html/draft-josefsson-ssh-chacha20-poly1305-openssh-00
+//
+// the methods here also implement padding, which RFC 4253 Section 6
+// also requires of stream ciphers.
+type chacha20Poly1305Cipher struct {
+	lengthKey  [32]byte
+	contentKey [32]byte
+	buf        []byte
+}
+
+func newChaCha20Cipher(key, unusedIV, unusedMACKey []byte, unusedAlgs directionAlgorithms) (packetCipher, error) {
+	if len(key) != 64 {
+		panic(len(key))
+	}
+
+	c := &chacha20Poly1305Cipher{
+		buf: make([]byte, 256),
+	}
+
+	copy(c.contentKey[:], key[:32])
+	copy(c.lengthKey[:], key[32:])
+	return c, nil
+}
+
+func (c *chacha20Poly1305Cipher) readCipherPacket(seqNum uint32, r io.Reader) ([]byte, error) {
+	nonce := make([]byte, 12)
+	binary.BigEndian.PutUint32(nonce[8:], seqNum)
+	s, err := chacha20.NewUnauthenticatedCipher(c.contentKey[:], nonce)
+	if err != nil {
+		return nil, err
+	}
+	var polyKey, discardBuf [32]byte
+	s.XORKeyStream(polyKey[:], polyKey[:])
+	s.XORKeyStream(discardBuf[:], discardBuf[:]) // skip the next 32 bytes
+
+	encryptedLength := c.buf[:4]
+	if _, err := io.ReadFull(r, encryptedLength); err != nil {
+		return nil, err
+	}
+
+	var lenBytes [4]byte
+	ls, err := chacha20.NewUnauthenticatedCipher(c.lengthKey[:], nonce)
+	if err != nil {
+		return nil, err
+	}
+	ls.XORKeyStream(lenBytes[:], encryptedLength)
+
+	length := binary.BigEndian.Uint32(lenBytes[:])
+	if length > maxPacket {
+		return nil, errors.New("ssh: invalid packet length, packet too large")
+	}
+
+	contentEnd := 4 + length
+	packetEnd := contentEnd + poly1305.TagSize
+	if uint32(cap(c.buf)) < packetEnd {
+		c.buf = make([]byte, packetEnd)
+		copy(c.buf[:], encryptedLength)
+	} else {
+		c.buf = c.buf[:packetEnd]
+	}
+
+	if _, err := io.ReadFull(r, c.buf[4:packetEnd]); err != nil {
+		return nil, err
+	}
+
+	var mac [poly1305.TagSize]byte
+	copy(mac[:], c.buf[contentEnd:packetEnd])
+	if !poly1305.Verify(&mac, c.buf[:contentEnd], &polyKey) {
+		return nil, errors.New("ssh: MAC failure")
+	}
+
+	plain := c.buf[4:contentEnd]
+	s.XORKeyStream(plain, plain)
+
+	if len(plain) == 0 {
+		return nil, errors.New("ssh: empty packet")
+	}
+
+	padding := plain[0]
+	if padding < 4 {
+		// padding is a byte, so it automatically satisfies
+		// the maximum size, which is 255.
+		return nil, fmt.Errorf("ssh: illegal padding %d", padding)
+	}
+
+	if int(padding)+1 >= len(plain) {
+		return nil, fmt.Errorf("ssh: padding %d too large", padding)
+	}
+
+	plain = plain[1 : len(plain)-int(padding)]
+
+	return plain, nil
+}
+
+func (c *chacha20Poly1305Cipher) writeCipherPacket(seqNum uint32, w io.Writer, rand io.Reader, payload []byte) error {
+	nonce := make([]byte, 12)
+	binary.BigEndian.PutUint32(nonce[8:], seqNum)
+	s, err := chacha20.NewUnauthenticatedCipher(c.contentKey[:], nonce)
+	if err != nil {
+		return err
+	}
+	var polyKey, discardBuf [32]byte
+	s.XORKeyStream(polyKey[:], polyKey[:])
+	s.XORKeyStream(discardBuf[:], discardBuf[:]) // skip the next 32 bytes
+
+	// There is no blocksize, so fall back to multiple of 8 byte
+	// padding, as described in RFC 4253, Sec 6.
+	const packetSizeMultiple = 8
+
+	padding := packetSizeMultiple - (1+len(payload))%packetSizeMultiple
+	if padding < 4 {
+		padding += packetSizeMultiple
+	}
+
+	// size (4 bytes), padding (1), payload, padding, tag.
+	totalLength := 4 + 1 + len(payload) + padding + poly1305.TagSize
+	if cap(c.buf) < totalLength {
+		c.buf = make([]byte, totalLength)
+	} else {
+		c.buf = c.buf[:totalLength]
+	}
+
+	binary.BigEndian.PutUint32(c.buf, uint32(1+len(payload)+padding))
+	ls, err := chacha20.NewUnauthenticatedCipher(c.lengthKey[:], nonce)
+	if err != nil {
+		return err
+	}
+	ls.XORKeyStream(c.buf, c.buf[:4])
+	c.buf[4] = byte(padding)
+	copy(c.buf[5:], payload)
+	packetEnd := 5 + len(payload) + padding
+	if _, err := io.ReadFull(rand, c.buf[5+len(payload):packetEnd]); err != nil {
+		return err
+	}
+
+	s.XORKeyStream(c.buf[4:], c.buf[4:packetEnd])
+
+	var mac [poly1305.TagSize]byte
+	poly1305.Sum(&mac, c.buf[:packetEnd], &polyKey)
+
+	copy(c.buf[packetEnd:], mac[:])
+
+	if _, err := w.Write(c.buf); err != nil {
+		return err
+	}
+	return nil
+}

pkg/proto/ssh/client.go

@@ -0,0 +1,282 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"net"
+	"os"
+	"sync"
+	"time"
+)
+
+// Client implements a traditional SSH client that supports shells,
+// subprocesses, TCP port/streamlocal forwarding and tunneled dialing.
+type Client struct {
+	Conn
+
+	handleForwardsOnce sync.Once // guards calling (*Client).handleForwards
+
+	forwards        forwardList // forwarded tcpip connections from the remote side
+	mu              sync.Mutex
+	channelHandlers map[string]chan NewChannel
+}
+
+// HandleChannelOpen returns a channel on which NewChannel requests
+// for the given type are sent. If the type already is being handled,
+// nil is returned. The channel is closed when the connection is closed.
+func (c *Client) HandleChannelOpen(channelType string) <-chan NewChannel {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.channelHandlers == nil {
+		// The SSH channel has been closed.
+		c := make(chan NewChannel)
+		close(c)
+		return c
+	}
+
+	ch := c.channelHandlers[channelType]
+	if ch != nil {
+		return nil
+	}
+
+	ch = make(chan NewChannel, chanSize)
+	c.channelHandlers[channelType] = ch
+	return ch
+}
+
+// NewClient creates a Client on top of the given connection.
+func NewClient(c Conn, chans <-chan NewChannel, reqs <-chan *Request) *Client {
+	conn := &Client{
+		Conn:            c,
+		channelHandlers: make(map[string]chan NewChannel, 1),
+	}
+
+	go conn.handleGlobalRequests(reqs)
+	go conn.handleChannelOpens(chans)
+	go func() {
+		conn.Wait()
+		conn.forwards.closeAll()
+	}()
+	return conn
+}
+
+// NewClientConn establishes an authenticated SSH connection using c
+// as the underlying transport.  The Request and NewChannel channels
+// must be serviced or the connection will hang.
+func NewClientConn(c net.Conn, addr string, config *ClientConfig) (Conn, <-chan NewChannel, <-chan *Request, error) {
+	fullConf := *config
+	fullConf.SetDefaults()
+	if fullConf.HostKeyCallback == nil {
+		c.Close()
+		return nil, nil, nil, errors.New("ssh: must specify HostKeyCallback")
+	}
+
+	conn := &connection{
+		sshConn: sshConn{conn: c, user: fullConf.User},
+	}
+
+	if err := conn.clientHandshake(addr, &fullConf); err != nil {
+		c.Close()
+		return nil, nil, nil, fmt.Errorf("ssh: handshake failed: %v", err)
+	}
+	conn.mux = newMux(conn.transport)
+	return conn, conn.mux.incomingChannels, conn.mux.incomingRequests, nil
+}
+
+// clientHandshake performs the client side key exchange. See RFC 4253 Section
+// 7.
+func (c *connection) clientHandshake(dialAddress string, config *ClientConfig) error {
+	if config.ClientVersion != "" {
+		c.clientVersion = []byte(config.ClientVersion)
+	} else {
+		c.clientVersion = []byte(packageVersion)
+	}
+	var err error
+	c.serverVersion, err = exchangeVersions(c.sshConn.conn, c.clientVersion)
+	if err != nil {
+		return err
+	}
+
+	c.transport = newClientTransport(
+		newTransport(c.sshConn.conn, config.Rand, true /* is client */),
+		c.clientVersion, c.serverVersion, config, dialAddress, c.sshConn.RemoteAddr())
+	if err := c.transport.waitSession(); err != nil {
+		return err
+	}
+
+	c.sessionID = c.transport.getSessionID()
+	return c.clientAuthenticate(config)
+}
+
+// verifyHostKeySignature verifies the host key obtained in the key exchange.
+// algo is the negotiated algorithm, and may be a certificate type.
+func verifyHostKeySignature(hostKey PublicKey, algo string, result *kexResult) error {
+	sig, rest, ok := parseSignatureBody(result.Signature)
+	if len(rest) > 0 || !ok {
+		return errors.New("ssh: signature parse error")
+	}
+
+	if a := underlyingAlgo(algo); sig.Format != a {
+		return fmt.Errorf("ssh: invalid signature algorithm %q, expected %q", sig.Format, a)
+	}
+
+	return hostKey.Verify(result.H, sig)
+}
+
+// NewSession opens a new Session for this client. (A session is a remote
+// execution of a program.)
+func (c *Client) NewSession() (*Session, error) {
+	ch, in, err := c.OpenChannel("session", nil)
+	if err != nil {
+		return nil, err
+	}
+	return newSession(ch, in)
+}
+
+func (c *Client) handleGlobalRequests(incoming <-chan *Request) {
+	for r := range incoming {
+		// This handles keepalive messages and matches
+		// the behaviour of OpenSSH.
+		r.Reply(false, nil)
+	}
+}
+
+// handleChannelOpens channel open messages from the remote side.
+func (c *Client) handleChannelOpens(in <-chan NewChannel) {
+	for ch := range in {
+		c.mu.Lock()
+		handler := c.channelHandlers[ch.ChannelType()]
+		c.mu.Unlock()
+
+		if handler != nil {
+			handler <- ch
+		} else {
+			ch.Reject(UnknownChannelType, fmt.Sprintf("unknown channel type: %v", ch.ChannelType()))
+		}
+	}
+
+	c.mu.Lock()
+	for _, ch := range c.channelHandlers {
+		close(ch)
+	}
+	c.channelHandlers = nil
+	c.mu.Unlock()
+}
+
+// Dial starts a client connection to the given SSH server. It is a
+// convenience function that connects to the given network address,
+// initiates the SSH handshake, and then sets up a Client.  For access
+// to incoming channels and requests, use net.Dial with NewClientConn
+// instead.
+func Dial(network, addr string, config *ClientConfig) (*Client, error) {
+	conn, err := net.DialTimeout(network, addr, config.Timeout)
+	if err != nil {
+		return nil, err
+	}
+	c, chans, reqs, err := NewClientConn(conn, addr, config)
+	if err != nil {
+		return nil, err
+	}
+	return NewClient(c, chans, reqs), nil
+}
+
+// HostKeyCallback is the function type used for verifying server
+// keys.  A HostKeyCallback must return nil if the host key is OK, or
+// an error to reject it. It receives the hostname as passed to Dial
+// or NewClientConn. The remote address is the RemoteAddr of the
+// net.Conn underlying the SSH connection.
+type HostKeyCallback func(hostname string, remote net.Addr, key PublicKey) error
+
+// BannerCallback is the function type used for treat the banner sent by
+// the server. A BannerCallback receives the message sent by the remote server.
+type BannerCallback func(message string) error
+
+// A ClientConfig structure is used to configure a Client. It must not be
+// modified after having been passed to an SSH function.
+type ClientConfig struct {
+	// Config contains configuration that is shared between clients and
+	// servers.
+	Config
+
+	// User contains the username to authenticate as.
+	User string
+
+	// Auth contains possible authentication methods to use with the
+	// server. Only the first instance of a particular RFC 4252 method will
+	// be used during authentication.
+	Auth []AuthMethod
+
+	// HostKeyCallback is called during the cryptographic
+	// handshake to validate the server's host key. The client
+	// configuration must supply this callback for the connection
+	// to succeed. The functions InsecureIgnoreHostKey or
+	// FixedHostKey can be used for simplistic host key checks.
+	HostKeyCallback HostKeyCallback
+
+	// BannerCallback is called during the SSH dance to display a custom
+	// server's message. The client configuration can supply this callback to
+	// handle it as wished. The function BannerDisplayStderr can be used for
+	// simplistic display on Stderr.
+	BannerCallback BannerCallback
+
+	// ClientVersion contains the version identification string that will
+	// be used for the connection. If empty, a reasonable default is used.
+	ClientVersion string
+
+	// HostKeyAlgorithms lists the public key algorithms that the client will
+	// accept from the server for host key authentication, in order of
+	// preference. If empty, a reasonable default is used. Any
+	// string returned from a PublicKey.Type method may be used, or
+	// any of the CertAlgo and KeyAlgo constants.
+	HostKeyAlgorithms []string
+
+	// Timeout is the maximum amount of time for the TCP connection to establish.
+	//
+	// A Timeout of zero means no timeout.
+	Timeout time.Duration
+}
+
+// InsecureIgnoreHostKey returns a function that can be used for
+// ClientConfig.HostKeyCallback to accept any host key. It should
+// not be used for production code.
+func InsecureIgnoreHostKey() HostKeyCallback {
+	return func(hostname string, remote net.Addr, key PublicKey) error {
+		return nil
+	}
+}
+
+type fixedHostKey struct {
+	key PublicKey
+}
+
+func (f *fixedHostKey) check(hostname string, remote net.Addr, key PublicKey) error {
+	if f.key == nil {
+		return fmt.Errorf("ssh: required host key was nil")
+	}
+	if !bytes.Equal(key.Marshal(), f.key.Marshal()) {
+		return fmt.Errorf("ssh: host key mismatch")
+	}
+	return nil
+}
+
+// FixedHostKey returns a function for use in
+// ClientConfig.HostKeyCallback to accept only a specific host key.
+func FixedHostKey(key PublicKey) HostKeyCallback {
+	hk := &fixedHostKey{key}
+	return hk.check
+}
+
+// BannerDisplayStderr returns a function that can be used for
+// ClientConfig.BannerCallback to display banners on os.Stderr.
+func BannerDisplayStderr() BannerCallback {
+	return func(banner string) error {
+		_, err := os.Stderr.WriteString(banner)
+
+		return err
+	}
+}

pkg/proto/ssh/client_auth.go

@@ -0,0 +1,761 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"strings"
+)
+
+type authResult int
+
+const (
+	authFailure authResult = iota
+	authPartialSuccess
+	authSuccess
+)
+
+// clientAuthenticate authenticates with the remote server. See RFC 4252.
+func (c *connection) clientAuthenticate(config *ClientConfig) error {
+	// initiate user auth session
+	if err := c.transport.writePacket(Marshal(&serviceRequestMsg{serviceUserAuth})); err != nil {
+		return err
+	}
+	packet, err := c.transport.readPacket()
+	if err != nil {
+		return err
+	}
+	// The server may choose to send a SSH_MSG_EXT_INFO at this point (if we
+	// advertised willingness to receive one, which we always do) or not. See
+	// RFC 8308, Section 2.4.
+	extensions := make(map[string][]byte)
+	if len(packet) > 0 && packet[0] == msgExtInfo {
+		var extInfo extInfoMsg
+		if err := Unmarshal(packet, &extInfo); err != nil {
+			return err
+		}
+		payload := extInfo.Payload
+		for i := uint32(0); i < extInfo.NumExtensions; i++ {
+			name, rest, ok := parseString(payload)
+			if !ok {
+				return parseError(msgExtInfo)
+			}
+			value, rest, ok := parseString(rest)
+			if !ok {
+				return parseError(msgExtInfo)
+			}
+			extensions[string(name)] = value
+			payload = rest
+		}
+		packet, err = c.transport.readPacket()
+		if err != nil {
+			return err
+		}
+	}
+	var serviceAccept serviceAcceptMsg
+	if err := Unmarshal(packet, &serviceAccept); err != nil {
+		return err
+	}
+
+	// during the authentication phase the client first attempts the "none" method
+	// then any untried methods suggested by the server.
+	var tried []string
+	var lastMethods []string
+
+	sessionID := c.transport.getSessionID()
+	for auth := AuthMethod(new(noneAuth)); auth != nil; {
+		ok, methods, err := auth.auth(sessionID, config.User, c.transport, config.Rand, extensions)
+		if err != nil {
+			// We return the error later if there is no other method left to
+			// try.
+			ok = authFailure
+		}
+		if ok == authSuccess {
+			// success
+			return nil
+		} else if ok == authFailure {
+			if m := auth.method(); !contains(tried, m) {
+				tried = append(tried, m)
+			}
+		}
+		if methods == nil {
+			methods = lastMethods
+		}
+		lastMethods = methods
+
+		auth = nil
+
+	findNext:
+		for _, a := range config.Auth {
+			candidateMethod := a.method()
+			if contains(tried, candidateMethod) {
+				continue
+			}
+			for _, meth := range methods {
+				if meth == candidateMethod {
+					auth = a
+					break findNext
+				}
+			}
+		}
+
+		if auth == nil && err != nil {
+			// We have an error and there are no other authentication methods to
+			// try, so we return it.
+			return err
+		}
+	}
+	return fmt.Errorf("ssh: unable to authenticate, attempted methods %v, no supported methods remain", tried)
+}
+
+func contains(list []string, e string) bool {
+	for _, s := range list {
+		if s == e {
+			return true
+		}
+	}
+	return false
+}
+
+// An AuthMethod represents an instance of an RFC 4252 authentication method.
+type AuthMethod interface {
+	// auth authenticates user over transport t.
+	// Returns true if authentication is successful.
+	// If authentication is not successful, a []string of alternative
+	// method names is returned. If the slice is nil, it will be ignored
+	// and the previous set of possible methods will be reused.
+	auth(session []byte, user string, p packetConn, rand io.Reader, extensions map[string][]byte) (authResult, []string, error)
+
+	// method returns the RFC 4252 method name.
+	method() string
+}
+
+// "none" authentication, RFC 4252 section 5.2.
+type noneAuth int
+
+func (n *noneAuth) auth(session []byte, user string, c packetConn, rand io.Reader, _ map[string][]byte) (authResult, []string, error) {
+	if err := c.writePacket(Marshal(&userAuthRequestMsg{
+		User:    user,
+		Service: serviceSSH,
+		Method:  "none",
+	})); err != nil {
+		return authFailure, nil, err
+	}
+
+	return handleAuthResponse(c)
+}
+
+func (n *noneAuth) method() string {
+	return "none"
+}
+
+// passwordCallback is an AuthMethod that fetches the password through
+// a function call, e.g. by prompting the user.
+type passwordCallback func() (password string, err error)
+
+func (cb passwordCallback) auth(session []byte, user string, c packetConn, rand io.Reader, _ map[string][]byte) (authResult, []string, error) {
+	type passwordAuthMsg struct {
+		User     string `sshtype:"50"`
+		Service  string
+		Method   string
+		Reply    bool
+		Password string
+	}
+
+	pw, err := cb()
+	// REVIEW NOTE: is there a need to support skipping a password attempt?
+	// The program may only find out that the user doesn't have a password
+	// when prompting.
+	if err != nil {
+		return authFailure, nil, err
+	}
+
+	if err := c.writePacket(Marshal(&passwordAuthMsg{
+		User:     user,
+		Service:  serviceSSH,
+		Method:   cb.method(),
+		Reply:    false,
+		Password: pw,
+	})); err != nil {
+		return authFailure, nil, err
+	}
+
+	return handleAuthResponse(c)
+}
+
+func (cb passwordCallback) method() string {
+	return "password"
+}
+
+// Password returns an AuthMethod using the given password.
+func Password(secret string) AuthMethod {
+	return passwordCallback(func() (string, error) { return secret, nil })
+}
+
+// PasswordCallback returns an AuthMethod that uses a callback for
+// fetching a password.
+func PasswordCallback(prompt func() (secret string, err error)) AuthMethod {
+	return passwordCallback(prompt)
+}
+
+type publickeyAuthMsg struct {
+	User    string `sshtype:"50"`
+	Service string
+	Method  string
+	// HasSig indicates to the receiver packet that the auth request is signed and
+	// should be used for authentication of the request.
+	HasSig   bool
+	Algoname string
+	PubKey   []byte
+	// Sig is tagged with "rest" so Marshal will exclude it during
+	// validateKey
+	Sig []byte `ssh:"rest"`
+}
+
+// publicKeyCallback is an AuthMethod that uses a set of key
+// pairs for authentication.
+type publicKeyCallback func() ([]Signer, error)
+
+func (cb publicKeyCallback) method() string {
+	return "publickey"
+}
+
+func pickSignatureAlgorithm(signer Signer, extensions map[string][]byte) (MultiAlgorithmSigner, string, error) {
+	var as MultiAlgorithmSigner
+	keyFormat := signer.PublicKey().Type()
+
+	// If the signer implements MultiAlgorithmSigner we use the algorithms it
+	// support, if it implements AlgorithmSigner we assume it supports all
+	// algorithms, otherwise only the key format one.
+	switch s := signer.(type) {
+	case MultiAlgorithmSigner:
+		as = s
+	case AlgorithmSigner:
+		as = &multiAlgorithmSigner{
+			AlgorithmSigner:     s,
+			supportedAlgorithms: algorithmsForKeyFormat(underlyingAlgo(keyFormat)),
+		}
+	default:
+		as = &multiAlgorithmSigner{
+			AlgorithmSigner:     algorithmSignerWrapper{signer},
+			supportedAlgorithms: []string{underlyingAlgo(keyFormat)},
+		}
+	}
+
+	getFallbackAlgo := func() (string, error) {
+		// Fallback to use if there is no "server-sig-algs" extension or a
+		// common algorithm cannot be found. We use the public key format if the
+		// MultiAlgorithmSigner supports it, otherwise we return an error.
+		if !contains(as.Algorithms(), underlyingAlgo(keyFormat)) {
+			return "", fmt.Errorf("ssh: no common public key signature algorithm, server only supports %q for key type %q, signer only supports %v",
+				underlyingAlgo(keyFormat), keyFormat, as.Algorithms())
+		}
+		return keyFormat, nil
+	}
+
+	extPayload, ok := extensions["server-sig-algs"]
+	if !ok {
+		// If there is no "server-sig-algs" extension use the fallback
+		// algorithm.
+		algo, err := getFallbackAlgo()
+		return as, algo, err
+	}
+
+	// The server-sig-algs extension only carries underlying signature
+	// algorithm, but we are trying to select a protocol-level public key
+	// algorithm, which might be a certificate type. Extend the list of server
+	// supported algorithms to include the corresponding certificate algorithms.
+	serverAlgos := strings.Split(string(extPayload), ",")
+	for _, algo := range serverAlgos {
+		if certAlgo, ok := certificateAlgo(algo); ok {
+			serverAlgos = append(serverAlgos, certAlgo)
+		}
+	}
+
+	// Filter algorithms based on those supported by MultiAlgorithmSigner.
+	var keyAlgos []string
+	for _, algo := range algorithmsForKeyFormat(keyFormat) {
+		if contains(as.Algorithms(), underlyingAlgo(algo)) {
+			keyAlgos = append(keyAlgos, algo)
+		}
+	}
+
+	algo, err := findCommon("public key signature algorithm", keyAlgos, serverAlgos)
+	if err != nil {
+		// If there is no overlap, return the fallback algorithm to support
+		// servers that fail to list all supported algorithms.
+		algo, err := getFallbackAlgo()
+		return as, algo, err
+	}
+	return as, algo, nil
+}
+
+func (cb publicKeyCallback) auth(session []byte, user string, c packetConn, rand io.Reader, extensions map[string][]byte) (authResult, []string, error) {
+	// Authentication is performed by sending an enquiry to test if a key is
+	// acceptable to the remote. If the key is acceptable, the client will
+	// attempt to authenticate with the valid key.  If not the client will repeat
+	// the process with the remaining keys.
+
+	signers, err := cb()
+	if err != nil {
+		return authFailure, nil, err
+	}
+	var methods []string
+	var errSigAlgo error
+	for _, signer := range signers {
+		pub := signer.PublicKey()
+		as, algo, err := pickSignatureAlgorithm(signer, extensions)
+		if err != nil && errSigAlgo == nil {
+			// If we cannot negotiate a signature algorithm store the first
+			// error so we can return it to provide a more meaningful message if
+			// no other signers work.
+			errSigAlgo = err
+			continue
+		}
+		ok, err := validateKey(pub, algo, user, c)
+		if err != nil {
+			return authFailure, nil, err
+		}
+		if !ok {
+			continue
+		}
+
+		pubKey := pub.Marshal()
+		data := buildDataSignedForAuth(session, userAuthRequestMsg{
+			User:    user,
+			Service: serviceSSH,
+			Method:  cb.method(),
+		}, algo, pubKey)
+		sign, err := as.SignWithAlgorithm(rand, data, underlyingAlgo(algo))
+		if err != nil {
+			return authFailure, nil, err
+		}
+
+		// manually wrap the serialized signature in a string
+		s := Marshal(sign)
+		sig := make([]byte, stringLength(len(s)))
+		marshalString(sig, s)
+		msg := publickeyAuthMsg{
+			User:     user,
+			Service:  serviceSSH,
+			Method:   cb.method(),
+			HasSig:   true,
+			Algoname: algo,
+			PubKey:   pubKey,
+			Sig:      sig,
+		}
+		p := Marshal(&msg)
+		if err := c.writePacket(p); err != nil {
+			return authFailure, nil, err
+		}
+		var success authResult
+		success, methods, err = handleAuthResponse(c)
+		if err != nil {
+			return authFailure, nil, err
+		}
+
+		// If authentication succeeds or the list of available methods does not
+		// contain the "publickey" method, do not attempt to authenticate with any
+		// other keys.  According to RFC 4252 Section 7, the latter can occur when
+		// additional authentication methods are required.
+		if success == authSuccess || !contains(methods, cb.method()) {
+			return success, methods, err
+		}
+	}
+
+	return authFailure, methods, errSigAlgo
+}
+
+// validateKey validates the key provided is acceptable to the server.
+func validateKey(key PublicKey, algo string, user string, c packetConn) (bool, error) {
+	pubKey := key.Marshal()
+	msg := publickeyAuthMsg{
+		User:     user,
+		Service:  serviceSSH,
+		Method:   "publickey",
+		HasSig:   false,
+		Algoname: algo,
+		PubKey:   pubKey,
+	}
+	if err := c.writePacket(Marshal(&msg)); err != nil {
+		return false, err
+	}
+
+	return confirmKeyAck(key, algo, c)
+}
+
+func confirmKeyAck(key PublicKey, algo string, c packetConn) (bool, error) {
+	pubKey := key.Marshal()
+
+	for {
+		packet, err := c.readPacket()
+		if err != nil {
+			return false, err
+		}
+		switch packet[0] {
+		case msgUserAuthBanner:
+			if err := handleBannerResponse(c, packet); err != nil {
+				return false, err
+			}
+		case msgUserAuthPubKeyOk:
+			var msg userAuthPubKeyOkMsg
+			if err := Unmarshal(packet, &msg); err != nil {
+				return false, err
+			}
+			if msg.Algo != algo || !bytes.Equal(msg.PubKey, pubKey) {
+				return false, nil
+			}
+			return true, nil
+		case msgUserAuthFailure:
+			return false, nil
+		default:
+			return false, unexpectedMessageError(msgUserAuthPubKeyOk, packet[0])
+		}
+	}
+}
+
+// PublicKeys returns an AuthMethod that uses the given key
+// pairs.
+func PublicKeys(signers ...Signer) AuthMethod {
+	return publicKeyCallback(func() ([]Signer, error) { return signers, nil })
+}
+
+// PublicKeysCallback returns an AuthMethod that runs the given
+// function to obtain a list of key pairs.
+func PublicKeysCallback(getSigners func() (signers []Signer, err error)) AuthMethod {
+	return publicKeyCallback(getSigners)
+}
+
+// handleAuthResponse returns whether the preceding authentication request succeeded
+// along with a list of remaining authentication methods to try next and
+// an error if an unexpected response was received.
+func handleAuthResponse(c packetConn) (authResult, []string, error) {
+	gotMsgExtInfo := false
+	for {
+		packet, err := c.readPacket()
+		if err != nil {
+			return authFailure, nil, err
+		}
+
+		switch packet[0] {
+		case msgUserAuthBanner:
+			if err := handleBannerResponse(c, packet); err != nil {
+				return authFailure, nil, err
+			}
+		case msgExtInfo:
+			// Ignore post-authentication RFC 8308 extensions, once.
+			if gotMsgExtInfo {
+				return authFailure, nil, unexpectedMessageError(msgUserAuthSuccess, packet[0])
+			}
+			gotMsgExtInfo = true
+		case msgUserAuthFailure:
+			var msg userAuthFailureMsg
+			if err := Unmarshal(packet, &msg); err != nil {
+				return authFailure, nil, err
+			}
+			if msg.PartialSuccess {
+				return authPartialSuccess, msg.Methods, nil
+			}
+			return authFailure, msg.Methods, nil
+		case msgUserAuthSuccess:
+			return authSuccess, nil, nil
+		default:
+			return authFailure, nil, unexpectedMessageError(msgUserAuthSuccess, packet[0])
+		}
+	}
+}
+
+func handleBannerResponse(c packetConn, packet []byte) error {
+	var msg userAuthBannerMsg
+	if err := Unmarshal(packet, &msg); err != nil {
+		return err
+	}
+
+	transport, ok := c.(*handshakeTransport)
+	if !ok {
+		return nil
+	}
+
+	if transport.bannerCallback != nil {
+		return transport.bannerCallback(msg.Message)
+	}
+
+	return nil
+}
+
+// KeyboardInteractiveChallenge should print questions, optionally
+// disabling echoing (e.g. for passwords), and return all the answers.
+// Challenge may be called multiple times in a single session. After
+// successful authentication, the server may send a challenge with no
+// questions, for which the name and instruction messages should be
+// printed.  RFC 4256 section 3.3 details how the UI should behave for
+// both CLI and GUI environments.
+type KeyboardInteractiveChallenge func(name, instruction string, questions []string, echos []bool) (answers []string, err error)
+
+// KeyboardInteractive returns an AuthMethod using a prompt/response
+// sequence controlled by the server.
+func KeyboardInteractive(challenge KeyboardInteractiveChallenge) AuthMethod {
+	return challenge
+}
+
+func (cb KeyboardInteractiveChallenge) method() string {
+	return "keyboard-interactive"
+}
+
+func (cb KeyboardInteractiveChallenge) auth(session []byte, user string, c packetConn, rand io.Reader, _ map[string][]byte) (authResult, []string, error) {
+	type initiateMsg struct {
+		User       string `sshtype:"50"`
+		Service    string
+		Method     string
+		Language   string
+		Submethods string
+	}
+
+	if err := c.writePacket(Marshal(&initiateMsg{
+		User:    user,
+		Service: serviceSSH,
+		Method:  "keyboard-interactive",
+	})); err != nil {
+		return authFailure, nil, err
+	}
+
+	gotMsgExtInfo := false
+	for {
+		packet, err := c.readPacket()
+		if err != nil {
+			return authFailure, nil, err
+		}
+
+		// like handleAuthResponse, but with less options.
+		switch packet[0] {
+		case msgUserAuthBanner:
+			if err := handleBannerResponse(c, packet); err != nil {
+				return authFailure, nil, err
+			}
+			continue
+		case msgExtInfo:
+			// Ignore post-authentication RFC 8308 extensions, once.
+			if gotMsgExtInfo {
+				return authFailure, nil, unexpectedMessageError(msgUserAuthInfoRequest, packet[0])
+			}
+			gotMsgExtInfo = true
+			continue
+		case msgUserAuthInfoRequest:
+			// OK
+		case msgUserAuthFailure:
+			var msg userAuthFailureMsg
+			if err := Unmarshal(packet, &msg); err != nil {
+				return authFailure, nil, err
+			}
+			if msg.PartialSuccess {
+				return authPartialSuccess, msg.Methods, nil
+			}
+			return authFailure, msg.Methods, nil
+		case msgUserAuthSuccess:
+			return authSuccess, nil, nil
+		default:
+			return authFailure, nil, unexpectedMessageError(msgUserAuthInfoRequest, packet[0])
+		}
+
+		var msg userAuthInfoRequestMsg
+		if err := Unmarshal(packet, &msg); err != nil {
+			return authFailure, nil, err
+		}
+
+		// Manually unpack the prompt/echo pairs.
+		rest := msg.Prompts
+		var prompts []string
+		var echos []bool
+		for i := 0; i < int(msg.NumPrompts); i++ {
+			prompt, r, ok := parseString(rest)
+			if !ok || len(r) == 0 {
+				return authFailure, nil, errors.New("ssh: prompt format error")
+			}
+			prompts = append(prompts, string(prompt))
+			echos = append(echos, r[0] != 0)
+			rest = r[1:]
+		}
+
+		if len(rest) != 0 {
+			return authFailure, nil, errors.New("ssh: extra data following keyboard-interactive pairs")
+		}
+
+		answers, err := cb(msg.Name, msg.Instruction, prompts, echos)
+		if err != nil {
+			return authFailure, nil, err
+		}
+
+		if len(answers) != len(prompts) {
+			return authFailure, nil, fmt.Errorf("ssh: incorrect number of answers from keyboard-interactive callback %d (expected %d)", len(answers), len(prompts))
+		}
+		responseLength := 1 + 4
+		for _, a := range answers {
+			responseLength += stringLength(len(a))
+		}
+		serialized := make([]byte, responseLength)
+		p := serialized
+		p[0] = msgUserAuthInfoResponse
+		p = p[1:]
+		p = marshalUint32(p, uint32(len(answers)))
+		for _, a := range answers {
+			p = marshalString(p, []byte(a))
+		}
+
+		if err := c.writePacket(serialized); err != nil {
+			return authFailure, nil, err
+		}
+	}
+}
+
+type retryableAuthMethod struct {
+	authMethod AuthMethod
+	maxTries   int
+}
+
+func (r *retryableAuthMethod) auth(session []byte, user string, c packetConn, rand io.Reader, extensions map[string][]byte) (ok authResult, methods []string, err error) {
+	for i := 0; r.maxTries <= 0 || i < r.maxTries; i++ {
+		ok, methods, err = r.authMethod.auth(session, user, c, rand, extensions)
+		if ok != authFailure || err != nil { // either success, partial success or error terminate
+			return ok, methods, err
+		}
+	}
+	return ok, methods, err
+}
+
+func (r *retryableAuthMethod) method() string {
+	return r.authMethod.method()
+}
+
+// RetryableAuthMethod is a decorator for other auth methods enabling them to
+// be retried up to maxTries before considering that AuthMethod itself failed.
+// If maxTries is <= 0, will retry indefinitely
+//
+// This is useful for interactive clients using challenge/response type
+// authentication (e.g. Keyboard-Interactive, Password, etc) where the user
+// could mistype their response resulting in the server issuing a
+// SSH_MSG_USERAUTH_FAILURE (rfc4252 #8 [password] and rfc4256 #3.4
+// [keyboard-interactive]); Without this decorator, the non-retryable
+// AuthMethod would be removed from future consideration, and never tried again
+// (and so the user would never be able to retry their entry).
+func RetryableAuthMethod(auth AuthMethod, maxTries int) AuthMethod {
+	return &retryableAuthMethod{authMethod: auth, maxTries: maxTries}
+}
+
+// GSSAPIWithMICAuthMethod is an AuthMethod with "gssapi-with-mic" authentication.
+// See RFC 4462 section 3
+// gssAPIClient is implementation of the GSSAPIClient interface, see the definition of the interface for details.
+// target is the server host you want to log in to.
+func GSSAPIWithMICAuthMethod(gssAPIClient GSSAPIClient, target string) AuthMethod {
+	if gssAPIClient == nil {
+		panic("gss-api client must be not nil with enable gssapi-with-mic")
+	}
+	return &gssAPIWithMICCallback{gssAPIClient: gssAPIClient, target: target}
+}
+
+type gssAPIWithMICCallback struct {
+	gssAPIClient GSSAPIClient
+	target       string
+}
+
+func (g *gssAPIWithMICCallback) auth(session []byte, user string, c packetConn, rand io.Reader, _ map[string][]byte) (authResult, []string, error) {
+	m := &userAuthRequestMsg{
+		User:    user,
+		Service: serviceSSH,
+		Method:  g.method(),
+	}
+	// The GSS-API authentication method is initiated when the client sends an SSH_MSG_USERAUTH_REQUEST.
+	// See RFC 4462 section 3.2.
+	m.Payload = appendU32(m.Payload, 1)
+	m.Payload = appendString(m.Payload, string(krb5OID))
+	if err := c.writePacket(Marshal(m)); err != nil {
+		return authFailure, nil, err
+	}
+	// The server responds to the SSH_MSG_USERAUTH_REQUEST with either an
+	// SSH_MSG_USERAUTH_FAILURE if none of the mechanisms are supported or
+	// with an SSH_MSG_USERAUTH_GSSAPI_RESPONSE.
+	// See RFC 4462 section 3.3.
+	// OpenSSH supports Kerberos V5 mechanism only for GSS-API authentication,so I don't want to check
+	// selected mech if it is valid.
+	packet, err := c.readPacket()
+	if err != nil {
+		return authFailure, nil, err
+	}
+	userAuthGSSAPIResp := &userAuthGSSAPIResponse{}
+	if err := Unmarshal(packet, userAuthGSSAPIResp); err != nil {
+		return authFailure, nil, err
+	}
+	// Start the loop into the exchange token.
+	// See RFC 4462 section 3.4.
+	var token []byte
+	defer g.gssAPIClient.DeleteSecContext()
+	for {
+		// Initiates the establishment of a security context between the application and a remote peer.
+		nextToken, needContinue, err := g.gssAPIClient.InitSecContext("host@"+g.target, token, false)
+		if err != nil {
+			return authFailure, nil, err
+		}
+		if len(nextToken) > 0 {
+			if err := c.writePacket(Marshal(&userAuthGSSAPIToken{
+				Token: nextToken,
+			})); err != nil {
+				return authFailure, nil, err
+			}
+		}
+		if !needContinue {
+			break
+		}
+		packet, err = c.readPacket()
+		if err != nil {
+			return authFailure, nil, err
+		}
+		switch packet[0] {
+		case msgUserAuthFailure:
+			var msg userAuthFailureMsg
+			if err := Unmarshal(packet, &msg); err != nil {
+				return authFailure, nil, err
+			}
+			if msg.PartialSuccess {
+				return authPartialSuccess, msg.Methods, nil
+			}
+			return authFailure, msg.Methods, nil
+		case msgUserAuthGSSAPIError:
+			userAuthGSSAPIErrorResp := &userAuthGSSAPIError{}
+			if err := Unmarshal(packet, userAuthGSSAPIErrorResp); err != nil {
+				return authFailure, nil, err
+			}
+			return authFailure, nil, fmt.Errorf("GSS-API Error:\n"+
+				"Major Status: %d\n"+
+				"Minor Status: %d\n"+
+				"Error Message: %s\n", userAuthGSSAPIErrorResp.MajorStatus, userAuthGSSAPIErrorResp.MinorStatus,
+				userAuthGSSAPIErrorResp.Message)
+		case msgUserAuthGSSAPIToken:
+			userAuthGSSAPITokenReq := &userAuthGSSAPIToken{}
+			if err := Unmarshal(packet, userAuthGSSAPITokenReq); err != nil {
+				return authFailure, nil, err
+			}
+			token = userAuthGSSAPITokenReq.Token
+		}
+	}
+	// Binding Encryption Keys.
+	// See RFC 4462 section 3.5.
+	micField := buildMIC(string(session), user, "ssh-connection", "gssapi-with-mic")
+	micToken, err := g.gssAPIClient.GetMIC(micField)
+	if err != nil {
+		return authFailure, nil, err
+	}
+	if err := c.writePacket(Marshal(&userAuthGSSAPIMIC{
+		MIC: micToken,
+	})); err != nil {
+		return authFailure, nil, err
+	}
+	return handleAuthResponse(c)
+}
+
+func (g *gssAPIWithMICCallback) method() string {
+	return "gssapi-with-mic"
+}

pkg/proto/ssh/common.go

@@ -0,0 +1,468 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"crypto"
+	"crypto/rand"
+	"fmt"
+	"io"
+	"math"
+	"sync"
+
+	_ "crypto/sha1"
+	_ "crypto/sha256"
+	_ "crypto/sha512"
+)
+
+// These are string constants in the SSH protocol.
+const (
+	compressionNone = "none"
+	serviceUserAuth = "ssh-userauth"
+	serviceSSH      = "ssh-connection"
+)
+
+// supportedCiphers lists ciphers we support but might not recommend.
+var supportedCiphers = []string{
+	"aes128-ctr", "aes192-ctr", "aes256-ctr",
+	"aes128-gcm@openssh.com", gcm256CipherID,
+	chacha20Poly1305ID,
+	"arcfour256", "arcfour128", "arcfour",
+	aes128cbcID,
+	tripledescbcID,
+}
+
+// preferredCiphers specifies the default preference for ciphers.
+var preferredCiphers = []string{
+	"aes128-gcm@openssh.com", gcm256CipherID,
+	chacha20Poly1305ID,
+	"aes128-ctr", "aes192-ctr", "aes256-ctr",
+}
+
+// supportedKexAlgos specifies the supported key-exchange algorithms in
+// preference order.
+var supportedKexAlgos = []string{
+	kexAlgoCurve25519SHA256, kexAlgoCurve25519SHA256LibSSH,
+	// P384 and P521 are not constant-time yet, but since we don't
+	// reuse ephemeral keys, using them for ECDH should be OK.
+	kexAlgoECDH256, kexAlgoECDH384, kexAlgoECDH521,
+	kexAlgoDH14SHA256, kexAlgoDH16SHA512, kexAlgoDH14SHA1,
+	kexAlgoDH1SHA1,
+}
+
+// serverForbiddenKexAlgos contains key exchange algorithms, that are forbidden
+// for the server half.
+var serverForbiddenKexAlgos = map[string]struct{}{
+	kexAlgoDHGEXSHA1:   {}, // server half implementation is only minimal to satisfy the automated tests
+	kexAlgoDHGEXSHA256: {}, // server half implementation is only minimal to satisfy the automated tests
+}
+
+// preferredKexAlgos specifies the default preference for key-exchange
+// algorithms in preference order. The diffie-hellman-group16-sha512 algorithm
+// is disabled by default because it is a bit slower than the others.
+var preferredKexAlgos = []string{
+	kexAlgoCurve25519SHA256, kexAlgoCurve25519SHA256LibSSH,
+	kexAlgoECDH256, kexAlgoECDH384, kexAlgoECDH521,
+	kexAlgoDH14SHA256, kexAlgoDH14SHA1,
+}
+
+// supportedHostKeyAlgos specifies the supported host-key algorithms (i.e. methods
+// of authenticating servers) in preference order.
+var supportedHostKeyAlgos = []string{
+	CertAlgoRSASHA256v01, CertAlgoRSASHA512v01,
+	CertAlgoRSAv01, CertAlgoDSAv01, CertAlgoECDSA256v01,
+	CertAlgoECDSA384v01, CertAlgoECDSA521v01, CertAlgoED25519v01,
+
+	KeyAlgoECDSA256, KeyAlgoECDSA384, KeyAlgoECDSA521,
+	KeyAlgoRSASHA256, KeyAlgoRSASHA512,
+	KeyAlgoRSA, KeyAlgoDSA,
+
+	KeyAlgoED25519,
+}
+
+// supportedMACs specifies a default set of MAC algorithms in preference order.
+// This is based on RFC 4253, section 6.4, but with hmac-md5 variants removed
+// because they have reached the end of their useful life.
+var supportedMACs = []string{
+	"hmac-sha2-256-etm@openssh.com", "hmac-sha2-512-etm@openssh.com", "hmac-sha2-256", "hmac-sha2-512", "hmac-sha1", "hmac-sha1-96",
+}
+
+var supportedCompressions = []string{compressionNone}
+
+// hashFuncs keeps the mapping of supported signature algorithms to their
+// respective hashes needed for signing and verification.
+var hashFuncs = map[string]crypto.Hash{
+	KeyAlgoRSA:       crypto.SHA1,
+	KeyAlgoRSASHA256: crypto.SHA256,
+	KeyAlgoRSASHA512: crypto.SHA512,
+	KeyAlgoDSA:       crypto.SHA1,
+	KeyAlgoECDSA256:  crypto.SHA256,
+	KeyAlgoECDSA384:  crypto.SHA384,
+	KeyAlgoECDSA521:  crypto.SHA512,
+	// KeyAlgoED25519 doesn't pre-hash.
+	KeyAlgoSKECDSA256: crypto.SHA256,
+	KeyAlgoSKED25519:  crypto.SHA256,
+}
+
+// algorithmsForKeyFormat returns the supported signature algorithms for a given
+// public key format (PublicKey.Type), in order of preference. See RFC 8332,
+// Section 2. See also the note in sendKexInit on backwards compatibility.
+func algorithmsForKeyFormat(keyFormat string) []string {
+	switch keyFormat {
+	case KeyAlgoRSA:
+		return []string{KeyAlgoRSASHA256, KeyAlgoRSASHA512, KeyAlgoRSA}
+	case CertAlgoRSAv01:
+		return []string{CertAlgoRSASHA256v01, CertAlgoRSASHA512v01, CertAlgoRSAv01}
+	default:
+		return []string{keyFormat}
+	}
+}
+
+// isRSA returns whether algo is a supported RSA algorithm, including certificate
+// algorithms.
+func isRSA(algo string) bool {
+	algos := algorithmsForKeyFormat(KeyAlgoRSA)
+	return contains(algos, underlyingAlgo(algo))
+}
+
+// supportedPubKeyAuthAlgos specifies the supported client public key
+// authentication algorithms. Note that this doesn't include certificate types
+// since those use the underlying algorithm. This list is sent to the client if
+// it supports the server-sig-algs extension. Order is irrelevant.
+var supportedPubKeyAuthAlgos = []string{
+	KeyAlgoED25519,
+	KeyAlgoSKED25519, KeyAlgoSKECDSA256,
+	KeyAlgoECDSA256, KeyAlgoECDSA384, KeyAlgoECDSA521,
+	KeyAlgoRSASHA256, KeyAlgoRSASHA512, KeyAlgoRSA,
+	KeyAlgoDSA,
+}
+
+// unexpectedMessageError results when the SSH message that we received didn't
+// match what we wanted.
+func unexpectedMessageError(expected, got uint8) error {
+	return fmt.Errorf("ssh: unexpected message type %d (expected %d)", got, expected)
+}
+
+// parseError results from a malformed SSH message.
+func parseError(tag uint8) error {
+	return fmt.Errorf("ssh: parse error in message type %d", tag)
+}
+
+func findCommon(what string, client []string, server []string) (common string, err error) {
+	for _, c := range client {
+		for _, s := range server {
+			if c == s {
+				return c, nil
+			}
+		}
+	}
+	return "", fmt.Errorf("ssh: no common algorithm for %s; client offered: %v, server offered: %v", what, client, server)
+}
+
+// directionAlgorithms records algorithm choices in one direction (either read or write)
+type directionAlgorithms struct {
+	Cipher      string
+	MAC         string
+	Compression string
+}
+
+// rekeyBytes returns a rekeying intervals in bytes.
+func (a *directionAlgorithms) rekeyBytes() int64 {
+	// According to RFC 4344 block ciphers should rekey after
+	// 2^(BLOCKSIZE/4) blocks. For all AES flavors BLOCKSIZE is
+	// 128.
+	switch a.Cipher {
+	case "aes128-ctr", "aes192-ctr", "aes256-ctr", gcm128CipherID, gcm256CipherID, aes128cbcID:
+		return 16 * (1 << 32)
+
+	}
+
+	// For others, stick with RFC 4253 recommendation to rekey after 1 Gb of data.
+	return 1 << 30
+}
+
+var aeadCiphers = map[string]bool{
+	gcm128CipherID:     true,
+	gcm256CipherID:     true,
+	chacha20Poly1305ID: true,
+}
+
+type algorithms struct {
+	kex     string
+	hostKey string
+	w       directionAlgorithms
+	r       directionAlgorithms
+}
+
+func findAgreedAlgorithms(isClient bool, clientKexInit, serverKexInit *kexInitMsg) (algs *algorithms, err error) {
+	result := &algorithms{}
+
+	result.kex, err = findCommon("key exchange", clientKexInit.KexAlgos, serverKexInit.KexAlgos)
+	if err != nil {
+		return
+	}
+
+	result.hostKey, err = findCommon("host key", clientKexInit.ServerHostKeyAlgos, serverKexInit.ServerHostKeyAlgos)
+	if err != nil {
+		return
+	}
+
+	stoc, ctos := &result.w, &result.r
+	if isClient {
+		ctos, stoc = stoc, ctos
+	}
+
+	ctos.Cipher, err = findCommon("client to server cipher", clientKexInit.CiphersClientServer, serverKexInit.CiphersClientServer)
+	if err != nil {
+		return
+	}
+
+	stoc.Cipher, err = findCommon("server to client cipher", clientKexInit.CiphersServerClient, serverKexInit.CiphersServerClient)
+	if err != nil {
+		return
+	}
+
+	if !aeadCiphers[ctos.Cipher] {
+		ctos.MAC, err = findCommon("client to server MAC", clientKexInit.MACsClientServer, serverKexInit.MACsClientServer)
+		if err != nil {
+			return
+		}
+	}
+
+	if !aeadCiphers[stoc.Cipher] {
+		stoc.MAC, err = findCommon("server to client MAC", clientKexInit.MACsServerClient, serverKexInit.MACsServerClient)
+		if err != nil {
+			return
+		}
+	}
+
+	ctos.Compression, err = findCommon("client to server compression", clientKexInit.CompressionClientServer, serverKexInit.CompressionClientServer)
+	if err != nil {
+		return
+	}
+
+	stoc.Compression, err = findCommon("server to client compression", clientKexInit.CompressionServerClient, serverKexInit.CompressionServerClient)
+	if err != nil {
+		return
+	}
+
+	return result, nil
+}
+
+// If rekeythreshold is too small, we can't make any progress sending
+// stuff.
+const minRekeyThreshold uint64 = 256
+
+// Config contains configuration data common to both ServerConfig and
+// ClientConfig.
+type Config struct {
+	// Rand provides the source of entropy for cryptographic
+	// primitives. If Rand is nil, the cryptographic random reader
+	// in package crypto/rand will be used.
+	Rand io.Reader
+
+	// The maximum number of bytes sent or received after which a
+	// new key is negotiated. It must be at least 256. If
+	// unspecified, a size suitable for the chosen cipher is used.
+	RekeyThreshold uint64
+
+	// The allowed key exchanges algorithms. If unspecified then a default set
+	// of algorithms is used. Unsupported values are silently ignored.
+	KeyExchanges []string
+
+	// The allowed cipher algorithms. If unspecified then a sensible default is
+	// used. Unsupported values are silently ignored.
+	Ciphers []string
+
+	// The allowed MAC algorithms. If unspecified then a sensible default is
+	// used. Unsupported values are silently ignored.
+	MACs []string
+}
+
+// SetDefaults sets sensible values for unset fields in config. This is
+// exported for testing: Configs passed to SSH functions are copied and have
+// default values set automatically.
+func (c *Config) SetDefaults() {
+	if c.Rand == nil {
+		c.Rand = rand.Reader
+	}
+	if c.Ciphers == nil {
+		c.Ciphers = preferredCiphers
+	}
+	var ciphers []string
+	for _, c := range c.Ciphers {
+		if cipherModes[c] != nil {
+			// Ignore the cipher if we have no cipherModes definition.
+			ciphers = append(ciphers, c)
+		}
+	}
+	c.Ciphers = ciphers
+
+	if c.KeyExchanges == nil {
+		c.KeyExchanges = preferredKexAlgos
+	}
+	var kexs []string
+	for _, k := range c.KeyExchanges {
+		if kexAlgoMap[k] != nil {
+			// Ignore the KEX if we have no kexAlgoMap definition.
+			kexs = append(kexs, k)
+		}
+	}
+	c.KeyExchanges = kexs
+
+	if c.MACs == nil {
+		c.MACs = supportedMACs
+	}
+	var macs []string
+	for _, m := range c.MACs {
+		if macModes[m] != nil {
+			// Ignore the MAC if we have no macModes definition.
+			macs = append(macs, m)
+		}
+	}
+	c.MACs = macs
+
+	if c.RekeyThreshold == 0 {
+		// cipher specific default
+	} else if c.RekeyThreshold < minRekeyThreshold {
+		c.RekeyThreshold = minRekeyThreshold
+	} else if c.RekeyThreshold >= math.MaxInt64 {
+		// Avoid weirdness if somebody uses -1 as a threshold.
+		c.RekeyThreshold = math.MaxInt64
+	}
+}
+
+// buildDataSignedForAuth returns the data that is signed in order to prove
+// possession of a private key. See RFC 4252, section 7. algo is the advertised
+// algorithm, and may be a certificate type.
+func buildDataSignedForAuth(sessionID []byte, req userAuthRequestMsg, algo string, pubKey []byte) []byte {
+	data := struct {
+		Session []byte
+		Type    byte
+		User    string
+		Service string
+		Method  string
+		Sign    bool
+		Algo    string
+		PubKey  []byte
+	}{
+		sessionID,
+		msgUserAuthRequest,
+		req.User,
+		req.Service,
+		req.Method,
+		true,
+		algo,
+		pubKey,
+	}
+	return Marshal(data)
+}
+
+func appendU16(buf []byte, n uint16) []byte {
+	return append(buf, byte(n>>8), byte(n))
+}
+
+func appendU32(buf []byte, n uint32) []byte {
+	return append(buf, byte(n>>24), byte(n>>16), byte(n>>8), byte(n))
+}
+
+func appendU64(buf []byte, n uint64) []byte {
+	return append(buf,
+		byte(n>>56), byte(n>>48), byte(n>>40), byte(n>>32),
+		byte(n>>24), byte(n>>16), byte(n>>8), byte(n))
+}
+
+func appendInt(buf []byte, n int) []byte {
+	return appendU32(buf, uint32(n))
+}
+
+func appendString(buf []byte, s string) []byte {
+	buf = appendU32(buf, uint32(len(s)))
+	buf = append(buf, s...)
+	return buf
+}
+
+func appendBool(buf []byte, b bool) []byte {
+	if b {
+		return append(buf, 1)
+	}
+	return append(buf, 0)
+}
+
+// newCond is a helper to hide the fact that there is no usable zero
+// value for sync.Cond.
+func newCond() *sync.Cond { return sync.NewCond(new(sync.Mutex)) }
+
+// window represents the buffer available to clients
+// wishing to write to a channel.
+type window struct {
+	*sync.Cond
+	win          uint32 // RFC 4254 5.2 says the window size can grow to 2^32-1
+	writeWaiters int
+	closed       bool
+}
+
+// add adds win to the amount of window available
+// for consumers.
+func (w *window) add(win uint32) bool {
+	// a zero sized window adjust is a noop.
+	if win == 0 {
+		return true
+	}
+	w.L.Lock()
+	if w.win+win < win {
+		w.L.Unlock()
+		return false
+	}
+	w.win += win
+	// It is unusual that multiple goroutines would be attempting to reserve
+	// window space, but not guaranteed. Use broadcast to notify all waiters
+	// that additional window is available.
+	w.Broadcast()
+	w.L.Unlock()
+	return true
+}
+
+// close sets the window to closed, so all reservations fail
+// immediately.
+func (w *window) close() {
+	w.L.Lock()
+	w.closed = true
+	w.Broadcast()
+	w.L.Unlock()
+}
+
+// reserve reserves win from the available window capacity.
+// If no capacity remains, reserve will block. reserve may
+// return less than requested.
+func (w *window) reserve(win uint32) (uint32, error) {
+	var err error
+	w.L.Lock()
+	w.writeWaiters++
+	w.Broadcast()
+	for w.win == 0 && !w.closed {
+		w.Wait()
+	}
+	w.writeWaiters--
+	if w.win < win {
+		win = w.win
+	}
+	w.win -= win
+	if w.closed {
+		err = io.EOF
+	}
+	w.L.Unlock()
+	return win, err
+}
+
+// waitWriterBlocked waits until some goroutine is blocked for further
+// writes. It is used in tests only.
+func (w *window) waitWriterBlocked() {
+	w.Cond.L.Lock()
+	for w.writeWaiters == 0 {
+		w.Cond.Wait()
+	}
+	w.Cond.L.Unlock()
+}

pkg/proto/ssh/connection.go

@@ -0,0 +1,143 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"fmt"
+	"net"
+)
+
+// OpenChannelError is returned if the other side rejects an
+// OpenChannel request.
+type OpenChannelError struct {
+	Reason  RejectionReason
+	Message string
+}
+
+func (e *OpenChannelError) Error() string {
+	return fmt.Sprintf("ssh: rejected: %s (%s)", e.Reason, e.Message)
+}
+
+// ConnMetadata holds metadata for the connection.
+type ConnMetadata interface {
+	// User returns the user ID for this connection.
+	User() string
+
+	// SessionID returns the session hash, also denoted by H.
+	SessionID() []byte
+
+	// ClientVersion returns the client's version string as hashed
+	// into the session ID.
+	ClientVersion() []byte
+
+	// ServerVersion returns the server's version string as hashed
+	// into the session ID.
+	ServerVersion() []byte
+
+	// RemoteAddr returns the remote address for this connection.
+	RemoteAddr() net.Addr
+
+	// LocalAddr returns the local address for this connection.
+	LocalAddr() net.Addr
+}
+
+// Conn represents an SSH connection for both server and client roles.
+// Conn is the basis for implementing an application layer, such
+// as ClientConn, which implements the traditional shell access for
+// clients.
+type Conn interface {
+	ConnMetadata
+
+	// SendRequest sends a global request, and returns the
+	// reply. If wantReply is true, it returns the response status
+	// and payload. See also RFC 4254, section 4.
+	SendRequest(name string, wantReply bool, payload []byte) (bool, []byte, error)
+
+	// OpenChannel tries to open an channel. If the request is
+	// rejected, it returns *OpenChannelError. On success it returns
+	// the SSH Channel and a Go channel for incoming, out-of-band
+	// requests. The Go channel must be serviced, or the
+	// connection will hang.
+	OpenChannel(name string, data []byte) (Channel, <-chan *Request, error)
+
+	// Close closes the underlying network connection
+	Close() error
+
+	// Wait blocks until the connection has shut down, and returns the
+	// error causing the shutdown.
+	Wait() error
+
+	// TODO(hanwen): consider exposing:
+	//   RequestKeyChange
+	//   Disconnect
+}
+
+// DiscardRequests consumes and rejects all requests from the
+// passed-in channel.
+func DiscardRequests(in <-chan *Request) {
+	for req := range in {
+		if req.WantReply {
+			req.Reply(false, nil)
+		}
+	}
+}
+
+// A connection represents an incoming connection.
+type connection struct {
+	transport *handshakeTransport
+	sshConn
+
+	// The connection protocol.
+	*mux
+}
+
+func (c *connection) Close() error {
+	return c.sshConn.conn.Close()
+}
+
+// sshConn provides net.Conn metadata, but disallows direct reads and
+// writes.
+type sshConn struct {
+	conn net.Conn
+
+	user          string
+	sessionID     []byte
+	clientVersion []byte
+	serverVersion []byte
+}
+
+func dup(src []byte) []byte {
+	dst := make([]byte, len(src))
+	copy(dst, src)
+	return dst
+}
+
+func (c *sshConn) User() string {
+	return c.user
+}
+
+func (c *sshConn) RemoteAddr() net.Addr {
+	return c.conn.RemoteAddr()
+}
+
+func (c *sshConn) Close() error {
+	return c.conn.Close()
+}
+
+func (c *sshConn) LocalAddr() net.Addr {
+	return c.conn.LocalAddr()
+}
+
+func (c *sshConn) SessionID() []byte {
+	return dup(c.sessionID)
+}
+
+func (c *sshConn) ClientVersion() []byte {
+	return dup(c.clientVersion)
+}
+
+func (c *sshConn) ServerVersion() []byte {
+	return dup(c.serverVersion)
+}

pkg/proto/ssh/doc.go

@@ -0,0 +1,23 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package ssh implements an SSH client and server.
+
+SSH is a transport security protocol, an authentication protocol and a
+family of application protocols. The most typical application level
+protocol is a remote shell and this is specifically implemented.  However,
+the multiplexed nature of SSH is exposed to users that wish to support
+others.
+
+References:
+
+	[PROTOCOL]: https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL?rev=HEAD
+	[PROTOCOL.certkeys]: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.certkeys?rev=HEAD
+	[SSH-PARAMETERS]:    http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xml#ssh-parameters-1
+
+This package does not fall under the stability promise of the Go language itself,
+so its API may be changed when pressing needs arise.
+*/
+package ssh // import "github.com/Neur0toxine/sshpoke/pkg/proto/ssh"

pkg/proto/ssh/handshake.go

@@ -0,0 +1,758 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"crypto/rand"
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"strings"
+	"sync"
+)
+
+// debugHandshake, if set, prints messages sent and received.  Key
+// exchange messages are printed as if DH were used, so the debug
+// messages are wrong when using ECDH.
+const debugHandshake = false
+
+// chanSize sets the amount of buffering SSH connections. This is
+// primarily for testing: setting chanSize=0 uncovers deadlocks more
+// quickly.
+const chanSize = 16
+
+// keyingTransport is a packet based transport that supports key
+// changes. It need not be thread-safe. It should pass through
+// msgNewKeys in both directions.
+type keyingTransport interface {
+	packetConn
+
+	// prepareKeyChange sets up a key change. The key change for a
+	// direction will be effected if a msgNewKeys message is sent
+	// or received.
+	prepareKeyChange(*algorithms, *kexResult) error
+}
+
+// handshakeTransport implements rekeying on top of a keyingTransport
+// and offers a thread-safe writePacket() interface.
+type handshakeTransport struct {
+	conn   keyingTransport
+	config *Config
+
+	serverVersion []byte
+	clientVersion []byte
+
+	// hostKeys is non-empty if we are the server. In that case,
+	// it contains all host keys that can be used to sign the
+	// connection.
+	hostKeys []Signer
+
+	// publicKeyAuthAlgorithms is non-empty if we are the server. In that case,
+	// it contains the supported client public key authentication algorithms.
+	publicKeyAuthAlgorithms []string
+
+	// hostKeyAlgorithms is non-empty if we are the client. In that case,
+	// we accept these key types from the server as host key.
+	hostKeyAlgorithms []string
+
+	// On read error, incoming is closed, and readError is set.
+	incoming  chan []byte
+	readError error
+
+	mu               sync.Mutex
+	writeError       error
+	sentInitPacket   []byte
+	sentInitMsg      *kexInitMsg
+	pendingPackets   [][]byte // Used when a key exchange is in progress.
+	writePacketsLeft uint32
+	writeBytesLeft   int64
+
+	// If the read loop wants to schedule a kex, it pings this
+	// channel, and the write loop will send out a kex
+	// message.
+	requestKex chan struct{}
+
+	// If the other side requests or confirms a kex, its kexInit
+	// packet is sent here for the write loop to find it.
+	startKex    chan *pendingKex
+	kexLoopDone chan struct{} // closed (with writeError non-nil) when kexLoop exits
+
+	// data for host key checking
+	hostKeyCallback HostKeyCallback
+	dialAddress     string
+	remoteAddr      net.Addr
+
+	// bannerCallback is non-empty if we are the client and it has been set in
+	// ClientConfig. In that case it is called during the user authentication
+	// dance to handle a custom server's message.
+	bannerCallback BannerCallback
+
+	// Algorithms agreed in the last key exchange.
+	algorithms *algorithms
+
+	// Counters exclusively owned by readLoop.
+	readPacketsLeft uint32
+	readBytesLeft   int64
+
+	// The session ID or nil if first kex did not complete yet.
+	sessionID []byte
+}
+
+type pendingKex struct {
+	otherInit []byte
+	done      chan error
+}
+
+func newHandshakeTransport(conn keyingTransport, config *Config, clientVersion, serverVersion []byte) *handshakeTransport {
+	t := &handshakeTransport{
+		conn:          conn,
+		serverVersion: serverVersion,
+		clientVersion: clientVersion,
+		incoming:      make(chan []byte, chanSize),
+		requestKex:    make(chan struct{}, 1),
+		startKex:      make(chan *pendingKex),
+		kexLoopDone:   make(chan struct{}),
+
+		config: config,
+	}
+	t.resetReadThresholds()
+	t.resetWriteThresholds()
+
+	// We always start with a mandatory key exchange.
+	t.requestKex <- struct{}{}
+	return t
+}
+
+func newClientTransport(conn keyingTransport, clientVersion, serverVersion []byte, config *ClientConfig, dialAddr string, addr net.Addr) *handshakeTransport {
+	t := newHandshakeTransport(conn, &config.Config, clientVersion, serverVersion)
+	t.dialAddress = dialAddr
+	t.remoteAddr = addr
+	t.hostKeyCallback = config.HostKeyCallback
+	t.bannerCallback = config.BannerCallback
+	if config.HostKeyAlgorithms != nil {
+		t.hostKeyAlgorithms = config.HostKeyAlgorithms
+	} else {
+		t.hostKeyAlgorithms = supportedHostKeyAlgos
+	}
+	go t.readLoop()
+	go t.kexLoop()
+	return t
+}
+
+func newServerTransport(conn keyingTransport, clientVersion, serverVersion []byte, config *ServerConfig) *handshakeTransport {
+	t := newHandshakeTransport(conn, &config.Config, clientVersion, serverVersion)
+	t.hostKeys = config.hostKeys
+	t.publicKeyAuthAlgorithms = config.PublicKeyAuthAlgorithms
+	go t.readLoop()
+	go t.kexLoop()
+	return t
+}
+
+func (t *handshakeTransport) getSessionID() []byte {
+	return t.sessionID
+}
+
+// waitSession waits for the session to be established. This should be
+// the first thing to call after instantiating handshakeTransport.
+func (t *handshakeTransport) waitSession() error {
+	p, err := t.readPacket()
+	if err != nil {
+		return err
+	}
+	if p[0] != msgNewKeys {
+		return fmt.Errorf("ssh: first packet should be msgNewKeys")
+	}
+
+	return nil
+}
+
+func (t *handshakeTransport) id() string {
+	if len(t.hostKeys) > 0 {
+		return "server"
+	}
+	return "client"
+}
+
+func (t *handshakeTransport) printPacket(p []byte, write bool) {
+	action := "got"
+	if write {
+		action = "sent"
+	}
+
+	if p[0] == msgChannelData || p[0] == msgChannelExtendedData {
+		log.Printf("%s %s data (packet %d bytes)", t.id(), action, len(p))
+	} else {
+		msg, err := decode(p)
+		log.Printf("%s %s %T %v (%v)", t.id(), action, msg, msg, err)
+	}
+}
+
+func (t *handshakeTransport) readPacket() ([]byte, error) {
+	p, ok := <-t.incoming
+	if !ok {
+		return nil, t.readError
+	}
+	return p, nil
+}
+
+func (t *handshakeTransport) readLoop() {
+	first := true
+	for {
+		p, err := t.readOnePacket(first)
+		first = false
+		if err != nil {
+			t.readError = err
+			close(t.incoming)
+			break
+		}
+		if p[0] == msgIgnore || p[0] == msgDebug {
+			continue
+		}
+		t.incoming <- p
+	}
+
+	// Stop writers too.
+	t.recordWriteError(t.readError)
+
+	// Unblock the writer should it wait for this.
+	close(t.startKex)
+
+	// Don't close t.requestKex; it's also written to from writePacket.
+}
+
+func (t *handshakeTransport) pushPacket(p []byte) error {
+	if debugHandshake {
+		t.printPacket(p, true)
+	}
+	return t.conn.writePacket(p)
+}
+
+func (t *handshakeTransport) getWriteError() error {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	return t.writeError
+}
+
+func (t *handshakeTransport) recordWriteError(err error) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if t.writeError == nil && err != nil {
+		t.writeError = err
+	}
+}
+
+func (t *handshakeTransport) requestKeyExchange() {
+	select {
+	case t.requestKex <- struct{}{}:
+	default:
+		// something already requested a kex, so do nothing.
+	}
+}
+
+func (t *handshakeTransport) resetWriteThresholds() {
+	t.writePacketsLeft = packetRekeyThreshold
+	if t.config.RekeyThreshold > 0 {
+		t.writeBytesLeft = int64(t.config.RekeyThreshold)
+	} else if t.algorithms != nil {
+		t.writeBytesLeft = t.algorithms.w.rekeyBytes()
+	} else {
+		t.writeBytesLeft = 1 << 30
+	}
+}
+
+func (t *handshakeTransport) kexLoop() {
+
+write:
+	for t.getWriteError() == nil {
+		var request *pendingKex
+		var sent bool
+
+		for request == nil || !sent {
+			var ok bool
+			select {
+			case request, ok = <-t.startKex:
+				if !ok {
+					break write
+				}
+			case <-t.requestKex:
+				break
+			}
+
+			if !sent {
+				if err := t.sendKexInit(); err != nil {
+					t.recordWriteError(err)
+					break
+				}
+				sent = true
+			}
+		}
+
+		if err := t.getWriteError(); err != nil {
+			if request != nil {
+				request.done <- err
+			}
+			break
+		}
+
+		// We're not servicing t.requestKex, but that is OK:
+		// we never block on sending to t.requestKex.
+
+		// We're not servicing t.startKex, but the remote end
+		// has just sent us a kexInitMsg, so it can't send
+		// another key change request, until we close the done
+		// channel on the pendingKex request.
+
+		err := t.enterKeyExchange(request.otherInit)
+
+		t.mu.Lock()
+		t.writeError = err
+		t.sentInitPacket = nil
+		t.sentInitMsg = nil
+
+		t.resetWriteThresholds()
+
+		// we have completed the key exchange. Since the
+		// reader is still blocked, it is safe to clear out
+		// the requestKex channel. This avoids the situation
+		// where: 1) we consumed our own request for the
+		// initial kex, and 2) the kex from the remote side
+		// caused another send on the requestKex channel,
+	clear:
+		for {
+			select {
+			case <-t.requestKex:
+				//
+			default:
+				break clear
+			}
+		}
+
+		request.done <- t.writeError
+
+		// kex finished. Push packets that we received while
+		// the kex was in progress. Don't look at t.startKex
+		// and don't increment writtenSinceKex: if we trigger
+		// another kex while we are still busy with the last
+		// one, things will become very confusing.
+		for _, p := range t.pendingPackets {
+			t.writeError = t.pushPacket(p)
+			if t.writeError != nil {
+				break
+			}
+		}
+		t.pendingPackets = t.pendingPackets[:0]
+		t.mu.Unlock()
+	}
+
+	// Unblock reader.
+	t.conn.Close()
+
+	// drain startKex channel. We don't service t.requestKex
+	// because nobody does blocking sends there.
+	for request := range t.startKex {
+		request.done <- t.getWriteError()
+	}
+
+	// Mark that the loop is done so that Close can return.
+	close(t.kexLoopDone)
+}
+
+// The protocol uses uint32 for packet counters, so we can't let them
+// reach 1<<32.  We will actually read and write more packets than
+// this, though: the other side may send more packets, and after we
+// hit this limit on writing we will send a few more packets for the
+// key exchange itself.
+const packetRekeyThreshold = (1 << 31)
+
+func (t *handshakeTransport) resetReadThresholds() {
+	t.readPacketsLeft = packetRekeyThreshold
+	if t.config.RekeyThreshold > 0 {
+		t.readBytesLeft = int64(t.config.RekeyThreshold)
+	} else if t.algorithms != nil {
+		t.readBytesLeft = t.algorithms.r.rekeyBytes()
+	} else {
+		t.readBytesLeft = 1 << 30
+	}
+}
+
+func (t *handshakeTransport) readOnePacket(first bool) ([]byte, error) {
+	p, err := t.conn.readPacket()
+	if err != nil {
+		return nil, err
+	}
+
+	if t.readPacketsLeft > 0 {
+		t.readPacketsLeft--
+	} else {
+		t.requestKeyExchange()
+	}
+
+	if t.readBytesLeft > 0 {
+		t.readBytesLeft -= int64(len(p))
+	} else {
+		t.requestKeyExchange()
+	}
+
+	if debugHandshake {
+		t.printPacket(p, false)
+	}
+
+	if first && p[0] != msgKexInit {
+		return nil, fmt.Errorf("ssh: first packet should be msgKexInit")
+	}
+
+	if p[0] != msgKexInit {
+		return p, nil
+	}
+
+	firstKex := t.sessionID == nil
+
+	kex := pendingKex{
+		done:      make(chan error, 1),
+		otherInit: p,
+	}
+	t.startKex <- &kex
+	err = <-kex.done
+
+	if debugHandshake {
+		log.Printf("%s exited key exchange (first %v), err %v", t.id(), firstKex, err)
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	t.resetReadThresholds()
+
+	// By default, a key exchange is hidden from higher layers by
+	// translating it into msgIgnore.
+	successPacket := []byte{msgIgnore}
+	if firstKex {
+		// sendKexInit() for the first kex waits for
+		// msgNewKeys so the authentication process is
+		// guaranteed to happen over an encrypted transport.
+		successPacket = []byte{msgNewKeys}
+	}
+
+	return successPacket, nil
+}
+
+// sendKexInit sends a key change message.
+func (t *handshakeTransport) sendKexInit() error {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if t.sentInitMsg != nil {
+		// kexInits may be sent either in response to the other side,
+		// or because our side wants to initiate a key change, so we
+		// may have already sent a kexInit. In that case, don't send a
+		// second kexInit.
+		return nil
+	}
+
+	msg := &kexInitMsg{
+		KexAlgos:                t.config.KeyExchanges,
+		CiphersClientServer:     t.config.Ciphers,
+		CiphersServerClient:     t.config.Ciphers,
+		MACsClientServer:        t.config.MACs,
+		MACsServerClient:        t.config.MACs,
+		CompressionClientServer: supportedCompressions,
+		CompressionServerClient: supportedCompressions,
+	}
+	io.ReadFull(rand.Reader, msg.Cookie[:])
+
+	isServer := len(t.hostKeys) > 0
+	if isServer {
+		for _, k := range t.hostKeys {
+			// If k is a MultiAlgorithmSigner, we restrict the signature
+			// algorithms. If k is a AlgorithmSigner, presume it supports all
+			// signature algorithms associated with the key format. If k is not
+			// an AlgorithmSigner, we can only assume it only supports the
+			// algorithms that matches the key format. (This means that Sign
+			// can't pick a different default).
+			keyFormat := k.PublicKey().Type()
+
+			switch s := k.(type) {
+			case MultiAlgorithmSigner:
+				for _, algo := range algorithmsForKeyFormat(keyFormat) {
+					if contains(s.Algorithms(), underlyingAlgo(algo)) {
+						msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, algo)
+					}
+				}
+			case AlgorithmSigner:
+				msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, algorithmsForKeyFormat(keyFormat)...)
+			default:
+				msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, keyFormat)
+			}
+		}
+	} else {
+		msg.ServerHostKeyAlgos = t.hostKeyAlgorithms
+
+		// As a client we opt in to receiving SSH_MSG_EXT_INFO so we know what
+		// algorithms the server supports for public key authentication. See RFC
+		// 8308, Section 2.1.
+		if firstKeyExchange := t.sessionID == nil; firstKeyExchange {
+			msg.KexAlgos = make([]string, 0, len(t.config.KeyExchanges)+1)
+			msg.KexAlgos = append(msg.KexAlgos, t.config.KeyExchanges...)
+			msg.KexAlgos = append(msg.KexAlgos, "ext-info-c")
+		}
+	}
+
+	packet := Marshal(msg)
+
+	// writePacket destroys the contents, so save a copy.
+	packetCopy := make([]byte, len(packet))
+	copy(packetCopy, packet)
+
+	if err := t.pushPacket(packetCopy); err != nil {
+		return err
+	}
+
+	t.sentInitMsg = msg
+	t.sentInitPacket = packet
+
+	return nil
+}
+
+func (t *handshakeTransport) writePacket(p []byte) error {
+	switch p[0] {
+	case msgKexInit:
+		return errors.New("ssh: only handshakeTransport can send kexInit")
+	case msgNewKeys:
+		return errors.New("ssh: only handshakeTransport can send newKeys")
+	}
+
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if t.writeError != nil {
+		return t.writeError
+	}
+
+	if t.sentInitMsg != nil {
+		// Copy the packet so the writer can reuse the buffer.
+		cp := make([]byte, len(p))
+		copy(cp, p)
+		t.pendingPackets = append(t.pendingPackets, cp)
+		return nil
+	}
+
+	if t.writeBytesLeft > 0 {
+		t.writeBytesLeft -= int64(len(p))
+	} else {
+		t.requestKeyExchange()
+	}
+
+	if t.writePacketsLeft > 0 {
+		t.writePacketsLeft--
+	} else {
+		t.requestKeyExchange()
+	}
+
+	if err := t.pushPacket(p); err != nil {
+		t.writeError = err
+	}
+
+	return nil
+}
+
+func (t *handshakeTransport) Close() error {
+	// Close the connection. This should cause the readLoop goroutine to wake up
+	// and close t.startKex, which will shut down kexLoop if running.
+	err := t.conn.Close()
+
+	// Wait for the kexLoop goroutine to complete.
+	// At that point we know that the readLoop goroutine is complete too,
+	// because kexLoop itself waits for readLoop to close the startKex channel.
+	<-t.kexLoopDone
+
+	return err
+}
+
+func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error {
+	if debugHandshake {
+		log.Printf("%s entered key exchange", t.id())
+	}
+
+	otherInit := &kexInitMsg{}
+	if err := Unmarshal(otherInitPacket, otherInit); err != nil {
+		return err
+	}
+
+	magics := handshakeMagics{
+		clientVersion: t.clientVersion,
+		serverVersion: t.serverVersion,
+		clientKexInit: otherInitPacket,
+		serverKexInit: t.sentInitPacket,
+	}
+
+	clientInit := otherInit
+	serverInit := t.sentInitMsg
+	isClient := len(t.hostKeys) == 0
+	if isClient {
+		clientInit, serverInit = serverInit, clientInit
+
+		magics.clientKexInit = t.sentInitPacket
+		magics.serverKexInit = otherInitPacket
+	}
+
+	var err error
+	t.algorithms, err = findAgreedAlgorithms(isClient, clientInit, serverInit)
+	if err != nil {
+		return err
+	}
+
+	// We don't send FirstKexFollows, but we handle receiving it.
+	//
+	// RFC 4253 section 7 defines the kex and the agreement method for
+	// first_kex_packet_follows. It states that the guessed packet
+	// should be ignored if the "kex algorithm and/or the host
+	// key algorithm is guessed wrong (server and client have
+	// different preferred algorithm), or if any of the other
+	// algorithms cannot be agreed upon". The other algorithms have
+	// already been checked above so the kex algorithm and host key
+	// algorithm are checked here.
+	if otherInit.FirstKexFollows && (clientInit.KexAlgos[0] != serverInit.KexAlgos[0] || clientInit.ServerHostKeyAlgos[0] != serverInit.ServerHostKeyAlgos[0]) {
+		// other side sent a kex message for the wrong algorithm,
+		// which we have to ignore.
+		if _, err := t.conn.readPacket(); err != nil {
+			return err
+		}
+	}
+
+	kex, ok := kexAlgoMap[t.algorithms.kex]
+	if !ok {
+		return fmt.Errorf("ssh: unexpected key exchange algorithm %v", t.algorithms.kex)
+	}
+
+	var result *kexResult
+	if len(t.hostKeys) > 0 {
+		result, err = t.server(kex, &magics)
+	} else {
+		result, err = t.client(kex, &magics)
+	}
+
+	if err != nil {
+		return err
+	}
+
+	firstKeyExchange := t.sessionID == nil
+	if firstKeyExchange {
+		t.sessionID = result.H
+	}
+	result.SessionID = t.sessionID
+
+	if err := t.conn.prepareKeyChange(t.algorithms, result); err != nil {
+		return err
+	}
+	if err = t.conn.writePacket([]byte{msgNewKeys}); err != nil {
+		return err
+	}
+
+	// On the server side, after the first SSH_MSG_NEWKEYS, send a SSH_MSG_EXT_INFO
+	// message with the server-sig-algs extension if the client supports it. See
+	// RFC 8308, Sections 2.4 and 3.1, and [PROTOCOL], Section 1.9.
+	if !isClient && firstKeyExchange && contains(clientInit.KexAlgos, "ext-info-c") {
+		supportedPubKeyAuthAlgosList := strings.Join(t.publicKeyAuthAlgorithms, ",")
+		extInfo := &extInfoMsg{
+			NumExtensions: 2,
+			Payload:       make([]byte, 0, 4+15+4+len(supportedPubKeyAuthAlgosList)+4+16+4+1),
+		}
+		extInfo.Payload = appendInt(extInfo.Payload, len("server-sig-algs"))
+		extInfo.Payload = append(extInfo.Payload, "server-sig-algs"...)
+		extInfo.Payload = appendInt(extInfo.Payload, len(supportedPubKeyAuthAlgosList))
+		extInfo.Payload = append(extInfo.Payload, supportedPubKeyAuthAlgosList...)
+		extInfo.Payload = appendInt(extInfo.Payload, len("ping@openssh.com"))
+		extInfo.Payload = append(extInfo.Payload, "ping@openssh.com"...)
+		extInfo.Payload = appendInt(extInfo.Payload, 1)
+		extInfo.Payload = append(extInfo.Payload, "0"...)
+		if err := t.conn.writePacket(Marshal(extInfo)); err != nil {
+			return err
+		}
+	}
+
+	if packet, err := t.conn.readPacket(); err != nil {
+		return err
+	} else if packet[0] != msgNewKeys {
+		return unexpectedMessageError(msgNewKeys, packet[0])
+	}
+
+	return nil
+}
+
+// algorithmSignerWrapper is an AlgorithmSigner that only supports the default
+// key format algorithm.
+//
+// This is technically a violation of the AlgorithmSigner interface, but it
+// should be unreachable given where we use this. Anyway, at least it returns an
+// error instead of panicing or producing an incorrect signature.
+type algorithmSignerWrapper struct {
+	Signer
+}
+
+func (a algorithmSignerWrapper) SignWithAlgorithm(rand io.Reader, data []byte, algorithm string) (*Signature, error) {
+	if algorithm != underlyingAlgo(a.PublicKey().Type()) {
+		return nil, errors.New("ssh: internal error: algorithmSignerWrapper invoked with non-default algorithm")
+	}
+	return a.Sign(rand, data)
+}
+
+func pickHostKey(hostKeys []Signer, algo string) AlgorithmSigner {
+	for _, k := range hostKeys {
+		if s, ok := k.(MultiAlgorithmSigner); ok {
+			if !contains(s.Algorithms(), underlyingAlgo(algo)) {
+				continue
+			}
+		}
+
+		if algo == k.PublicKey().Type() {
+			return algorithmSignerWrapper{k}
+		}
+
+		k, ok := k.(AlgorithmSigner)
+		if !ok {
+			continue
+		}
+		for _, a := range algorithmsForKeyFormat(k.PublicKey().Type()) {
+			if algo == a {
+				return k
+			}
+		}
+	}
+	return nil
+}
+
+func (t *handshakeTransport) server(kex kexAlgorithm, magics *handshakeMagics) (*kexResult, error) {
+	hostKey := pickHostKey(t.hostKeys, t.algorithms.hostKey)
+	if hostKey == nil {
+		return nil, errors.New("ssh: internal error: negotiated unsupported signature type")
+	}
+
+	r, err := kex.Server(t.conn, t.config.Rand, magics, hostKey, t.algorithms.hostKey)
+	return r, err
+}
+
+func (t *handshakeTransport) client(kex kexAlgorithm, magics *handshakeMagics) (*kexResult, error) {
+	result, err := kex.Client(t.conn, t.config.Rand, magics)
+	if err != nil {
+		return nil, err
+	}
+
+	hostKey, err := ParsePublicKey(result.HostKey)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := verifyHostKeySignature(hostKey, t.algorithms.hostKey, result); err != nil {
+		return nil, err
+	}
+
+	err = t.hostKeyCallback(t.dialAddress, t.remoteAddr, hostKey)
+	if err != nil {
+		return nil, err
+	}
+
+	return result, nil
+}

pkg/proto/ssh/internal/bcrypt_pbkdf/bcrypt_pbkdf.go

@@ -0,0 +1,93 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package bcrypt_pbkdf implements bcrypt_pbkdf(3) from OpenBSD.
+//
+// See https://flak.tedunangst.com/post/bcrypt-pbkdf and
+// https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/lib/libutil/bcrypt_pbkdf.c.
+package bcrypt_pbkdf
+
+import (
+	"crypto/sha512"
+	"errors"
+	"golang.org/x/crypto/blowfish"
+)
+
+const blockSize = 32
+
+// Key derives a key from the password, salt and rounds count, returning a
+// []byte of length keyLen that can be used as cryptographic key.
+func Key(password, salt []byte, rounds, keyLen int) ([]byte, error) {
+	if rounds < 1 {
+		return nil, errors.New("bcrypt_pbkdf: number of rounds is too small")
+	}
+	if len(password) == 0 {
+		return nil, errors.New("bcrypt_pbkdf: empty password")
+	}
+	if len(salt) == 0 || len(salt) > 1<<20 {
+		return nil, errors.New("bcrypt_pbkdf: bad salt length")
+	}
+	if keyLen > 1024 {
+		return nil, errors.New("bcrypt_pbkdf: keyLen is too large")
+	}
+
+	numBlocks := (keyLen + blockSize - 1) / blockSize
+	key := make([]byte, numBlocks*blockSize)
+
+	h := sha512.New()
+	h.Write(password)
+	shapass := h.Sum(nil)
+
+	shasalt := make([]byte, 0, sha512.Size)
+	cnt, tmp := make([]byte, 4), make([]byte, blockSize)
+	for block := 1; block <= numBlocks; block++ {
+		h.Reset()
+		h.Write(salt)
+		cnt[0] = byte(block >> 24)
+		cnt[1] = byte(block >> 16)
+		cnt[2] = byte(block >> 8)
+		cnt[3] = byte(block)
+		h.Write(cnt)
+		bcryptHash(tmp, shapass, h.Sum(shasalt))
+
+		out := make([]byte, blockSize)
+		copy(out, tmp)
+		for i := 2; i <= rounds; i++ {
+			h.Reset()
+			h.Write(tmp)
+			bcryptHash(tmp, shapass, h.Sum(shasalt))
+			for j := 0; j < len(out); j++ {
+				out[j] ^= tmp[j]
+			}
+		}
+
+		for i, v := range out {
+			key[i*numBlocks+(block-1)] = v
+		}
+	}
+	return key[:keyLen], nil
+}
+
+var magic = []byte("OxychromaticBlowfishSwatDynamite")
+
+func bcryptHash(out, shapass, shasalt []byte) {
+	c, err := blowfish.NewSaltedCipher(shapass, shasalt)
+	if err != nil {
+		panic(err)
+	}
+	for i := 0; i < 64; i++ {
+		blowfish.ExpandKey(shasalt, c)
+		blowfish.ExpandKey(shapass, c)
+	}
+	copy(out, magic)
+	for i := 0; i < 32; i += 8 {
+		for j := 0; j < 64; j++ {
+			c.Encrypt(out[i:i+8], out[i:i+8])
+		}
+	}
+	// Swap bytes due to different endianness.
+	for i := 0; i < 32; i += 4 {
+		out[i+3], out[i+2], out[i+1], out[i] = out[i], out[i+1], out[i+2], out[i+3]
+	}
+}

pkg/proto/ssh/internal/poly1305/bits_compat.go

@@ -0,0 +1,39 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !go1.13
+
+package poly1305
+
+// Generic fallbacks for the math/bits intrinsics, copied from
+// src/math/bits/bits.go. They were added in Go 1.12, but Add64 and Sum64 had
+// variable time fallbacks until Go 1.13.
+
+func bitsAdd64(x, y, carry uint64) (sum, carryOut uint64) {
+	sum = x + y + carry
+	carryOut = ((x & y) | ((x | y) &^ sum)) >> 63
+	return
+}
+
+func bitsSub64(x, y, borrow uint64) (diff, borrowOut uint64) {
+	diff = x - y - borrow
+	borrowOut = ((^x & y) | (^(x ^ y) & diff)) >> 63
+	return
+}
+
+func bitsMul64(x, y uint64) (hi, lo uint64) {
+	const mask32 = 1<<32 - 1
+	x0 := x & mask32
+	x1 := x >> 32
+	y0 := y & mask32
+	y1 := y >> 32
+	w0 := x0 * y0
+	t := x1*y0 + w0>>32
+	w1 := t & mask32
+	w2 := t >> 32
+	w1 += x0 * y1
+	hi = x1*y1 + w2 + w1>>32
+	lo = x * y
+	return
+}

pkg/proto/ssh/internal/poly1305/bits_go1.13.go

@@ -0,0 +1,21 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.13
+
+package poly1305
+
+import "math/bits"
+
+func bitsAdd64(x, y, carry uint64) (sum, carryOut uint64) {
+	return bits.Add64(x, y, carry)
+}
+
+func bitsSub64(x, y, borrow uint64) (diff, borrowOut uint64) {
+	return bits.Sub64(x, y, borrow)
+}
+
+func bitsMul64(x, y uint64) (hi, lo uint64) {
+	return bits.Mul64(x, y)
+}

pkg/proto/ssh/internal/poly1305/mac_noasm.go

@@ -0,0 +1,9 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build (!amd64 && !ppc64le && !s390x) || !gc || purego
+
+package poly1305
+
+type mac struct{ macGeneric }

pkg/proto/ssh/internal/poly1305/poly1305.go

@@ -0,0 +1,99 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package poly1305 implements Poly1305 one-time message authentication code as
+// specified in https://cr.yp.to/mac/poly1305-20050329.pdf.
+//
+// Poly1305 is a fast, one-time authentication function. It is infeasible for an
+// attacker to generate an authenticator for a message without the key. However, a
+// key must only be used for a single message. Authenticating two different
+// messages with the same key allows an attacker to forge authenticators for other
+// messages with the same key.
+//
+// Poly1305 was originally coupled with AES in order to make Poly1305-AES. AES was
+// used with a fixed key in order to generate one-time keys from an nonce.
+// However, in this package AES isn't used and the one-time key is specified
+// directly.
+package poly1305
+
+import "crypto/subtle"
+
+// TagSize is the size, in bytes, of a poly1305 authenticator.
+const TagSize = 16
+
+// Sum generates an authenticator for msg using a one-time key and puts the
+// 16-byte result into out. Authenticating two different messages with the same
+// key allows an attacker to forge messages at will.
+func Sum(out *[16]byte, m []byte, key *[32]byte) {
+	h := New(key)
+	h.Write(m)
+	h.Sum(out[:0])
+}
+
+// Verify returns true if mac is a valid authenticator for m with the given key.
+func Verify(mac *[16]byte, m []byte, key *[32]byte) bool {
+	var tmp [16]byte
+	Sum(&tmp, m, key)
+	return subtle.ConstantTimeCompare(tmp[:], mac[:]) == 1
+}
+
+// New returns a new MAC computing an authentication
+// tag of all data written to it with the given key.
+// This allows writing the message progressively instead
+// of passing it as a single slice. Common users should use
+// the Sum function instead.
+//
+// The key must be unique for each message, as authenticating
+// two different messages with the same key allows an attacker
+// to forge messages at will.
+func New(key *[32]byte) *MAC {
+	m := &MAC{}
+	initialize(key, &m.macState)
+	return m
+}
+
+// MAC is an io.Writer computing an authentication tag
+// of the data written to it.
+//
+// MAC cannot be used like common hash.Hash implementations,
+// because using a poly1305 key twice breaks its security.
+// Therefore writing data to a running MAC after calling
+// Sum or Verify causes it to panic.
+type MAC struct {
+	mac // platform-dependent implementation
+
+	finalized bool
+}
+
+// Size returns the number of bytes Sum will return.
+func (h *MAC) Size() int { return TagSize }
+
+// Write adds more data to the running message authentication code.
+// It never returns an error.
+//
+// It must not be called after the first call of Sum or Verify.
+func (h *MAC) Write(p []byte) (n int, err error) {
+	if h.finalized {
+		panic("poly1305: write to MAC after Sum or Verify")
+	}
+	return h.mac.Write(p)
+}
+
+// Sum computes the authenticator of all data written to the
+// message authentication code.
+func (h *MAC) Sum(b []byte) []byte {
+	var mac [TagSize]byte
+	h.mac.Sum(&mac)
+	h.finalized = true
+	return append(b, mac[:]...)
+}
+
+// Verify returns whether the authenticator of all data written to
+// the message authentication code matches the expected value.
+func (h *MAC) Verify(expected []byte) bool {
+	var mac [TagSize]byte
+	h.mac.Sum(&mac)
+	h.finalized = true
+	return subtle.ConstantTimeCompare(expected, mac[:]) == 1
+}

pkg/proto/ssh/internal/poly1305/sum_amd64.go

@@ -0,0 +1,47 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build gc && !purego
+
+package poly1305
+
+//go:noescape
+func update(state *macState, msg []byte)
+
+// mac is a wrapper for macGeneric that redirects calls that would have gone to
+// updateGeneric to update.
+//
+// Its Write and Sum methods are otherwise identical to the macGeneric ones, but
+// using function pointers would carry a major performance cost.
+type mac struct{ macGeneric }
+
+func (h *mac) Write(p []byte) (int, error) {
+	nn := len(p)
+	if h.offset > 0 {
+		n := copy(h.buffer[h.offset:], p)
+		if h.offset+n < TagSize {
+			h.offset += n
+			return nn, nil
+		}
+		p = p[n:]
+		h.offset = 0
+		update(&h.macState, h.buffer[:])
+	}
+	if n := len(p) - (len(p) % TagSize); n > 0 {
+		update(&h.macState, p[:n])
+		p = p[n:]
+	}
+	if len(p) > 0 {
+		h.offset += copy(h.buffer[h.offset:], p)
+	}
+	return nn, nil
+}
+
+func (h *mac) Sum(out *[16]byte) {
+	state := h.macState
+	if h.offset > 0 {
+		update(&state, h.buffer[:h.offset])
+	}
+	finalize(out, &state.h, &state.s)
+}

pkg/proto/ssh/internal/poly1305/sum_amd64.s

@@ -0,0 +1,108 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build gc && !purego
+
+#include "textflag.h"
+
+#define POLY1305_ADD(msg, h0, h1, h2) \
+	ADDQ 0(msg), h0;  \
+	ADCQ 8(msg), h1;  \
+	ADCQ $1, h2;      \
+	LEAQ 16(msg), msg
+
+#define POLY1305_MUL(h0, h1, h2, r0, r1, t0, t1, t2, t3) \
+	MOVQ  r0, AX;                  \
+	MULQ  h0;                      \
+	MOVQ  AX, t0;                  \
+	MOVQ  DX, t1;                  \
+	MOVQ  r0, AX;                  \
+	MULQ  h1;                      \
+	ADDQ  AX, t1;                  \
+	ADCQ  $0, DX;                  \
+	MOVQ  r0, t2;                  \
+	IMULQ h2, t2;                  \
+	ADDQ  DX, t2;                  \
+	                               \
+	MOVQ  r1, AX;                  \
+	MULQ  h0;                      \
+	ADDQ  AX, t1;                  \
+	ADCQ  $0, DX;                  \
+	MOVQ  DX, h0;                  \
+	MOVQ  r1, t3;                  \
+	IMULQ h2, t3;                  \
+	MOVQ  r1, AX;                  \
+	MULQ  h1;                      \
+	ADDQ  AX, t2;                  \
+	ADCQ  DX, t3;                  \
+	ADDQ  h0, t2;                  \
+	ADCQ  $0, t3;                  \
+	                               \
+	MOVQ  t0, h0;                  \
+	MOVQ  t1, h1;                  \
+	MOVQ  t2, h2;                  \
+	ANDQ  $3, h2;                  \
+	MOVQ  t2, t0;                  \
+	ANDQ  $0xFFFFFFFFFFFFFFFC, t0; \
+	ADDQ  t0, h0;                  \
+	ADCQ  t3, h1;                  \
+	ADCQ  $0, h2;                  \
+	SHRQ  $2, t3, t2;              \
+	SHRQ  $2, t3;                  \
+	ADDQ  t2, h0;                  \
+	ADCQ  t3, h1;                  \
+	ADCQ  $0, h2
+
+// func update(state *[7]uint64, msg []byte)
+TEXT ·update(SB), $0-32
+	MOVQ state+0(FP), DI
+	MOVQ msg_base+8(FP), SI
+	MOVQ msg_len+16(FP), R15
+
+	MOVQ 0(DI), R8   // h0
+	MOVQ 8(DI), R9   // h1
+	MOVQ 16(DI), R10 // h2
+	MOVQ 24(DI), R11 // r0
+	MOVQ 32(DI), R12 // r1
+
+	CMPQ R15, $16
+	JB   bytes_between_0_and_15
+
+loop:
+	POLY1305_ADD(SI, R8, R9, R10)
+
+multiply:
+	POLY1305_MUL(R8, R9, R10, R11, R12, BX, CX, R13, R14)
+	SUBQ $16, R15
+	CMPQ R15, $16
+	JAE  loop
+
+bytes_between_0_and_15:
+	TESTQ R15, R15
+	JZ    done
+	MOVQ  $1, BX
+	XORQ  CX, CX
+	XORQ  R13, R13
+	ADDQ  R15, SI
+
+flush_buffer:
+	SHLQ $8, BX, CX
+	SHLQ $8, BX
+	MOVB -1(SI), R13
+	XORQ R13, BX
+	DECQ SI
+	DECQ R15
+	JNZ  flush_buffer
+
+	ADDQ BX, R8
+	ADCQ CX, R9
+	ADCQ $0, R10
+	MOVQ $16, R15
+	JMP  multiply
+
+done:
+	MOVQ R8, 0(DI)
+	MOVQ R9, 8(DI)
+	MOVQ R10, 16(DI)
+	RET

pkg/proto/ssh/internal/poly1305/sum_generic.go

@@ -0,0 +1,309 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file provides the generic implementation of Sum and MAC. Other files
+// might provide optimized assembly implementations of some of this code.
+
+package poly1305
+
+import "encoding/binary"
+
+// Poly1305 [RFC 7539] is a relatively simple algorithm: the authentication tag
+// for a 64 bytes message is approximately
+//
+//     s + m[0:16] * r⁴ + m[16:32] * r³ + m[32:48] * r² + m[48:64] * r  mod  2¹³⁰ - 5
+//
+// for some secret r and s. It can be computed sequentially like
+//
+//     for len(msg) > 0:
+//         h += read(msg, 16)
+//         h *= r
+//         h %= 2¹³⁰ - 5
+//     return h + s
+//
+// All the complexity is about doing performant constant-time math on numbers
+// larger than any available numeric type.
+
+func sumGeneric(out *[TagSize]byte, msg []byte, key *[32]byte) {
+	h := newMACGeneric(key)
+	h.Write(msg)
+	h.Sum(out)
+}
+
+func newMACGeneric(key *[32]byte) macGeneric {
+	m := macGeneric{}
+	initialize(key, &m.macState)
+	return m
+}
+
+// macState holds numbers in saturated 64-bit little-endian limbs. That is,
+// the value of [x0, x1, x2] is x[0] + x[1] * 2⁶⁴ + x[2] * 2¹²⁸.
+type macState struct {
+	// h is the main accumulator. It is to be interpreted modulo 2¹³⁰ - 5, but
+	// can grow larger during and after rounds. It must, however, remain below
+	// 2 * (2¹³⁰ - 5).
+	h [3]uint64
+	// r and s are the private key components.
+	r [2]uint64
+	s [2]uint64
+}
+
+type macGeneric struct {
+	macState
+
+	buffer [TagSize]byte
+	offset int
+}
+
+// Write splits the incoming message into TagSize chunks, and passes them to
+// update. It buffers incomplete chunks.
+func (h *macGeneric) Write(p []byte) (int, error) {
+	nn := len(p)
+	if h.offset > 0 {
+		n := copy(h.buffer[h.offset:], p)
+		if h.offset+n < TagSize {
+			h.offset += n
+			return nn, nil
+		}
+		p = p[n:]
+		h.offset = 0
+		updateGeneric(&h.macState, h.buffer[:])
+	}
+	if n := len(p) - (len(p) % TagSize); n > 0 {
+		updateGeneric(&h.macState, p[:n])
+		p = p[n:]
+	}
+	if len(p) > 0 {
+		h.offset += copy(h.buffer[h.offset:], p)
+	}
+	return nn, nil
+}
+
+// Sum flushes the last incomplete chunk from the buffer, if any, and generates
+// the MAC output. It does not modify its state, in order to allow for multiple
+// calls to Sum, even if no Write is allowed after Sum.
+func (h *macGeneric) Sum(out *[TagSize]byte) {
+	state := h.macState
+	if h.offset > 0 {
+		updateGeneric(&state, h.buffer[:h.offset])
+	}
+	finalize(out, &state.h, &state.s)
+}
+
+// [rMask0, rMask1] is the specified Poly1305 clamping mask in little-endian. It
+// clears some bits of the secret coefficient to make it possible to implement
+// multiplication more efficiently.
+const (
+	rMask0 = 0x0FFFFFFC0FFFFFFF
+	rMask1 = 0x0FFFFFFC0FFFFFFC
+)
+
+// initialize loads the 256-bit key into the two 128-bit secret values r and s.
+func initialize(key *[32]byte, m *macState) {
+	m.r[0] = binary.LittleEndian.Uint64(key[0:8]) & rMask0
+	m.r[1] = binary.LittleEndian.Uint64(key[8:16]) & rMask1
+	m.s[0] = binary.LittleEndian.Uint64(key[16:24])
+	m.s[1] = binary.LittleEndian.Uint64(key[24:32])
+}
+
+// uint128 holds a 128-bit number as two 64-bit limbs, for use with the
+// bits.Mul64 and bits.Add64 intrinsics.
+type uint128 struct {
+	lo, hi uint64
+}
+
+func mul64(a, b uint64) uint128 {
+	hi, lo := bitsMul64(a, b)
+	return uint128{lo, hi}
+}
+
+func add128(a, b uint128) uint128 {
+	lo, c := bitsAdd64(a.lo, b.lo, 0)
+	hi, c := bitsAdd64(a.hi, b.hi, c)
+	if c != 0 {
+		panic("poly1305: unexpected overflow")
+	}
+	return uint128{lo, hi}
+}
+
+func shiftRightBy2(a uint128) uint128 {
+	a.lo = a.lo>>2 | (a.hi&3)<<62
+	a.hi = a.hi >> 2
+	return a
+}
+
+// updateGeneric absorbs msg into the state.h accumulator. For each chunk m of
+// 128 bits of message, it computes
+//
+//	h₊ = (h + m) * r  mod  2¹³⁰ - 5
+//
+// If the msg length is not a multiple of TagSize, it assumes the last
+// incomplete chunk is the final one.
+func updateGeneric(state *macState, msg []byte) {
+	h0, h1, h2 := state.h[0], state.h[1], state.h[2]
+	r0, r1 := state.r[0], state.r[1]
+
+	for len(msg) > 0 {
+		var c uint64
+
+		// For the first step, h + m, we use a chain of bits.Add64 intrinsics.
+		// The resulting value of h might exceed 2¹³⁰ - 5, but will be partially
+		// reduced at the end of the multiplication below.
+		//
+		// The spec requires us to set a bit just above the message size, not to
+		// hide leading zeroes. For full chunks, that's 1 << 128, so we can just
+		// add 1 to the most significant (2¹²⁸) limb, h2.
+		if len(msg) >= TagSize {
+			h0, c = bitsAdd64(h0, binary.LittleEndian.Uint64(msg[0:8]), 0)
+			h1, c = bitsAdd64(h1, binary.LittleEndian.Uint64(msg[8:16]), c)
+			h2 += c + 1
+
+			msg = msg[TagSize:]
+		} else {
+			var buf [TagSize]byte
+			copy(buf[:], msg)
+			buf[len(msg)] = 1
+
+			h0, c = bitsAdd64(h0, binary.LittleEndian.Uint64(buf[0:8]), 0)
+			h1, c = bitsAdd64(h1, binary.LittleEndian.Uint64(buf[8:16]), c)
+			h2 += c
+
+			msg = nil
+		}
+
+		// Multiplication of big number limbs is similar to elementary school
+		// columnar multiplication. Instead of digits, there are 64-bit limbs.
+		//
+		// We are multiplying a 3 limbs number, h, by a 2 limbs number, r.
+		//
+		//                        h2    h1    h0  x
+		//                              r1    r0  =
+		//                       ----------------
+		//                      h2r0  h1r0  h0r0     <-- individual 128-bit products
+		//            +   h2r1  h1r1  h0r1
+		//               ------------------------
+		//                 m3    m2    m1    m0      <-- result in 128-bit overlapping limbs
+		//               ------------------------
+		//         m3.hi m2.hi m1.hi m0.hi           <-- carry propagation
+		//     +         m3.lo m2.lo m1.lo m0.lo
+		//        -------------------------------
+		//           t4    t3    t2    t1    t0      <-- final result in 64-bit limbs
+		//
+		// The main difference from pen-and-paper multiplication is that we do
+		// carry propagation in a separate step, as if we wrote two digit sums
+		// at first (the 128-bit limbs), and then carried the tens all at once.
+
+		h0r0 := mul64(h0, r0)
+		h1r0 := mul64(h1, r0)
+		h2r0 := mul64(h2, r0)
+		h0r1 := mul64(h0, r1)
+		h1r1 := mul64(h1, r1)
+		h2r1 := mul64(h2, r1)
+
+		// Since h2 is known to be at most 7 (5 + 1 + 1), and r0 and r1 have their
+		// top 4 bits cleared by rMask{0,1}, we know that their product is not going
+		// to overflow 64 bits, so we can ignore the high part of the products.
+		//
+		// This also means that the product doesn't have a fifth limb (t4).
+		if h2r0.hi != 0 {
+			panic("poly1305: unexpected overflow")
+		}
+		if h2r1.hi != 0 {
+			panic("poly1305: unexpected overflow")
+		}
+
+		m0 := h0r0
+		m1 := add128(h1r0, h0r1) // These two additions don't overflow thanks again
+		m2 := add128(h2r0, h1r1) // to the 4 masked bits at the top of r0 and r1.
+		m3 := h2r1
+
+		t0 := m0.lo
+		t1, c := bitsAdd64(m1.lo, m0.hi, 0)
+		t2, c := bitsAdd64(m2.lo, m1.hi, c)
+		t3, _ := bitsAdd64(m3.lo, m2.hi, c)
+
+		// Now we have the result as 4 64-bit limbs, and we need to reduce it
+		// modulo 2¹³⁰ - 5. The special shape of this Crandall prime lets us do
+		// a cheap partial reduction according to the reduction identity
+		//
+		//     c * 2¹³⁰ + n  =  c * 5 + n  mod  2¹³⁰ - 5
+		//
+		// because 2¹³⁰ = 5 mod 2¹³⁰ - 5. Partial reduction since the result is
+		// likely to be larger than 2¹³⁰ - 5, but still small enough to fit the
+		// assumptions we make about h in the rest of the code.
+		//
+		// See also https://speakerdeck.com/gtank/engineering-prime-numbers?slide=23
+
+		// We split the final result at the 2¹³⁰ mark into h and cc, the carry.
+		// Note that the carry bits are effectively shifted left by 2, in other
+		// words, cc = c * 4 for the c in the reduction identity.
+		h0, h1, h2 = t0, t1, t2&maskLow2Bits
+		cc := uint128{t2 & maskNotLow2Bits, t3}
+
+		// To add c * 5 to h, we first add cc = c * 4, and then add (cc >> 2) = c.
+
+		h0, c = bitsAdd64(h0, cc.lo, 0)
+		h1, c = bitsAdd64(h1, cc.hi, c)
+		h2 += c
+
+		cc = shiftRightBy2(cc)
+
+		h0, c = bitsAdd64(h0, cc.lo, 0)
+		h1, c = bitsAdd64(h1, cc.hi, c)
+		h2 += c
+
+		// h2 is at most 3 + 1 + 1 = 5, making the whole of h at most
+		//
+		//     5 * 2¹²⁸ + (2¹²⁸ - 1) = 6 * 2¹²⁸ - 1
+	}
+
+	state.h[0], state.h[1], state.h[2] = h0, h1, h2
+}
+
+const (
+	maskLow2Bits    uint64 = 0x0000000000000003
+	maskNotLow2Bits uint64 = ^maskLow2Bits
+)
+
+// select64 returns x if v == 1 and y if v == 0, in constant time.
+func select64(v, x, y uint64) uint64 { return ^(v-1)&x | (v-1)&y }
+
+// [p0, p1, p2] is 2¹³⁰ - 5 in little endian order.
+const (
+	p0 = 0xFFFFFFFFFFFFFFFB
+	p1 = 0xFFFFFFFFFFFFFFFF
+	p2 = 0x0000000000000003
+)
+
+// finalize completes the modular reduction of h and computes
+//
+//	out = h + s  mod  2¹²⁸
+func finalize(out *[TagSize]byte, h *[3]uint64, s *[2]uint64) {
+	h0, h1, h2 := h[0], h[1], h[2]
+
+	// After the partial reduction in updateGeneric, h might be more than
+	// 2¹³⁰ - 5, but will be less than 2 * (2¹³⁰ - 5). To complete the reduction
+	// in constant time, we compute t = h - (2¹³⁰ - 5), and select h as the
+	// result if the subtraction underflows, and t otherwise.
+
+	hMinusP0, b := bitsSub64(h0, p0, 0)
+	hMinusP1, b := bitsSub64(h1, p1, b)
+	_, b = bitsSub64(h2, p2, b)
+
+	// h = h if h < p else h - p
+	h0 = select64(b, h0, hMinusP0)
+	h1 = select64(b, h1, hMinusP1)
+
+	// Finally, we compute the last Poly1305 step
+	//
+	//     tag = h + s  mod  2¹²⁸
+	//
+	// by just doing a wide addition with the 128 low bits of h and discarding
+	// the overflow.
+	h0, c := bitsAdd64(h0, s[0], 0)
+	h1, _ = bitsAdd64(h1, s[1], c)
+
+	binary.LittleEndian.PutUint64(out[0:8], h0)
+	binary.LittleEndian.PutUint64(out[8:16], h1)
+}

pkg/proto/ssh/internal/poly1305/sum_ppc64le.go

@@ -0,0 +1,47 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build gc && !purego
+
+package poly1305
+
+//go:noescape
+func update(state *macState, msg []byte)
+
+// mac is a wrapper for macGeneric that redirects calls that would have gone to
+// updateGeneric to update.
+//
+// Its Write and Sum methods are otherwise identical to the macGeneric ones, but
+// using function pointers would carry a major performance cost.
+type mac struct{ macGeneric }
+
+func (h *mac) Write(p []byte) (int, error) {
+	nn := len(p)
+	if h.offset > 0 {
+		n := copy(h.buffer[h.offset:], p)
+		if h.offset+n < TagSize {
+			h.offset += n
+			return nn, nil
+		}
+		p = p[n:]
+		h.offset = 0
+		update(&h.macState, h.buffer[:])
+	}
+	if n := len(p) - (len(p) % TagSize); n > 0 {
+		update(&h.macState, p[:n])
+		p = p[n:]
+	}
+	if len(p) > 0 {
+		h.offset += copy(h.buffer[h.offset:], p)
+	}
+	return nn, nil
+}
+
+func (h *mac) Sum(out *[16]byte) {
+	state := h.macState
+	if h.offset > 0 {
+		update(&state, h.buffer[:h.offset])
+	}
+	finalize(out, &state.h, &state.s)
+}

pkg/proto/ssh/internal/poly1305/sum_ppc64le.s

@@ -0,0 +1,181 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build gc && !purego
+
+#include "textflag.h"
+
+// This was ported from the amd64 implementation.
+
+#define POLY1305_ADD(msg, h0, h1, h2, t0, t1, t2) \
+	MOVD (msg), t0;  \
+	MOVD 8(msg), t1; \
+	MOVD $1, t2;     \
+	ADDC t0, h0, h0; \
+	ADDE t1, h1, h1; \
+	ADDE t2, h2;     \
+	ADD  $16, msg
+
+#define POLY1305_MUL(h0, h1, h2, r0, r1, t0, t1, t2, t3, t4, t5) \
+	MULLD  r0, h0, t0;  \
+	MULLD  r0, h1, t4;  \
+	MULHDU r0, h0, t1;  \
+	MULHDU r0, h1, t5;  \
+	ADDC   t4, t1, t1;  \
+	MULLD  r0, h2, t2;  \
+	ADDZE  t5;          \
+	MULHDU r1, h0, t4;  \
+	MULLD  r1, h0, h0;  \
+	ADD    t5, t2, t2;  \
+	ADDC   h0, t1, t1;  \
+	MULLD  h2, r1, t3;  \
+	ADDZE  t4, h0;      \
+	MULHDU r1, h1, t5;  \
+	MULLD  r1, h1, t4;  \
+	ADDC   t4, t2, t2;  \
+	ADDE   t5, t3, t3;  \
+	ADDC   h0, t2, t2;  \
+	MOVD   $-4, t4;     \
+	MOVD   t0, h0;      \
+	MOVD   t1, h1;      \
+	ADDZE  t3;          \
+	ANDCC  $3, t2, h2;  \
+	AND    t2, t4, t0;  \
+	ADDC   t0, h0, h0;  \
+	ADDE   t3, h1, h1;  \
+	SLD    $62, t3, t4; \
+	SRD    $2, t2;      \
+	ADDZE  h2;          \
+	OR     t4, t2, t2;  \
+	SRD    $2, t3;      \
+	ADDC   t2, h0, h0;  \
+	ADDE   t3, h1, h1;  \
+	ADDZE  h2
+
+DATA ·poly1305Mask<>+0x00(SB)/8, $0x0FFFFFFC0FFFFFFF
+DATA ·poly1305Mask<>+0x08(SB)/8, $0x0FFFFFFC0FFFFFFC
+GLOBL ·poly1305Mask<>(SB), RODATA, $16
+
+// func update(state *[7]uint64, msg []byte)
+TEXT ·update(SB), $0-32
+	MOVD state+0(FP), R3
+	MOVD msg_base+8(FP), R4
+	MOVD msg_len+16(FP), R5
+
+	MOVD 0(R3), R8   // h0
+	MOVD 8(R3), R9   // h1
+	MOVD 16(R3), R10 // h2
+	MOVD 24(R3), R11 // r0
+	MOVD 32(R3), R12 // r1
+
+	CMP R5, $16
+	BLT bytes_between_0_and_15
+
+loop:
+	POLY1305_ADD(R4, R8, R9, R10, R20, R21, R22)
+
+multiply:
+	POLY1305_MUL(R8, R9, R10, R11, R12, R16, R17, R18, R14, R20, R21)
+	ADD $-16, R5
+	CMP R5, $16
+	BGE loop
+
+bytes_between_0_and_15:
+	CMP  R5, $0
+	BEQ  done
+	MOVD $0, R16 // h0
+	MOVD $0, R17 // h1
+
+flush_buffer:
+	CMP R5, $8
+	BLE just1
+
+	MOVD $8, R21
+	SUB  R21, R5, R21
+
+	// Greater than 8 -- load the rightmost remaining bytes in msg
+	// and put into R17 (h1)
+	MOVD (R4)(R21), R17
+	MOVD $16, R22
+
+	// Find the offset to those bytes
+	SUB R5, R22, R22
+	SLD $3, R22
+
+	// Shift to get only the bytes in msg
+	SRD R22, R17, R17
+
+	// Put 1 at high end
+	MOVD $1, R23
+	SLD  $3, R21
+	SLD  R21, R23, R23
+	OR   R23, R17, R17
+
+	// Remainder is 8
+	MOVD $8, R5
+
+just1:
+	CMP R5, $8
+	BLT less8
+
+	// Exactly 8
+	MOVD (R4), R16
+
+	CMP R17, $0
+
+	// Check if we've already set R17; if not
+	// set 1 to indicate end of msg.
+	BNE  carry
+	MOVD $1, R17
+	BR   carry
+
+less8:
+	MOVD  $0, R16   // h0
+	MOVD  $0, R22   // shift count
+	CMP   R5, $4
+	BLT   less4
+	MOVWZ (R4), R16
+	ADD   $4, R4
+	ADD   $-4, R5
+	MOVD  $32, R22
+
+less4:
+	CMP   R5, $2
+	BLT   less2
+	MOVHZ (R4), R21
+	SLD   R22, R21, R21
+	OR    R16, R21, R16
+	ADD   $16, R22
+	ADD   $-2, R5
+	ADD   $2, R4
+
+less2:
+	CMP   R5, $0
+	BEQ   insert1
+	MOVBZ (R4), R21
+	SLD   R22, R21, R21
+	OR    R16, R21, R16
+	ADD   $8, R22
+
+insert1:
+	// Insert 1 at end of msg
+	MOVD $1, R21
+	SLD  R22, R21, R21
+	OR   R16, R21, R16
+
+carry:
+	// Add new values to h0, h1, h2
+	ADDC  R16, R8
+	ADDE  R17, R9
+	ADDZE R10, R10
+	MOVD  $16, R5
+	ADD   R5, R4
+	BR    multiply
+
+done:
+	// Save h0, h1, h2 in state
+	MOVD R8, 0(R3)
+	MOVD R9, 8(R3)
+	MOVD R10, 16(R3)
+	RET

pkg/proto/ssh/internal/poly1305/sum_s390x.go

@@ -0,0 +1,76 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build gc && !purego
+
+package poly1305
+
+import (
+	"golang.org/x/sys/cpu"
+)
+
+// updateVX is an assembly implementation of Poly1305 that uses vector
+// instructions. It must only be called if the vector facility (vx) is
+// available.
+//
+//go:noescape
+func updateVX(state *macState, msg []byte)
+
+// mac is a replacement for macGeneric that uses a larger buffer and redirects
+// calls that would have gone to updateGeneric to updateVX if the vector
+// facility is installed.
+//
+// A larger buffer is required for good performance because the vector
+// implementation has a higher fixed cost per call than the generic
+// implementation.
+type mac struct {
+	macState
+
+	buffer [16 * TagSize]byte // size must be a multiple of block size (16)
+	offset int
+}
+
+func (h *mac) Write(p []byte) (int, error) {
+	nn := len(p)
+	if h.offset > 0 {
+		n := copy(h.buffer[h.offset:], p)
+		if h.offset+n < len(h.buffer) {
+			h.offset += n
+			return nn, nil
+		}
+		p = p[n:]
+		h.offset = 0
+		if cpu.S390X.HasVX {
+			updateVX(&h.macState, h.buffer[:])
+		} else {
+			updateGeneric(&h.macState, h.buffer[:])
+		}
+	}
+
+	tail := len(p) % len(h.buffer) // number of bytes to copy into buffer
+	body := len(p) - tail          // number of bytes to process now
+	if body > 0 {
+		if cpu.S390X.HasVX {
+			updateVX(&h.macState, p[:body])
+		} else {
+			updateGeneric(&h.macState, p[:body])
+		}
+	}
+	h.offset = copy(h.buffer[:], p[body:]) // copy tail bytes - can be 0
+	return nn, nil
+}
+
+func (h *mac) Sum(out *[TagSize]byte) {
+	state := h.macState
+	remainder := h.buffer[:h.offset]
+
+	// Use the generic implementation if we have 2 or fewer blocks left
+	// to sum. The vector implementation has a higher startup time.
+	if cpu.S390X.HasVX && len(remainder) > 2*TagSize {
+		updateVX(&state, remainder)
+	} else if len(remainder) > 0 {
+		updateGeneric(&state, remainder)
+	}
+	finalize(out, &state.h, &state.s)
+}

pkg/proto/ssh/internal/poly1305/sum_s390x.s

@@ -0,0 +1,503 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build gc && !purego
+
+#include "textflag.h"
+
+// This implementation of Poly1305 uses the vector facility (vx)
+// to process up to 2 blocks (32 bytes) per iteration using an
+// algorithm based on the one described in:
+//
+// NEON crypto, Daniel J. Bernstein & Peter Schwabe
+// https://cryptojedi.org/papers/neoncrypto-20120320.pdf
+//
+// This algorithm uses 5 26-bit limbs to represent a 130-bit
+// value. These limbs are, for the most part, zero extended and
+// placed into 64-bit vector register elements. Each vector
+// register is 128-bits wide and so holds 2 of these elements.
+// Using 26-bit limbs allows us plenty of headroom to accommodate
+// accumulations before and after multiplication without
+// overflowing either 32-bits (before multiplication) or 64-bits
+// (after multiplication).
+//
+// In order to parallelise the operations required to calculate
+// the sum we use two separate accumulators and then sum those
+// in an extra final step. For compatibility with the generic
+// implementation we perform this summation at the end of every
+// updateVX call.
+//
+// To use two accumulators we must multiply the message blocks
+// by r² rather than r. Only the final message block should be
+// multiplied by r.
+//
+// Example:
+//
+// We want to calculate the sum (h) for a 64 byte message (m):
+//
+//   h = m[0:16]r⁴ + m[16:32]r³ + m[32:48]r² + m[48:64]r
+//
+// To do this we split the calculation into the even indices
+// and odd indices of the message. These form our SIMD 'lanes':
+//
+//   h = m[ 0:16]r⁴ + m[32:48]r² +   <- lane 0
+//       m[16:32]r³ + m[48:64]r      <- lane 1
+//
+// To calculate this iteratively we refactor so that both lanes
+// are written in terms of r² and r:
+//
+//   h = (m[ 0:16]r² + m[32:48])r² + <- lane 0
+//       (m[16:32]r² + m[48:64])r    <- lane 1
+//                ^             ^
+//                |             coefficients for second iteration
+//                coefficients for first iteration
+//
+// So in this case we would have two iterations. In the first
+// both lanes are multiplied by r². In the second only the
+// first lane is multiplied by r² and the second lane is
+// instead multiplied by r. This gives use the odd and even
+// powers of r that we need from the original equation.
+//
+// Notation:
+//
+//   h - accumulator
+//   r - key
+//   m - message
+//
+//   [a, b]       - SIMD register holding two 64-bit values
+//   [a, b, c, d] - SIMD register holding four 32-bit values
+//   xᵢ[n]        - limb n of variable x with bit width i
+//
+// Limbs are expressed in little endian order, so for 26-bit
+// limbs x₂₆[4] will be the most significant limb and x₂₆[0]
+// will be the least significant limb.
+
+// masking constants
+#define MOD24 V0 // [0x0000000000ffffff, 0x0000000000ffffff] - mask low 24-bits
+#define MOD26 V1 // [0x0000000003ffffff, 0x0000000003ffffff] - mask low 26-bits
+
+// expansion constants (see EXPAND macro)
+#define EX0 V2
+#define EX1 V3
+#define EX2 V4
+
+// key (r², r or 1 depending on context)
+#define R_0 V5
+#define R_1 V6
+#define R_2 V7
+#define R_3 V8
+#define R_4 V9
+
+// precalculated coefficients (5r², 5r or 0 depending on context)
+#define R5_1 V10
+#define R5_2 V11
+#define R5_3 V12
+#define R5_4 V13
+
+// message block (m)
+#define M_0 V14
+#define M_1 V15
+#define M_2 V16
+#define M_3 V17
+#define M_4 V18
+
+// accumulator (h)
+#define H_0 V19
+#define H_1 V20
+#define H_2 V21
+#define H_3 V22
+#define H_4 V23
+
+// temporary registers (for short-lived values)
+#define T_0 V24
+#define T_1 V25
+#define T_2 V26
+#define T_3 V27
+#define T_4 V28
+
+GLOBL ·constants<>(SB), RODATA, $0x30
+// EX0
+DATA ·constants<>+0x00(SB)/8, $0x0006050403020100
+DATA ·constants<>+0x08(SB)/8, $0x1016151413121110
+// EX1
+DATA ·constants<>+0x10(SB)/8, $0x060c0b0a09080706
+DATA ·constants<>+0x18(SB)/8, $0x161c1b1a19181716
+// EX2
+DATA ·constants<>+0x20(SB)/8, $0x0d0d0d0d0d0f0e0d
+DATA ·constants<>+0x28(SB)/8, $0x1d1d1d1d1d1f1e1d
+
+// MULTIPLY multiplies each lane of f and g, partially reduced
+// modulo 2¹³⁰ - 5. The result, h, consists of partial products
+// in each lane that need to be reduced further to produce the
+// final result.
+//
+//   h₁₃₀ = (f₁₃₀g₁₃₀) % 2¹³⁰ + (5f₁₃₀g₁₃₀) / 2¹³⁰
+//
+// Note that the multiplication by 5 of the high bits is
+// achieved by precalculating the multiplication of four of the
+// g coefficients by 5. These are g51-g54.
+#define MULTIPLY(f0, f1, f2, f3, f4, g0, g1, g2, g3, g4, g51, g52, g53, g54, h0, h1, h2, h3, h4) \
+	VMLOF  f0, g0, h0        \
+	VMLOF  f0, g3, h3        \
+	VMLOF  f0, g1, h1        \
+	VMLOF  f0, g4, h4        \
+	VMLOF  f0, g2, h2        \
+	VMLOF  f1, g54, T_0      \
+	VMLOF  f1, g2, T_3       \
+	VMLOF  f1, g0, T_1       \
+	VMLOF  f1, g3, T_4       \
+	VMLOF  f1, g1, T_2       \
+	VMALOF f2, g53, h0, h0   \
+	VMALOF f2, g1, h3, h3    \
+	VMALOF f2, g54, h1, h1   \
+	VMALOF f2, g2, h4, h4    \
+	VMALOF f2, g0, h2, h2    \
+	VMALOF f3, g52, T_0, T_0 \
+	VMALOF f3, g0, T_3, T_3  \
+	VMALOF f3, g53, T_1, T_1 \
+	VMALOF f3, g1, T_4, T_4  \
+	VMALOF f3, g54, T_2, T_2 \
+	VMALOF f4, g51, h0, h0   \
+	VMALOF f4, g54, h3, h3   \
+	VMALOF f4, g52, h1, h1   \
+	VMALOF f4, g0, h4, h4    \
+	VMALOF f4, g53, h2, h2   \
+	VAG    T_0, h0, h0       \
+	VAG    T_3, h3, h3       \
+	VAG    T_1, h1, h1       \
+	VAG    T_4, h4, h4       \
+	VAG    T_2, h2, h2
+
+// REDUCE performs the following carry operations in four
+// stages, as specified in Bernstein & Schwabe:
+//
+//   1: h₂₆[0]->h₂₆[1] h₂₆[3]->h₂₆[4]
+//   2: h₂₆[1]->h₂₆[2] h₂₆[4]->h₂₆[0]
+//   3: h₂₆[0]->h₂₆[1] h₂₆[2]->h₂₆[3]
+//   4: h₂₆[3]->h₂₆[4]
+//
+// The result is that all of the limbs are limited to 26-bits
+// except for h₂₆[1] and h₂₆[4] which are limited to 27-bits.
+//
+// Note that although each limb is aligned at 26-bit intervals
+// they may contain values that exceed 2²⁶ - 1, hence the need
+// to carry the excess bits in each limb.
+#define REDUCE(h0, h1, h2, h3, h4) \
+	VESRLG $26, h0, T_0  \
+	VESRLG $26, h3, T_1  \
+	VN     MOD26, h0, h0 \
+	VN     MOD26, h3, h3 \
+	VAG    T_0, h1, h1   \
+	VAG    T_1, h4, h4   \
+	VESRLG $26, h1, T_2  \
+	VESRLG $26, h4, T_3  \
+	VN     MOD26, h1, h1 \
+	VN     MOD26, h4, h4 \
+	VESLG  $2, T_3, T_4  \
+	VAG    T_3, T_4, T_4 \
+	VAG    T_2, h2, h2   \
+	VAG    T_4, h0, h0   \
+	VESRLG $26, h2, T_0  \
+	VESRLG $26, h0, T_1  \
+	VN     MOD26, h2, h2 \
+	VN     MOD26, h0, h0 \
+	VAG    T_0, h3, h3   \
+	VAG    T_1, h1, h1   \
+	VESRLG $26, h3, T_2  \
+	VN     MOD26, h3, h3 \
+	VAG    T_2, h4, h4
+
+// EXPAND splits the 128-bit little-endian values in0 and in1
+// into 26-bit big-endian limbs and places the results into
+// the first and second lane of d₂₆[0:4] respectively.
+//
+// The EX0, EX1 and EX2 constants are arrays of byte indices
+// for permutation. The permutation both reverses the bytes
+// in the input and ensures the bytes are copied into the
+// destination limb ready to be shifted into their final
+// position.
+#define EXPAND(in0, in1, d0, d1, d2, d3, d4) \
+	VPERM  in0, in1, EX0, d0 \
+	VPERM  in0, in1, EX1, d2 \
+	VPERM  in0, in1, EX2, d4 \
+	VESRLG $26, d0, d1       \
+	VESRLG $30, d2, d3       \
+	VESRLG $4, d2, d2        \
+	VN     MOD26, d0, d0     \ // [in0₂₆[0], in1₂₆[0]]
+	VN     MOD26, d3, d3     \ // [in0₂₆[3], in1₂₆[3]]
+	VN     MOD26, d1, d1     \ // [in0₂₆[1], in1₂₆[1]]
+	VN     MOD24, d4, d4     \ // [in0₂₆[4], in1₂₆[4]]
+	VN     MOD26, d2, d2     // [in0₂₆[2], in1₂₆[2]]
+
+// func updateVX(state *macState, msg []byte)
+TEXT ·updateVX(SB), NOSPLIT, $0
+	MOVD state+0(FP), R1
+	LMG  msg+8(FP), R2, R3 // R2=msg_base, R3=msg_len
+
+	// load EX0, EX1 and EX2
+	MOVD $·constants<>(SB), R5
+	VLM  (R5), EX0, EX2
+
+	// generate masks
+	VGMG $(64-24), $63, MOD24 // [0x00ffffff, 0x00ffffff]
+	VGMG $(64-26), $63, MOD26 // [0x03ffffff, 0x03ffffff]
+
+	// load h (accumulator) and r (key) from state
+	VZERO T_1               // [0, 0]
+	VL    0(R1), T_0        // [h₆₄[0], h₆₄[1]]
+	VLEG  $0, 16(R1), T_1   // [h₆₄[2], 0]
+	VL    24(R1), T_2       // [r₆₄[0], r₆₄[1]]
+	VPDI  $0, T_0, T_2, T_3 // [h₆₄[0], r₆₄[0]]
+	VPDI  $5, T_0, T_2, T_4 // [h₆₄[1], r₆₄[1]]
+
+	// unpack h and r into 26-bit limbs
+	// note: h₆₄[2] may have the low 3 bits set, so h₂₆[4] is a 27-bit value
+	VN     MOD26, T_3, H_0            // [h₂₆[0], r₂₆[0]]
+	VZERO  H_1                        // [0, 0]
+	VZERO  H_3                        // [0, 0]
+	VGMG   $(64-12-14), $(63-12), T_0 // [0x03fff000, 0x03fff000] - 26-bit mask with low 12 bits masked out
+	VESLG  $24, T_1, T_1              // [h₆₄[2]<<24, 0]
+	VERIMG $-26&63, T_3, MOD26, H_1   // [h₂₆[1], r₂₆[1]]
+	VESRLG $+52&63, T_3, H_2          // [h₂₆[2], r₂₆[2]] - low 12 bits only
+	VERIMG $-14&63, T_4, MOD26, H_3   // [h₂₆[1], r₂₆[1]]
+	VESRLG $40, T_4, H_4              // [h₂₆[4], r₂₆[4]] - low 24 bits only
+	VERIMG $+12&63, T_4, T_0, H_2     // [h₂₆[2], r₂₆[2]] - complete
+	VO     T_1, H_4, H_4              // [h₂₆[4], r₂₆[4]] - complete
+
+	// replicate r across all 4 vector elements
+	VREPF $3, H_0, R_0 // [r₂₆[0], r₂₆[0], r₂₆[0], r₂₆[0]]
+	VREPF $3, H_1, R_1 // [r₂₆[1], r₂₆[1], r₂₆[1], r₂₆[1]]
+	VREPF $3, H_2, R_2 // [r₂₆[2], r₂₆[2], r₂₆[2], r₂₆[2]]
+	VREPF $3, H_3, R_3 // [r₂₆[3], r₂₆[3], r₂₆[3], r₂₆[3]]
+	VREPF $3, H_4, R_4 // [r₂₆[4], r₂₆[4], r₂₆[4], r₂₆[4]]
+
+	// zero out lane 1 of h
+	VLEIG $1, $0, H_0 // [h₂₆[0], 0]
+	VLEIG $1, $0, H_1 // [h₂₆[1], 0]
+	VLEIG $1, $0, H_2 // [h₂₆[2], 0]
+	VLEIG $1, $0, H_3 // [h₂₆[3], 0]
+	VLEIG $1, $0, H_4 // [h₂₆[4], 0]
+
+	// calculate 5r (ignore least significant limb)
+	VREPIF $5, T_0
+	VMLF   T_0, R_1, R5_1 // [5r₂₆[1], 5r₂₆[1], 5r₂₆[1], 5r₂₆[1]]
+	VMLF   T_0, R_2, R5_2 // [5r₂₆[2], 5r₂₆[2], 5r₂₆[2], 5r₂₆[2]]
+	VMLF   T_0, R_3, R5_3 // [5r₂₆[3], 5r₂₆[3], 5r₂₆[3], 5r₂₆[3]]
+	VMLF   T_0, R_4, R5_4 // [5r₂₆[4], 5r₂₆[4], 5r₂₆[4], 5r₂₆[4]]
+
+	// skip r² calculation if we are only calculating one block
+	CMPBLE R3, $16, skip
+
+	// calculate r²
+	MULTIPLY(R_0, R_1, R_2, R_3, R_4, R_0, R_1, R_2, R_3, R_4, R5_1, R5_2, R5_3, R5_4, M_0, M_1, M_2, M_3, M_4)
+	REDUCE(M_0, M_1, M_2, M_3, M_4)
+	VGBM   $0x0f0f, T_0
+	VERIMG $0, M_0, T_0, R_0 // [r₂₆[0], r²₂₆[0], r₂₆[0], r²₂₆[0]]
+	VERIMG $0, M_1, T_0, R_1 // [r₂₆[1], r²₂₆[1], r₂₆[1], r²₂₆[1]]
+	VERIMG $0, M_2, T_0, R_2 // [r₂₆[2], r²₂₆[2], r₂₆[2], r²₂₆[2]]
+	VERIMG $0, M_3, T_0, R_3 // [r₂₆[3], r²₂₆[3], r₂₆[3], r²₂₆[3]]
+	VERIMG $0, M_4, T_0, R_4 // [r₂₆[4], r²₂₆[4], r₂₆[4], r²₂₆[4]]
+
+	// calculate 5r² (ignore least significant limb)
+	VREPIF $5, T_0
+	VMLF   T_0, R_1, R5_1 // [5r₂₆[1], 5r²₂₆[1], 5r₂₆[1], 5r²₂₆[1]]
+	VMLF   T_0, R_2, R5_2 // [5r₂₆[2], 5r²₂₆[2], 5r₂₆[2], 5r²₂₆[2]]
+	VMLF   T_0, R_3, R5_3 // [5r₂₆[3], 5r²₂₆[3], 5r₂₆[3], 5r²₂₆[3]]
+	VMLF   T_0, R_4, R5_4 // [5r₂₆[4], 5r²₂₆[4], 5r₂₆[4], 5r²₂₆[4]]
+
+loop:
+	CMPBLE R3, $32, b2 // 2 or fewer blocks remaining, need to change key coefficients
+
+	// load next 2 blocks from message
+	VLM (R2), T_0, T_1
+
+	// update message slice
+	SUB  $32, R3
+	MOVD $32(R2), R2
+
+	// unpack message blocks into 26-bit big-endian limbs
+	EXPAND(T_0, T_1, M_0, M_1, M_2, M_3, M_4)
+
+	// add 2¹²⁸ to each message block value
+	VLEIB $4, $1, M_4
+	VLEIB $12, $1, M_4
+
+multiply:
+	// accumulate the incoming message
+	VAG H_0, M_0, M_0
+	VAG H_3, M_3, M_3
+	VAG H_1, M_1, M_1
+	VAG H_4, M_4, M_4
+	VAG H_2, M_2, M_2
+
+	// multiply the accumulator by the key coefficient
+	MULTIPLY(M_0, M_1, M_2, M_3, M_4, R_0, R_1, R_2, R_3, R_4, R5_1, R5_2, R5_3, R5_4, H_0, H_1, H_2, H_3, H_4)
+
+	// carry and partially reduce the partial products
+	REDUCE(H_0, H_1, H_2, H_3, H_4)
+
+	CMPBNE R3, $0, loop
+
+finish:
+	// sum lane 0 and lane 1 and put the result in lane 1
+	VZERO  T_0
+	VSUMQG H_0, T_0, H_0
+	VSUMQG H_3, T_0, H_3
+	VSUMQG H_1, T_0, H_1
+	VSUMQG H_4, T_0, H_4
+	VSUMQG H_2, T_0, H_2
+
+	// reduce again after summation
+	// TODO(mundaym): there might be a more efficient way to do this
+	// now that we only have 1 active lane. For example, we could
+	// simultaneously pack the values as we reduce them.
+	REDUCE(H_0, H_1, H_2, H_3, H_4)
+
+	// carry h[1] through to h[4] so that only h[4] can exceed 2²⁶ - 1
+	// TODO(mundaym): in testing this final carry was unnecessary.
+	// Needs a proof before it can be removed though.
+	VESRLG $26, H_1, T_1
+	VN     MOD26, H_1, H_1
+	VAQ    T_1, H_2, H_2
+	VESRLG $26, H_2, T_2
+	VN     MOD26, H_2, H_2
+	VAQ    T_2, H_3, H_3
+	VESRLG $26, H_3, T_3
+	VN     MOD26, H_3, H_3
+	VAQ    T_3, H_4, H_4
+
+	// h is now < 2(2¹³⁰ - 5)
+	// Pack each lane in h₂₆[0:4] into h₁₂₈[0:1].
+	VESLG $26, H_1, H_1
+	VESLG $26, H_3, H_3
+	VO    H_0, H_1, H_0
+	VO    H_2, H_3, H_2
+	VESLG $4, H_2, H_2
+	VLEIB $7, $48, H_1
+	VSLB  H_1, H_2, H_2
+	VO    H_0, H_2, H_0
+	VLEIB $7, $104, H_1
+	VSLB  H_1, H_4, H_3
+	VO    H_3, H_0, H_0
+	VLEIB $7, $24, H_1
+	VSRLB H_1, H_4, H_1
+
+	// update state
+	VSTEG $1, H_0, 0(R1)
+	VSTEG $0, H_0, 8(R1)
+	VSTEG $1, H_1, 16(R1)
+	RET
+
+b2:  // 2 or fewer blocks remaining
+	CMPBLE R3, $16, b1
+
+	// Load the 2 remaining blocks (17-32 bytes remaining).
+	MOVD $-17(R3), R0    // index of final byte to load modulo 16
+	VL   (R2), T_0       // load full 16 byte block
+	VLL  R0, 16(R2), T_1 // load final (possibly partial) block and pad with zeros to 16 bytes
+
+	// The Poly1305 algorithm requires that a 1 bit be appended to
+	// each message block. If the final block is less than 16 bytes
+	// long then it is easiest to insert the 1 before the message
+	// block is split into 26-bit limbs. If, on the other hand, the
+	// final message block is 16 bytes long then we append the 1 bit
+	// after expansion as normal.
+	MOVBZ  $1, R0
+	MOVD   $-16(R3), R3   // index of byte in last block to insert 1 at (could be 16)
+	CMPBEQ R3, $16, 2(PC) // skip the insertion if the final block is 16 bytes long
+	VLVGB  R3, R0, T_1    // insert 1 into the byte at index R3
+
+	// Split both blocks into 26-bit limbs in the appropriate lanes.
+	EXPAND(T_0, T_1, M_0, M_1, M_2, M_3, M_4)
+
+	// Append a 1 byte to the end of the second to last block.
+	VLEIB $4, $1, M_4
+
+	// Append a 1 byte to the end of the last block only if it is a
+	// full 16 byte block.
+	CMPBNE R3, $16, 2(PC)
+	VLEIB  $12, $1, M_4
+
+	// Finally, set up the coefficients for the final multiplication.
+	// We have previously saved r and 5r in the 32-bit even indexes
+	// of the R_[0-4] and R5_[1-4] coefficient registers.
+	//
+	// We want lane 0 to be multiplied by r² so that can be kept the
+	// same. We want lane 1 to be multiplied by r so we need to move
+	// the saved r value into the 32-bit odd index in lane 1 by
+	// rotating the 64-bit lane by 32.
+	VGBM   $0x00ff, T_0         // [0, 0xffffffffffffffff] - mask lane 1 only
+	VERIMG $32, R_0, T_0, R_0   // [_,  r²₂₆[0], _,  r₂₆[0]]
+	VERIMG $32, R_1, T_0, R_1   // [_,  r²₂₆[1], _,  r₂₆[1]]
+	VERIMG $32, R_2, T_0, R_2   // [_,  r²₂₆[2], _,  r₂₆[2]]
+	VERIMG $32, R_3, T_0, R_3   // [_,  r²₂₆[3], _,  r₂₆[3]]
+	VERIMG $32, R_4, T_0, R_4   // [_,  r²₂₆[4], _,  r₂₆[4]]
+	VERIMG $32, R5_1, T_0, R5_1 // [_, 5r²₂₆[1], _, 5r₂₆[1]]
+	VERIMG $32, R5_2, T_0, R5_2 // [_, 5r²₂₆[2], _, 5r₂₆[2]]
+	VERIMG $32, R5_3, T_0, R5_3 // [_, 5r²₂₆[3], _, 5r₂₆[3]]
+	VERIMG $32, R5_4, T_0, R5_4 // [_, 5r²₂₆[4], _, 5r₂₆[4]]
+
+	MOVD $0, R3
+	BR   multiply
+
+skip:
+	CMPBEQ R3, $0, finish
+
+b1:  // 1 block remaining
+
+	// Load the final block (1-16 bytes). This will be placed into
+	// lane 0.
+	MOVD $-1(R3), R0
+	VLL  R0, (R2), T_0 // pad to 16 bytes with zeros
+
+	// The Poly1305 algorithm requires that a 1 bit be appended to
+	// each message block. If the final block is less than 16 bytes
+	// long then it is easiest to insert the 1 before the message
+	// block is split into 26-bit limbs. If, on the other hand, the
+	// final message block is 16 bytes long then we append the 1 bit
+	// after expansion as normal.
+	MOVBZ  $1, R0
+	CMPBEQ R3, $16, 2(PC)
+	VLVGB  R3, R0, T_0
+
+	// Set the message block in lane 1 to the value 0 so that it
+	// can be accumulated without affecting the final result.
+	VZERO T_1
+
+	// Split the final message block into 26-bit limbs in lane 0.
+	// Lane 1 will be contain 0.
+	EXPAND(T_0, T_1, M_0, M_1, M_2, M_3, M_4)
+
+	// Append a 1 byte to the end of the last block only if it is a
+	// full 16 byte block.
+	CMPBNE R3, $16, 2(PC)
+	VLEIB  $4, $1, M_4
+
+	// We have previously saved r and 5r in the 32-bit even indexes
+	// of the R_[0-4] and R5_[1-4] coefficient registers.
+	//
+	// We want lane 0 to be multiplied by r so we need to move the
+	// saved r value into the 32-bit odd index in lane 0. We want
+	// lane 1 to be set to the value 1. This makes multiplication
+	// a no-op. We do this by setting lane 1 in every register to 0
+	// and then just setting the 32-bit index 3 in R_0 to 1.
+	VZERO T_0
+	MOVD  $0, R0
+	MOVD  $0x10111213, R12
+	VLVGP R12, R0, T_1         // [_, 0x10111213, _, 0x00000000]
+	VPERM T_0, R_0, T_1, R_0   // [_,  r₂₆[0], _, 0]
+	VPERM T_0, R_1, T_1, R_1   // [_,  r₂₆[1], _, 0]
+	VPERM T_0, R_2, T_1, R_2   // [_,  r₂₆[2], _, 0]
+	VPERM T_0, R_3, T_1, R_3   // [_,  r₂₆[3], _, 0]
+	VPERM T_0, R_4, T_1, R_4   // [_,  r₂₆[4], _, 0]
+	VPERM T_0, R5_1, T_1, R5_1 // [_, 5r₂₆[1], _, 0]
+	VPERM T_0, R5_2, T_1, R5_2 // [_, 5r₂₆[2], _, 0]
+	VPERM T_0, R5_3, T_1, R5_3 // [_, 5r₂₆[3], _, 0]
+	VPERM T_0, R5_4, T_1, R5_4 // [_, 5r₂₆[4], _, 0]
+
+	// Set the value of lane 1 to be 1.
+	VLEIF $3, $1, R_0 // [_,  r₂₆[0], _, 1]
+
+	MOVD $0, R3
+	BR   multiply

pkg/proto/ssh/kex.go

@@ -0,0 +1,786 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/subtle"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"math/big"
+
+	"golang.org/x/crypto/curve25519"
+)
+
+const (
+	kexAlgoDH1SHA1                = "diffie-hellman-group1-sha1"
+	kexAlgoDH14SHA1               = "diffie-hellman-group14-sha1"
+	kexAlgoDH14SHA256             = "diffie-hellman-group14-sha256"
+	kexAlgoDH16SHA512             = "diffie-hellman-group16-sha512"
+	kexAlgoECDH256                = "ecdh-sha2-nistp256"
+	kexAlgoECDH384                = "ecdh-sha2-nistp384"
+	kexAlgoECDH521                = "ecdh-sha2-nistp521"
+	kexAlgoCurve25519SHA256LibSSH = "curve25519-sha256@libssh.org"
+	kexAlgoCurve25519SHA256       = "curve25519-sha256"
+
+	// For the following kex only the client half contains a production
+	// ready implementation. The server half only consists of a minimal
+	// implementation to satisfy the automated tests.
+	kexAlgoDHGEXSHA1   = "diffie-hellman-group-exchange-sha1"
+	kexAlgoDHGEXSHA256 = "diffie-hellman-group-exchange-sha256"
+)
+
+// kexResult captures the outcome of a key exchange.
+type kexResult struct {
+	// Session hash. See also RFC 4253, section 8.
+	H []byte
+
+	// Shared secret. See also RFC 4253, section 8.
+	K []byte
+
+	// Host key as hashed into H.
+	HostKey []byte
+
+	// Signature of H.
+	Signature []byte
+
+	// A cryptographic hash function that matches the security
+	// level of the key exchange algorithm. It is used for
+	// calculating H, and for deriving keys from H and K.
+	Hash crypto.Hash
+
+	// The session ID, which is the first H computed. This is used
+	// to derive key material inside the transport.
+	SessionID []byte
+}
+
+// handshakeMagics contains data that is always included in the
+// session hash.
+type handshakeMagics struct {
+	clientVersion, serverVersion []byte
+	clientKexInit, serverKexInit []byte
+}
+
+func (m *handshakeMagics) write(w io.Writer) {
+	writeString(w, m.clientVersion)
+	writeString(w, m.serverVersion)
+	writeString(w, m.clientKexInit)
+	writeString(w, m.serverKexInit)
+}
+
+// kexAlgorithm abstracts different key exchange algorithms.
+type kexAlgorithm interface {
+	// Server runs server-side key agreement, signing the result
+	// with a hostkey. algo is the negotiated algorithm, and may
+	// be a certificate type.
+	Server(p packetConn, rand io.Reader, magics *handshakeMagics, s AlgorithmSigner, algo string) (*kexResult, error)
+
+	// Client runs the client-side key agreement. Caller is
+	// responsible for verifying the host key signature.
+	Client(p packetConn, rand io.Reader, magics *handshakeMagics) (*kexResult, error)
+}
+
+// dhGroup is a multiplicative group suitable for implementing Diffie-Hellman key agreement.
+type dhGroup struct {
+	g, p, pMinus1 *big.Int
+	hashFunc      crypto.Hash
+}
+
+func (group *dhGroup) diffieHellman(theirPublic, myPrivate *big.Int) (*big.Int, error) {
+	if theirPublic.Cmp(bigOne) <= 0 || theirPublic.Cmp(group.pMinus1) >= 0 {
+		return nil, errors.New("ssh: DH parameter out of bounds")
+	}
+	return new(big.Int).Exp(theirPublic, myPrivate, group.p), nil
+}
+
+func (group *dhGroup) Client(c packetConn, randSource io.Reader, magics *handshakeMagics) (*kexResult, error) {
+	var x *big.Int
+	for {
+		var err error
+		if x, err = rand.Int(randSource, group.pMinus1); err != nil {
+			return nil, err
+		}
+		if x.Sign() > 0 {
+			break
+		}
+	}
+
+	X := new(big.Int).Exp(group.g, x, group.p)
+	kexDHInit := kexDHInitMsg{
+		X: X,
+	}
+	if err := c.writePacket(Marshal(&kexDHInit)); err != nil {
+		return nil, err
+	}
+
+	packet, err := c.readPacket()
+	if err != nil {
+		return nil, err
+	}
+
+	var kexDHReply kexDHReplyMsg
+	if err = Unmarshal(packet, &kexDHReply); err != nil {
+		return nil, err
+	}
+
+	ki, err := group.diffieHellman(kexDHReply.Y, x)
+	if err != nil {
+		return nil, err
+	}
+
+	h := group.hashFunc.New()
+	magics.write(h)
+	writeString(h, kexDHReply.HostKey)
+	writeInt(h, X)
+	writeInt(h, kexDHReply.Y)
+	K := make([]byte, intLength(ki))
+	marshalInt(K, ki)
+	h.Write(K)
+
+	return &kexResult{
+		H:         h.Sum(nil),
+		K:         K,
+		HostKey:   kexDHReply.HostKey,
+		Signature: kexDHReply.Signature,
+		Hash:      group.hashFunc,
+	}, nil
+}
+
+func (group *dhGroup) Server(c packetConn, randSource io.Reader, magics *handshakeMagics, priv AlgorithmSigner, algo string) (result *kexResult, err error) {
+	packet, err := c.readPacket()
+	if err != nil {
+		return
+	}
+	var kexDHInit kexDHInitMsg
+	if err = Unmarshal(packet, &kexDHInit); err != nil {
+		return
+	}
+
+	var y *big.Int
+	for {
+		if y, err = rand.Int(randSource, group.pMinus1); err != nil {
+			return
+		}
+		if y.Sign() > 0 {
+			break
+		}
+	}
+
+	Y := new(big.Int).Exp(group.g, y, group.p)
+	ki, err := group.diffieHellman(kexDHInit.X, y)
+	if err != nil {
+		return nil, err
+	}
+
+	hostKeyBytes := priv.PublicKey().Marshal()
+
+	h := group.hashFunc.New()
+	magics.write(h)
+	writeString(h, hostKeyBytes)
+	writeInt(h, kexDHInit.X)
+	writeInt(h, Y)
+
+	K := make([]byte, intLength(ki))
+	marshalInt(K, ki)
+	h.Write(K)
+
+	H := h.Sum(nil)
+
+	// H is already a hash, but the hostkey signing will apply its
+	// own key-specific hash algorithm.
+	sig, err := signAndMarshal(priv, randSource, H, algo)
+	if err != nil {
+		return nil, err
+	}
+
+	kexDHReply := kexDHReplyMsg{
+		HostKey:   hostKeyBytes,
+		Y:         Y,
+		Signature: sig,
+	}
+	packet = Marshal(&kexDHReply)
+
+	err = c.writePacket(packet)
+	return &kexResult{
+		H:         H,
+		K:         K,
+		HostKey:   hostKeyBytes,
+		Signature: sig,
+		Hash:      group.hashFunc,
+	}, err
+}
+
+// ecdh performs Elliptic Curve Diffie-Hellman key exchange as
+// described in RFC 5656, section 4.
+type ecdh struct {
+	curve elliptic.Curve
+}
+
+func (kex *ecdh) Client(c packetConn, rand io.Reader, magics *handshakeMagics) (*kexResult, error) {
+	ephKey, err := ecdsa.GenerateKey(kex.curve, rand)
+	if err != nil {
+		return nil, err
+	}
+
+	kexInit := kexECDHInitMsg{
+		ClientPubKey: elliptic.Marshal(kex.curve, ephKey.PublicKey.X, ephKey.PublicKey.Y),
+	}
+
+	serialized := Marshal(&kexInit)
+	if err := c.writePacket(serialized); err != nil {
+		return nil, err
+	}
+
+	packet, err := c.readPacket()
+	if err != nil {
+		return nil, err
+	}
+
+	var reply kexECDHReplyMsg
+	if err = Unmarshal(packet, &reply); err != nil {
+		return nil, err
+	}
+
+	x, y, err := unmarshalECKey(kex.curve, reply.EphemeralPubKey)
+	if err != nil {
+		return nil, err
+	}
+
+	// generate shared secret
+	secret, _ := kex.curve.ScalarMult(x, y, ephKey.D.Bytes())
+
+	h := ecHash(kex.curve).New()
+	magics.write(h)
+	writeString(h, reply.HostKey)
+	writeString(h, kexInit.ClientPubKey)
+	writeString(h, reply.EphemeralPubKey)
+	K := make([]byte, intLength(secret))
+	marshalInt(K, secret)
+	h.Write(K)
+
+	return &kexResult{
+		H:         h.Sum(nil),
+		K:         K,
+		HostKey:   reply.HostKey,
+		Signature: reply.Signature,
+		Hash:      ecHash(kex.curve),
+	}, nil
+}
+
+// unmarshalECKey parses and checks an EC key.
+func unmarshalECKey(curve elliptic.Curve, pubkey []byte) (x, y *big.Int, err error) {
+	x, y = elliptic.Unmarshal(curve, pubkey)
+	if x == nil {
+		return nil, nil, errors.New("ssh: elliptic.Unmarshal failure")
+	}
+	if !validateECPublicKey(curve, x, y) {
+		return nil, nil, errors.New("ssh: public key not on curve")
+	}
+	return x, y, nil
+}
+
+// validateECPublicKey checks that the point is a valid public key for
+// the given curve. See [SEC1], 3.2.2
+func validateECPublicKey(curve elliptic.Curve, x, y *big.Int) bool {
+	if x.Sign() == 0 && y.Sign() == 0 {
+		return false
+	}
+
+	if x.Cmp(curve.Params().P) >= 0 {
+		return false
+	}
+
+	if y.Cmp(curve.Params().P) >= 0 {
+		return false
+	}
+
+	if !curve.IsOnCurve(x, y) {
+		return false
+	}
+
+	// We don't check if N * PubKey == 0, since
+	//
+	// - the NIST curves have cofactor = 1, so this is implicit.
+	// (We don't foresee an implementation that supports non NIST
+	// curves)
+	//
+	// - for ephemeral keys, we don't need to worry about small
+	// subgroup attacks.
+	return true
+}
+
+func (kex *ecdh) Server(c packetConn, rand io.Reader, magics *handshakeMagics, priv AlgorithmSigner, algo string) (result *kexResult, err error) {
+	packet, err := c.readPacket()
+	if err != nil {
+		return nil, err
+	}
+
+	var kexECDHInit kexECDHInitMsg
+	if err = Unmarshal(packet, &kexECDHInit); err != nil {
+		return nil, err
+	}
+
+	clientX, clientY, err := unmarshalECKey(kex.curve, kexECDHInit.ClientPubKey)
+	if err != nil {
+		return nil, err
+	}
+
+	// We could cache this key across multiple users/multiple
+	// connection attempts, but the benefit is small. OpenSSH
+	// generates a new key for each incoming connection.
+	ephKey, err := ecdsa.GenerateKey(kex.curve, rand)
+	if err != nil {
+		return nil, err
+	}
+
+	hostKeyBytes := priv.PublicKey().Marshal()
+
+	serializedEphKey := elliptic.Marshal(kex.curve, ephKey.PublicKey.X, ephKey.PublicKey.Y)
+
+	// generate shared secret
+	secret, _ := kex.curve.ScalarMult(clientX, clientY, ephKey.D.Bytes())
+
+	h := ecHash(kex.curve).New()
+	magics.write(h)
+	writeString(h, hostKeyBytes)
+	writeString(h, kexECDHInit.ClientPubKey)
+	writeString(h, serializedEphKey)
+
+	K := make([]byte, intLength(secret))
+	marshalInt(K, secret)
+	h.Write(K)
+
+	H := h.Sum(nil)
+
+	// H is already a hash, but the hostkey signing will apply its
+	// own key-specific hash algorithm.
+	sig, err := signAndMarshal(priv, rand, H, algo)
+	if err != nil {
+		return nil, err
+	}
+
+	reply := kexECDHReplyMsg{
+		EphemeralPubKey: serializedEphKey,
+		HostKey:         hostKeyBytes,
+		Signature:       sig,
+	}
+
+	serialized := Marshal(&reply)
+	if err := c.writePacket(serialized); err != nil {
+		return nil, err
+	}
+
+	return &kexResult{
+		H:         H,
+		K:         K,
+		HostKey:   reply.HostKey,
+		Signature: sig,
+		Hash:      ecHash(kex.curve),
+	}, nil
+}
+
+// ecHash returns the hash to match the given elliptic curve, see RFC
+// 5656, section 6.2.1
+func ecHash(curve elliptic.Curve) crypto.Hash {
+	bitSize := curve.Params().BitSize
+	switch {
+	case bitSize <= 256:
+		return crypto.SHA256
+	case bitSize <= 384:
+		return crypto.SHA384
+	}
+	return crypto.SHA512
+}
+
+var kexAlgoMap = map[string]kexAlgorithm{}
+
+func init() {
+	// This is the group called diffie-hellman-group1-sha1 in
+	// RFC 4253 and Oakley Group 2 in RFC 2409.
+	p, _ := new(big.Int).SetString("FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFF", 16)
+	kexAlgoMap[kexAlgoDH1SHA1] = &dhGroup{
+		g:        new(big.Int).SetInt64(2),
+		p:        p,
+		pMinus1:  new(big.Int).Sub(p, bigOne),
+		hashFunc: crypto.SHA1,
+	}
+
+	// This are the groups called diffie-hellman-group14-sha1 and
+	// diffie-hellman-group14-sha256 in RFC 4253 and RFC 8268,
+	// and Oakley Group 14 in RFC 3526.
+	p, _ = new(big.Int).SetString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
+	group14 := &dhGroup{
+		g:       new(big.Int).SetInt64(2),
+		p:       p,
+		pMinus1: new(big.Int).Sub(p, bigOne),
+	}
+
+	kexAlgoMap[kexAlgoDH14SHA1] = &dhGroup{
+		g: group14.g, p: group14.p, pMinus1: group14.pMinus1,
+		hashFunc: crypto.SHA1,
+	}
+	kexAlgoMap[kexAlgoDH14SHA256] = &dhGroup{
+		g: group14.g, p: group14.p, pMinus1: group14.pMinus1,
+		hashFunc: crypto.SHA256,
+	}
+
+	// This is the group called diffie-hellman-group16-sha512 in RFC
+	// 8268 and Oakley Group 16 in RFC 3526.
+	p, _ = new(big.Int).SetString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
+
+	kexAlgoMap[kexAlgoDH16SHA512] = &dhGroup{
+		g:        new(big.Int).SetInt64(2),
+		p:        p,
+		pMinus1:  new(big.Int).Sub(p, bigOne),
+		hashFunc: crypto.SHA512,
+	}
+
+	kexAlgoMap[kexAlgoECDH521] = &ecdh{elliptic.P521()}
+	kexAlgoMap[kexAlgoECDH384] = &ecdh{elliptic.P384()}
+	kexAlgoMap[kexAlgoECDH256] = &ecdh{elliptic.P256()}
+	kexAlgoMap[kexAlgoCurve25519SHA256] = &curve25519sha256{}
+	kexAlgoMap[kexAlgoCurve25519SHA256LibSSH] = &curve25519sha256{}
+	kexAlgoMap[kexAlgoDHGEXSHA1] = &dhGEXSHA{hashFunc: crypto.SHA1}
+	kexAlgoMap[kexAlgoDHGEXSHA256] = &dhGEXSHA{hashFunc: crypto.SHA256}
+}
+
+// curve25519sha256 implements the curve25519-sha256 (formerly known as
+// curve25519-sha256@libssh.org) key exchange method, as described in RFC 8731.
+type curve25519sha256 struct{}
+
+type curve25519KeyPair struct {
+	priv [32]byte
+	pub  [32]byte
+}
+
+func (kp *curve25519KeyPair) generate(rand io.Reader) error {
+	if _, err := io.ReadFull(rand, kp.priv[:]); err != nil {
+		return err
+	}
+	curve25519.ScalarBaseMult(&kp.pub, &kp.priv)
+	return nil
+}
+
+// curve25519Zeros is just an array of 32 zero bytes so that we have something
+// convenient to compare against in order to reject curve25519 points with the
+// wrong order.
+var curve25519Zeros [32]byte
+
+func (kex *curve25519sha256) Client(c packetConn, rand io.Reader, magics *handshakeMagics) (*kexResult, error) {
+	var kp curve25519KeyPair
+	if err := kp.generate(rand); err != nil {
+		return nil, err
+	}
+	if err := c.writePacket(Marshal(&kexECDHInitMsg{kp.pub[:]})); err != nil {
+		return nil, err
+	}
+
+	packet, err := c.readPacket()
+	if err != nil {
+		return nil, err
+	}
+
+	var reply kexECDHReplyMsg
+	if err = Unmarshal(packet, &reply); err != nil {
+		return nil, err
+	}
+	if len(reply.EphemeralPubKey) != 32 {
+		return nil, errors.New("ssh: peer's curve25519 public value has wrong length")
+	}
+
+	var servPub, secret [32]byte
+	copy(servPub[:], reply.EphemeralPubKey)
+	curve25519.ScalarMult(&secret, &kp.priv, &servPub)
+	if subtle.ConstantTimeCompare(secret[:], curve25519Zeros[:]) == 1 {
+		return nil, errors.New("ssh: peer's curve25519 public value has wrong order")
+	}
+
+	h := crypto.SHA256.New()
+	magics.write(h)
+	writeString(h, reply.HostKey)
+	writeString(h, kp.pub[:])
+	writeString(h, reply.EphemeralPubKey)
+
+	ki := new(big.Int).SetBytes(secret[:])
+	K := make([]byte, intLength(ki))
+	marshalInt(K, ki)
+	h.Write(K)
+
+	return &kexResult{
+		H:         h.Sum(nil),
+		K:         K,
+		HostKey:   reply.HostKey,
+		Signature: reply.Signature,
+		Hash:      crypto.SHA256,
+	}, nil
+}
+
+func (kex *curve25519sha256) Server(c packetConn, rand io.Reader, magics *handshakeMagics, priv AlgorithmSigner, algo string) (result *kexResult, err error) {
+	packet, err := c.readPacket()
+	if err != nil {
+		return
+	}
+	var kexInit kexECDHInitMsg
+	if err = Unmarshal(packet, &kexInit); err != nil {
+		return
+	}
+
+	if len(kexInit.ClientPubKey) != 32 {
+		return nil, errors.New("ssh: peer's curve25519 public value has wrong length")
+	}
+
+	var kp curve25519KeyPair
+	if err := kp.generate(rand); err != nil {
+		return nil, err
+	}
+
+	var clientPub, secret [32]byte
+	copy(clientPub[:], kexInit.ClientPubKey)
+	curve25519.ScalarMult(&secret, &kp.priv, &clientPub)
+	if subtle.ConstantTimeCompare(secret[:], curve25519Zeros[:]) == 1 {
+		return nil, errors.New("ssh: peer's curve25519 public value has wrong order")
+	}
+
+	hostKeyBytes := priv.PublicKey().Marshal()
+
+	h := crypto.SHA256.New()
+	magics.write(h)
+	writeString(h, hostKeyBytes)
+	writeString(h, kexInit.ClientPubKey)
+	writeString(h, kp.pub[:])
+
+	ki := new(big.Int).SetBytes(secret[:])
+	K := make([]byte, intLength(ki))
+	marshalInt(K, ki)
+	h.Write(K)
+
+	H := h.Sum(nil)
+
+	sig, err := signAndMarshal(priv, rand, H, algo)
+	if err != nil {
+		return nil, err
+	}
+
+	reply := kexECDHReplyMsg{
+		EphemeralPubKey: kp.pub[:],
+		HostKey:         hostKeyBytes,
+		Signature:       sig,
+	}
+	if err := c.writePacket(Marshal(&reply)); err != nil {
+		return nil, err
+	}
+	return &kexResult{
+		H:         H,
+		K:         K,
+		HostKey:   hostKeyBytes,
+		Signature: sig,
+		Hash:      crypto.SHA256,
+	}, nil
+}
+
+// dhGEXSHA implements the diffie-hellman-group-exchange-sha1 and
+// diffie-hellman-group-exchange-sha256 key agreement protocols,
+// as described in RFC 4419
+type dhGEXSHA struct {
+	hashFunc crypto.Hash
+}
+
+const (
+	dhGroupExchangeMinimumBits   = 2048
+	dhGroupExchangePreferredBits = 2048
+	dhGroupExchangeMaximumBits   = 8192
+)
+
+func (gex *dhGEXSHA) Client(c packetConn, randSource io.Reader, magics *handshakeMagics) (*kexResult, error) {
+	// Send GexRequest
+	kexDHGexRequest := kexDHGexRequestMsg{
+		MinBits:      dhGroupExchangeMinimumBits,
+		PreferedBits: dhGroupExchangePreferredBits,
+		MaxBits:      dhGroupExchangeMaximumBits,
+	}
+	if err := c.writePacket(Marshal(&kexDHGexRequest)); err != nil {
+		return nil, err
+	}
+
+	// Receive GexGroup
+	packet, err := c.readPacket()
+	if err != nil {
+		return nil, err
+	}
+
+	var msg kexDHGexGroupMsg
+	if err = Unmarshal(packet, &msg); err != nil {
+		return nil, err
+	}
+
+	// reject if p's bit length < dhGroupExchangeMinimumBits or > dhGroupExchangeMaximumBits
+	if msg.P.BitLen() < dhGroupExchangeMinimumBits || msg.P.BitLen() > dhGroupExchangeMaximumBits {
+		return nil, fmt.Errorf("ssh: server-generated gex p is out of range (%d bits)", msg.P.BitLen())
+	}
+
+	// Check if g is safe by verifying that 1 < g < p-1
+	pMinusOne := new(big.Int).Sub(msg.P, bigOne)
+	if msg.G.Cmp(bigOne) <= 0 || msg.G.Cmp(pMinusOne) >= 0 {
+		return nil, fmt.Errorf("ssh: server provided gex g is not safe")
+	}
+
+	// Send GexInit
+	pHalf := new(big.Int).Rsh(msg.P, 1)
+	x, err := rand.Int(randSource, pHalf)
+	if err != nil {
+		return nil, err
+	}
+	X := new(big.Int).Exp(msg.G, x, msg.P)
+	kexDHGexInit := kexDHGexInitMsg{
+		X: X,
+	}
+	if err := c.writePacket(Marshal(&kexDHGexInit)); err != nil {
+		return nil, err
+	}
+
+	// Receive GexReply
+	packet, err = c.readPacket()
+	if err != nil {
+		return nil, err
+	}
+
+	var kexDHGexReply kexDHGexReplyMsg
+	if err = Unmarshal(packet, &kexDHGexReply); err != nil {
+		return nil, err
+	}
+
+	if kexDHGexReply.Y.Cmp(bigOne) <= 0 || kexDHGexReply.Y.Cmp(pMinusOne) >= 0 {
+		return nil, errors.New("ssh: DH parameter out of bounds")
+	}
+	kInt := new(big.Int).Exp(kexDHGexReply.Y, x, msg.P)
+
+	// Check if k is safe by verifying that k > 1 and k < p - 1
+	if kInt.Cmp(bigOne) <= 0 || kInt.Cmp(pMinusOne) >= 0 {
+		return nil, fmt.Errorf("ssh: derived k is not safe")
+	}
+
+	h := gex.hashFunc.New()
+	magics.write(h)
+	writeString(h, kexDHGexReply.HostKey)
+	binary.Write(h, binary.BigEndian, uint32(dhGroupExchangeMinimumBits))
+	binary.Write(h, binary.BigEndian, uint32(dhGroupExchangePreferredBits))
+	binary.Write(h, binary.BigEndian, uint32(dhGroupExchangeMaximumBits))
+	writeInt(h, msg.P)
+	writeInt(h, msg.G)
+	writeInt(h, X)
+	writeInt(h, kexDHGexReply.Y)
+	K := make([]byte, intLength(kInt))
+	marshalInt(K, kInt)
+	h.Write(K)
+
+	return &kexResult{
+		H:         h.Sum(nil),
+		K:         K,
+		HostKey:   kexDHGexReply.HostKey,
+		Signature: kexDHGexReply.Signature,
+		Hash:      gex.hashFunc,
+	}, nil
+}
+
+// Server half implementation of the Diffie Hellman Key Exchange with SHA1 and SHA256.
+//
+// This is a minimal implementation to satisfy the automated tests.
+func (gex dhGEXSHA) Server(c packetConn, randSource io.Reader, magics *handshakeMagics, priv AlgorithmSigner, algo string) (result *kexResult, err error) {
+	// Receive GexRequest
+	packet, err := c.readPacket()
+	if err != nil {
+		return
+	}
+	var kexDHGexRequest kexDHGexRequestMsg
+	if err = Unmarshal(packet, &kexDHGexRequest); err != nil {
+		return
+	}
+
+	// Send GexGroup
+	// This is the group called diffie-hellman-group14-sha1 in RFC
+	// 4253 and Oakley Group 14 in RFC 3526.
+	p, _ := new(big.Int).SetString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
+	g := big.NewInt(2)
+
+	msg := &kexDHGexGroupMsg{
+		P: p,
+		G: g,
+	}
+	if err := c.writePacket(Marshal(msg)); err != nil {
+		return nil, err
+	}
+
+	// Receive GexInit
+	packet, err = c.readPacket()
+	if err != nil {
+		return
+	}
+	var kexDHGexInit kexDHGexInitMsg
+	if err = Unmarshal(packet, &kexDHGexInit); err != nil {
+		return
+	}
+
+	pHalf := new(big.Int).Rsh(p, 1)
+
+	y, err := rand.Int(randSource, pHalf)
+	if err != nil {
+		return
+	}
+	Y := new(big.Int).Exp(g, y, p)
+
+	pMinusOne := new(big.Int).Sub(p, bigOne)
+	if kexDHGexInit.X.Cmp(bigOne) <= 0 || kexDHGexInit.X.Cmp(pMinusOne) >= 0 {
+		return nil, errors.New("ssh: DH parameter out of bounds")
+	}
+	kInt := new(big.Int).Exp(kexDHGexInit.X, y, p)
+
+	hostKeyBytes := priv.PublicKey().Marshal()
+
+	h := gex.hashFunc.New()
+	magics.write(h)
+	writeString(h, hostKeyBytes)
+	binary.Write(h, binary.BigEndian, uint32(dhGroupExchangeMinimumBits))
+	binary.Write(h, binary.BigEndian, uint32(dhGroupExchangePreferredBits))
+	binary.Write(h, binary.BigEndian, uint32(dhGroupExchangeMaximumBits))
+	writeInt(h, p)
+	writeInt(h, g)
+	writeInt(h, kexDHGexInit.X)
+	writeInt(h, Y)
+
+	K := make([]byte, intLength(kInt))
+	marshalInt(K, kInt)
+	h.Write(K)
+
+	H := h.Sum(nil)
+
+	// H is already a hash, but the hostkey signing will apply its
+	// own key-specific hash algorithm.
+	sig, err := signAndMarshal(priv, randSource, H, algo)
+	if err != nil {
+		return nil, err
+	}
+
+	kexDHGexReply := kexDHGexReplyMsg{
+		HostKey:   hostKeyBytes,
+		Y:         Y,
+		Signature: sig,
+	}
+	packet = Marshal(&kexDHGexReply)
+
+	err = c.writePacket(packet)
+
+	return &kexResult{
+		H:         H,
+		K:         K,
+		HostKey:   hostKeyBytes,
+		Signature: sig,
+		Hash:      gex.hashFunc,
+	}, err
+}

pkg/proto/ssh/knownhosts/knownhosts.go

@@ -0,0 +1,540 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package knownhosts implements a parser for the OpenSSH known_hosts
+// host key database, and provides utility functions for writing
+// OpenSSH compliant known_hosts files.
+package knownhosts
+
+import (
+	"bufio"
+	"bytes"
+	"crypto/hmac"
+	"crypto/rand"
+	"crypto/sha1"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"os"
+	"strings"
+
+	"github.com/Neur0toxine/sshpoke/pkg/proto/ssh"
+)
+
+// See the sshd manpage
+// (http://man.openbsd.org/sshd#SSH_KNOWN_HOSTS_FILE_FORMAT) for
+// background.
+
+type addr struct{ host, port string }
+
+func (a *addr) String() string {
+	h := a.host
+	if strings.Contains(h, ":") {
+		h = "[" + h + "]"
+	}
+	return h + ":" + a.port
+}
+
+type matcher interface {
+	match(addr) bool
+}
+
+type hostPattern struct {
+	negate bool
+	addr   addr
+}
+
+func (p *hostPattern) String() string {
+	n := ""
+	if p.negate {
+		n = "!"
+	}
+
+	return n + p.addr.String()
+}
+
+type hostPatterns []hostPattern
+
+func (ps hostPatterns) match(a addr) bool {
+	matched := false
+	for _, p := range ps {
+		if !p.match(a) {
+			continue
+		}
+		if p.negate {
+			return false
+		}
+		matched = true
+	}
+	return matched
+}
+
+// See
+// https://android.googlesource.com/platform/external/openssh/+/ab28f5495c85297e7a597c1ba62e996416da7c7e/addrmatch.c
+// The matching of * has no regard for separators, unlike filesystem globs
+func wildcardMatch(pat []byte, str []byte) bool {
+	for {
+		if len(pat) == 0 {
+			return len(str) == 0
+		}
+		if len(str) == 0 {
+			return false
+		}
+
+		if pat[0] == '*' {
+			if len(pat) == 1 {
+				return true
+			}
+
+			for j := range str {
+				if wildcardMatch(pat[1:], str[j:]) {
+					return true
+				}
+			}
+			return false
+		}
+
+		if pat[0] == '?' || pat[0] == str[0] {
+			pat = pat[1:]
+			str = str[1:]
+		} else {
+			return false
+		}
+	}
+}
+
+func (p *hostPattern) match(a addr) bool {
+	return wildcardMatch([]byte(p.addr.host), []byte(a.host)) && p.addr.port == a.port
+}
+
+type keyDBLine struct {
+	cert     bool
+	matcher  matcher
+	knownKey KnownKey
+}
+
+func serialize(k ssh.PublicKey) string {
+	return k.Type() + " " + base64.StdEncoding.EncodeToString(k.Marshal())
+}
+
+func (l *keyDBLine) match(a addr) bool {
+	return l.matcher.match(a)
+}
+
+type hostKeyDB struct {
+	// Serialized version of revoked keys
+	revoked map[string]*KnownKey
+	lines   []keyDBLine
+}
+
+func newHostKeyDB() *hostKeyDB {
+	db := &hostKeyDB{
+		revoked: make(map[string]*KnownKey),
+	}
+
+	return db
+}
+
+func keyEq(a, b ssh.PublicKey) bool {
+	return bytes.Equal(a.Marshal(), b.Marshal())
+}
+
+// IsHostAuthority can be used as a callback in ssh.CertChecker
+func (db *hostKeyDB) IsHostAuthority(remote ssh.PublicKey, address string) bool {
+	h, p, err := net.SplitHostPort(address)
+	if err != nil {
+		return false
+	}
+	a := addr{host: h, port: p}
+
+	for _, l := range db.lines {
+		if l.cert && keyEq(l.knownKey.Key, remote) && l.match(a) {
+			return true
+		}
+	}
+	return false
+}
+
+// IsRevoked can be used as a callback in ssh.CertChecker
+func (db *hostKeyDB) IsRevoked(key *ssh.Certificate) bool {
+	_, ok := db.revoked[string(key.Marshal())]
+	return ok
+}
+
+const markerCert = "@cert-authority"
+const markerRevoked = "@revoked"
+
+func nextWord(line []byte) (string, []byte) {
+	i := bytes.IndexAny(line, "\t ")
+	if i == -1 {
+		return string(line), nil
+	}
+
+	return string(line[:i]), bytes.TrimSpace(line[i:])
+}
+
+func parseLine(line []byte) (marker, host string, key ssh.PublicKey, err error) {
+	if w, next := nextWord(line); w == markerCert || w == markerRevoked {
+		marker = w
+		line = next
+	}
+
+	host, line = nextWord(line)
+	if len(line) == 0 {
+		return "", "", nil, errors.New("knownhosts: missing host pattern")
+	}
+
+	// ignore the keytype as it's in the key blob anyway.
+	_, line = nextWord(line)
+	if len(line) == 0 {
+		return "", "", nil, errors.New("knownhosts: missing key type pattern")
+	}
+
+	keyBlob, _ := nextWord(line)
+
+	keyBytes, err := base64.StdEncoding.DecodeString(keyBlob)
+	if err != nil {
+		return "", "", nil, err
+	}
+	key, err = ssh.ParsePublicKey(keyBytes)
+	if err != nil {
+		return "", "", nil, err
+	}
+
+	return marker, host, key, nil
+}
+
+func (db *hostKeyDB) parseLine(line []byte, filename string, linenum int) error {
+	marker, pattern, key, err := parseLine(line)
+	if err != nil {
+		return err
+	}
+
+	if marker == markerRevoked {
+		db.revoked[string(key.Marshal())] = &KnownKey{
+			Key:      key,
+			Filename: filename,
+			Line:     linenum,
+		}
+
+		return nil
+	}
+
+	entry := keyDBLine{
+		cert: marker == markerCert,
+		knownKey: KnownKey{
+			Filename: filename,
+			Line:     linenum,
+			Key:      key,
+		},
+	}
+
+	if pattern[0] == '|' {
+		entry.matcher, err = newHashedHost(pattern)
+	} else {
+		entry.matcher, err = newHostnameMatcher(pattern)
+	}
+
+	if err != nil {
+		return err
+	}
+
+	db.lines = append(db.lines, entry)
+	return nil
+}
+
+func newHostnameMatcher(pattern string) (matcher, error) {
+	var hps hostPatterns
+	for _, p := range strings.Split(pattern, ",") {
+		if len(p) == 0 {
+			continue
+		}
+
+		var a addr
+		var negate bool
+		if p[0] == '!' {
+			negate = true
+			p = p[1:]
+		}
+
+		if len(p) == 0 {
+			return nil, errors.New("knownhosts: negation without following hostname")
+		}
+
+		var err error
+		if p[0] == '[' {
+			a.host, a.port, err = net.SplitHostPort(p)
+			if err != nil {
+				return nil, err
+			}
+		} else {
+			a.host, a.port, err = net.SplitHostPort(p)
+			if err != nil {
+				a.host = p
+				a.port = "22"
+			}
+		}
+		hps = append(hps, hostPattern{
+			negate: negate,
+			addr:   a,
+		})
+	}
+	return hps, nil
+}
+
+// KnownKey represents a key declared in a known_hosts file.
+type KnownKey struct {
+	Key      ssh.PublicKey
+	Filename string
+	Line     int
+}
+
+func (k *KnownKey) String() string {
+	return fmt.Sprintf("%s:%d: %s", k.Filename, k.Line, serialize(k.Key))
+}
+
+// KeyError is returned if we did not find the key in the host key
+// database, or there was a mismatch.  Typically, in batch
+// applications, this should be interpreted as failure. Interactive
+// applications can offer an interactive prompt to the user.
+type KeyError struct {
+	// Want holds the accepted host keys. For each key algorithm,
+	// there can be one hostkey.  If Want is empty, the host is
+	// unknown. If Want is non-empty, there was a mismatch, which
+	// can signify a MITM attack.
+	Want []KnownKey
+}
+
+func (u *KeyError) Error() string {
+	if len(u.Want) == 0 {
+		return "knownhosts: key is unknown"
+	}
+	return "knownhosts: key mismatch"
+}
+
+// RevokedError is returned if we found a key that was revoked.
+type RevokedError struct {
+	Revoked KnownKey
+}
+
+func (r *RevokedError) Error() string {
+	return "knownhosts: key is revoked"
+}
+
+// check checks a key against the host database. This should not be
+// used for verifying certificates.
+func (db *hostKeyDB) check(address string, remote net.Addr, remoteKey ssh.PublicKey) error {
+	if revoked := db.revoked[string(remoteKey.Marshal())]; revoked != nil {
+		return &RevokedError{Revoked: *revoked}
+	}
+
+	host, port, err := net.SplitHostPort(remote.String())
+	if err != nil {
+		return fmt.Errorf("knownhosts: SplitHostPort(%s): %v", remote, err)
+	}
+
+	hostToCheck := addr{host, port}
+	if address != "" {
+		// Give preference to the hostname if available.
+		host, port, err := net.SplitHostPort(address)
+		if err != nil {
+			return fmt.Errorf("knownhosts: SplitHostPort(%s): %v", address, err)
+		}
+
+		hostToCheck = addr{host, port}
+	}
+
+	return db.checkAddr(hostToCheck, remoteKey)
+}
+
+// checkAddr checks if we can find the given public key for the
+// given address.  If we only find an entry for the IP address,
+// or only the hostname, then this still succeeds.
+func (db *hostKeyDB) checkAddr(a addr, remoteKey ssh.PublicKey) error {
+	// TODO(hanwen): are these the right semantics? What if there
+	// is just a key for the IP address, but not for the
+	// hostname?
+
+	// Algorithm => key.
+	knownKeys := map[string]KnownKey{}
+	for _, l := range db.lines {
+		if l.match(a) {
+			typ := l.knownKey.Key.Type()
+			if _, ok := knownKeys[typ]; !ok {
+				knownKeys[typ] = l.knownKey
+			}
+		}
+	}
+
+	keyErr := &KeyError{}
+	for _, v := range knownKeys {
+		keyErr.Want = append(keyErr.Want, v)
+	}
+
+	// Unknown remote host.
+	if len(knownKeys) == 0 {
+		return keyErr
+	}
+
+	// If the remote host starts using a different, unknown key type, we
+	// also interpret that as a mismatch.
+	if known, ok := knownKeys[remoteKey.Type()]; !ok || !keyEq(known.Key, remoteKey) {
+		return keyErr
+	}
+
+	return nil
+}
+
+// The Read function parses file contents.
+func (db *hostKeyDB) Read(r io.Reader, filename string) error {
+	scanner := bufio.NewScanner(r)
+
+	lineNum := 0
+	for scanner.Scan() {
+		lineNum++
+		line := scanner.Bytes()
+		line = bytes.TrimSpace(line)
+		if len(line) == 0 || line[0] == '#' {
+			continue
+		}
+
+		if err := db.parseLine(line, filename, lineNum); err != nil {
+			return fmt.Errorf("knownhosts: %s:%d: %v", filename, lineNum, err)
+		}
+	}
+	return scanner.Err()
+}
+
+// New creates a host key callback from the given OpenSSH host key
+// files. The returned callback is for use in
+// ssh.ClientConfig.HostKeyCallback. By preference, the key check
+// operates on the hostname if available, i.e. if a server changes its
+// IP address, the host key check will still succeed, even though a
+// record of the new IP address is not available.
+func New(files ...string) (ssh.HostKeyCallback, error) {
+	db := newHostKeyDB()
+	for _, fn := range files {
+		f, err := os.Open(fn)
+		if err != nil {
+			return nil, err
+		}
+		defer f.Close()
+		if err := db.Read(f, fn); err != nil {
+			return nil, err
+		}
+	}
+
+	var certChecker ssh.CertChecker
+	certChecker.IsHostAuthority = db.IsHostAuthority
+	certChecker.IsRevoked = db.IsRevoked
+	certChecker.HostKeyFallback = db.check
+
+	return certChecker.CheckHostKey, nil
+}
+
+// Normalize normalizes an address into the form used in known_hosts
+func Normalize(address string) string {
+	host, port, err := net.SplitHostPort(address)
+	if err != nil {
+		host = address
+		port = "22"
+	}
+	entry := host
+	if port != "22" {
+		entry = "[" + entry + "]:" + port
+	} else if strings.Contains(host, ":") && !strings.HasPrefix(host, "[") {
+		entry = "[" + entry + "]"
+	}
+	return entry
+}
+
+// Line returns a line to add append to the known_hosts files.
+func Line(addresses []string, key ssh.PublicKey) string {
+	var trimmed []string
+	for _, a := range addresses {
+		trimmed = append(trimmed, Normalize(a))
+	}
+
+	return strings.Join(trimmed, ",") + " " + serialize(key)
+}
+
+// HashHostname hashes the given hostname. The hostname is not
+// normalized before hashing.
+func HashHostname(hostname string) string {
+	// TODO(hanwen): check if we can safely normalize this always.
+	salt := make([]byte, sha1.Size)
+
+	_, err := rand.Read(salt)
+	if err != nil {
+		panic(fmt.Sprintf("crypto/rand failure %v", err))
+	}
+
+	hash := hashHost(hostname, salt)
+	return encodeHash(sha1HashType, salt, hash)
+}
+
+func decodeHash(encoded string) (hashType string, salt, hash []byte, err error) {
+	if len(encoded) == 0 || encoded[0] != '|' {
+		err = errors.New("knownhosts: hashed host must start with '|'")
+		return
+	}
+	components := strings.Split(encoded, "|")
+	if len(components) != 4 {
+		err = fmt.Errorf("knownhosts: got %d components, want 3", len(components))
+		return
+	}
+
+	hashType = components[1]
+	if salt, err = base64.StdEncoding.DecodeString(components[2]); err != nil {
+		return
+	}
+	if hash, err = base64.StdEncoding.DecodeString(components[3]); err != nil {
+		return
+	}
+	return
+}
+
+func encodeHash(typ string, salt []byte, hash []byte) string {
+	return strings.Join([]string{"",
+		typ,
+		base64.StdEncoding.EncodeToString(salt),
+		base64.StdEncoding.EncodeToString(hash),
+	}, "|")
+}
+
+// See https://android.googlesource.com/platform/external/openssh/+/ab28f5495c85297e7a597c1ba62e996416da7c7e/hostfile.c#120
+func hashHost(hostname string, salt []byte) []byte {
+	mac := hmac.New(sha1.New, salt)
+	mac.Write([]byte(hostname))
+	return mac.Sum(nil)
+}
+
+type hashedHost struct {
+	salt []byte
+	hash []byte
+}
+
+const sha1HashType = "1"
+
+func newHashedHost(encoded string) (*hashedHost, error) {
+	typ, salt, hash, err := decodeHash(encoded)
+	if err != nil {
+		return nil, err
+	}
+
+	// The type field seems for future algorithm agility, but it's
+	// actually hardcoded in openssh currently, see
+	// https://android.googlesource.com/platform/external/openssh/+/ab28f5495c85297e7a597c1ba62e996416da7c7e/hostfile.c#120
+	if typ != sha1HashType {
+		return nil, fmt.Errorf("knownhosts: got hash type %s, must be '1'", typ)
+	}
+
+	return &hashedHost{salt: salt, hash: hash}, nil
+}
+
+func (h *hashedHost) match(a addr) bool {
+	return bytes.Equal(hashHost(Normalize(a.String()), h.salt), h.hash)
+}

pkg/proto/ssh/mac.go

@@ -0,0 +1,68 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+// Message authentication support
+
+import (
+	"crypto/hmac"
+	"crypto/sha1"
+	"crypto/sha256"
+	"crypto/sha512"
+	"hash"
+)
+
+type macMode struct {
+	keySize int
+	etm     bool
+	new     func(key []byte) hash.Hash
+}
+
+// truncatingMAC wraps around a hash.Hash and truncates the output digest to
+// a given size.
+type truncatingMAC struct {
+	length int
+	hmac   hash.Hash
+}
+
+func (t truncatingMAC) Write(data []byte) (int, error) {
+	return t.hmac.Write(data)
+}
+
+func (t truncatingMAC) Sum(in []byte) []byte {
+	out := t.hmac.Sum(in)
+	return out[:len(in)+t.length]
+}
+
+func (t truncatingMAC) Reset() {
+	t.hmac.Reset()
+}
+
+func (t truncatingMAC) Size() int {
+	return t.length
+}
+
+func (t truncatingMAC) BlockSize() int { return t.hmac.BlockSize() }
+
+var macModes = map[string]*macMode{
+	"hmac-sha2-512-etm@openssh.com": {64, true, func(key []byte) hash.Hash {
+		return hmac.New(sha512.New, key)
+	}},
+	"hmac-sha2-256-etm@openssh.com": {32, true, func(key []byte) hash.Hash {
+		return hmac.New(sha256.New, key)
+	}},
+	"hmac-sha2-512": {64, false, func(key []byte) hash.Hash {
+		return hmac.New(sha512.New, key)
+	}},
+	"hmac-sha2-256": {32, false, func(key []byte) hash.Hash {
+		return hmac.New(sha256.New, key)
+	}},
+	"hmac-sha1": {20, false, func(key []byte) hash.Hash {
+		return hmac.New(sha1.New, key)
+	}},
+	"hmac-sha1-96": {20, false, func(key []byte) hash.Hash {
+		return truncatingMAC{12, hmac.New(sha1.New, key)}
+	}},
+}

pkg/proto/ssh/messages.go

@@ -0,0 +1,891 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"math/big"
+	"reflect"
+	"strconv"
+	"strings"
+)
+
+// These are SSH message type numbers. They are scattered around several
+// documents but many were taken from [SSH-PARAMETERS].
+const (
+	msgIgnore        = 2
+	msgUnimplemented = 3
+	msgDebug         = 4
+	msgNewKeys       = 21
+)
+
+// SSH messages:
+//
+// These structures mirror the wire format of the corresponding SSH messages.
+// They are marshaled using reflection with the marshal and unmarshal functions
+// in this file. The only wrinkle is that a final member of type []byte with a
+// ssh tag of "rest" receives the remainder of a packet when unmarshaling.
+
+// See RFC 4253, section 11.1.
+const msgDisconnect = 1
+
+// disconnectMsg is the message that signals a disconnect. It is also
+// the error type returned from mux.Wait()
+type disconnectMsg struct {
+	Reason   uint32 `sshtype:"1"`
+	Message  string
+	Language string
+}
+
+func (d *disconnectMsg) Error() string {
+	return fmt.Sprintf("ssh: disconnect, reason %d: %s", d.Reason, d.Message)
+}
+
+// See RFC 4253, section 7.1.
+const msgKexInit = 20
+
+type kexInitMsg struct {
+	Cookie                  [16]byte `sshtype:"20"`
+	KexAlgos                []string
+	ServerHostKeyAlgos      []string
+	CiphersClientServer     []string
+	CiphersServerClient     []string
+	MACsClientServer        []string
+	MACsServerClient        []string
+	CompressionClientServer []string
+	CompressionServerClient []string
+	LanguagesClientServer   []string
+	LanguagesServerClient   []string
+	FirstKexFollows         bool
+	Reserved                uint32
+}
+
+// See RFC 4253, section 8.
+
+// Diffie-Hellman
+const msgKexDHInit = 30
+
+type kexDHInitMsg struct {
+	X *big.Int `sshtype:"30"`
+}
+
+const msgKexECDHInit = 30
+
+type kexECDHInitMsg struct {
+	ClientPubKey []byte `sshtype:"30"`
+}
+
+const msgKexECDHReply = 31
+
+type kexECDHReplyMsg struct {
+	HostKey         []byte `sshtype:"31"`
+	EphemeralPubKey []byte
+	Signature       []byte
+}
+
+const msgKexDHReply = 31
+
+type kexDHReplyMsg struct {
+	HostKey   []byte `sshtype:"31"`
+	Y         *big.Int
+	Signature []byte
+}
+
+// See RFC 4419, section 5.
+const msgKexDHGexGroup = 31
+
+type kexDHGexGroupMsg struct {
+	P *big.Int `sshtype:"31"`
+	G *big.Int
+}
+
+const msgKexDHGexInit = 32
+
+type kexDHGexInitMsg struct {
+	X *big.Int `sshtype:"32"`
+}
+
+const msgKexDHGexReply = 33
+
+type kexDHGexReplyMsg struct {
+	HostKey   []byte `sshtype:"33"`
+	Y         *big.Int
+	Signature []byte
+}
+
+const msgKexDHGexRequest = 34
+
+type kexDHGexRequestMsg struct {
+	MinBits      uint32 `sshtype:"34"`
+	PreferedBits uint32
+	MaxBits      uint32
+}
+
+// See RFC 4253, section 10.
+const msgServiceRequest = 5
+
+type serviceRequestMsg struct {
+	Service string `sshtype:"5"`
+}
+
+// See RFC 4253, section 10.
+const msgServiceAccept = 6
+
+type serviceAcceptMsg struct {
+	Service string `sshtype:"6"`
+}
+
+// See RFC 8308, section 2.3
+const msgExtInfo = 7
+
+type extInfoMsg struct {
+	NumExtensions uint32 `sshtype:"7"`
+	Payload       []byte `ssh:"rest"`
+}
+
+// See RFC 4252, section 5.
+const msgUserAuthRequest = 50
+
+type userAuthRequestMsg struct {
+	User    string `sshtype:"50"`
+	Service string
+	Method  string
+	Payload []byte `ssh:"rest"`
+}
+
+// Used for debug printouts of packets.
+type userAuthSuccessMsg struct {
+}
+
+// See RFC 4252, section 5.1
+const msgUserAuthFailure = 51
+
+type userAuthFailureMsg struct {
+	Methods        []string `sshtype:"51"`
+	PartialSuccess bool
+}
+
+// See RFC 4252, section 5.1
+const msgUserAuthSuccess = 52
+
+// See RFC 4252, section 5.4
+const msgUserAuthBanner = 53
+
+type userAuthBannerMsg struct {
+	Message string `sshtype:"53"`
+	// unused, but required to allow message parsing
+	Language string
+}
+
+// See RFC 4256, section 3.2
+const msgUserAuthInfoRequest = 60
+const msgUserAuthInfoResponse = 61
+
+type userAuthInfoRequestMsg struct {
+	Name        string `sshtype:"60"`
+	Instruction string
+	Language    string
+	NumPrompts  uint32
+	Prompts     []byte `ssh:"rest"`
+}
+
+// See RFC 4254, section 5.1.
+const msgChannelOpen = 90
+
+type channelOpenMsg struct {
+	ChanType         string `sshtype:"90"`
+	PeersID          uint32
+	PeersWindow      uint32
+	MaxPacketSize    uint32
+	TypeSpecificData []byte `ssh:"rest"`
+}
+
+const msgChannelExtendedData = 95
+const msgChannelData = 94
+
+// Used for debug print outs of packets.
+type channelDataMsg struct {
+	PeersID uint32 `sshtype:"94"`
+	Length  uint32
+	Rest    []byte `ssh:"rest"`
+}
+
+// See RFC 4254, section 5.1.
+const msgChannelOpenConfirm = 91
+
+type channelOpenConfirmMsg struct {
+	PeersID          uint32 `sshtype:"91"`
+	MyID             uint32
+	MyWindow         uint32
+	MaxPacketSize    uint32
+	TypeSpecificData []byte `ssh:"rest"`
+}
+
+// See RFC 4254, section 5.1.
+const msgChannelOpenFailure = 92
+
+type channelOpenFailureMsg struct {
+	PeersID  uint32 `sshtype:"92"`
+	Reason   RejectionReason
+	Message  string
+	Language string
+}
+
+const msgChannelRequest = 98
+
+type channelRequestMsg struct {
+	PeersID             uint32 `sshtype:"98"`
+	Request             string
+	WantReply           bool
+	RequestSpecificData []byte `ssh:"rest"`
+}
+
+// See RFC 4254, section 5.4.
+const msgChannelSuccess = 99
+
+type channelRequestSuccessMsg struct {
+	PeersID uint32 `sshtype:"99"`
+}
+
+// See RFC 4254, section 5.4.
+const msgChannelFailure = 100
+
+type channelRequestFailureMsg struct {
+	PeersID uint32 `sshtype:"100"`
+}
+
+// See RFC 4254, section 5.3
+const msgChannelClose = 97
+
+type channelCloseMsg struct {
+	PeersID uint32 `sshtype:"97"`
+}
+
+// See RFC 4254, section 5.3
+const msgChannelEOF = 96
+
+type channelEOFMsg struct {
+	PeersID uint32 `sshtype:"96"`
+}
+
+// See RFC 4254, section 4
+const msgGlobalRequest = 80
+
+type globalRequestMsg struct {
+	Type      string `sshtype:"80"`
+	WantReply bool
+	Data      []byte `ssh:"rest"`
+}
+
+// See RFC 4254, section 4
+const msgRequestSuccess = 81
+
+type globalRequestSuccessMsg struct {
+	Data []byte `ssh:"rest" sshtype:"81"`
+}
+
+// See RFC 4254, section 4
+const msgRequestFailure = 82
+
+type globalRequestFailureMsg struct {
+	Data []byte `ssh:"rest" sshtype:"82"`
+}
+
+// See RFC 4254, section 5.2
+const msgChannelWindowAdjust = 93
+
+type windowAdjustMsg struct {
+	PeersID         uint32 `sshtype:"93"`
+	AdditionalBytes uint32
+}
+
+// See RFC 4252, section 7
+const msgUserAuthPubKeyOk = 60
+
+type userAuthPubKeyOkMsg struct {
+	Algo   string `sshtype:"60"`
+	PubKey []byte
+}
+
+// See RFC 4462, section 3
+const msgUserAuthGSSAPIResponse = 60
+
+type userAuthGSSAPIResponse struct {
+	SupportMech []byte `sshtype:"60"`
+}
+
+const msgUserAuthGSSAPIToken = 61
+
+type userAuthGSSAPIToken struct {
+	Token []byte `sshtype:"61"`
+}
+
+const msgUserAuthGSSAPIMIC = 66
+
+type userAuthGSSAPIMIC struct {
+	MIC []byte `sshtype:"66"`
+}
+
+// See RFC 4462, section 3.9
+const msgUserAuthGSSAPIErrTok = 64
+
+type userAuthGSSAPIErrTok struct {
+	ErrorToken []byte `sshtype:"64"`
+}
+
+// See RFC 4462, section 3.8
+const msgUserAuthGSSAPIError = 65
+
+type userAuthGSSAPIError struct {
+	MajorStatus uint32 `sshtype:"65"`
+	MinorStatus uint32
+	Message     string
+	LanguageTag string
+}
+
+// Transport layer OpenSSH extension. See [PROTOCOL], section 1.9
+const msgPing = 192
+
+type pingMsg struct {
+	Data string `sshtype:"192"`
+}
+
+// Transport layer OpenSSH extension. See [PROTOCOL], section 1.9
+const msgPong = 193
+
+type pongMsg struct {
+	Data string `sshtype:"193"`
+}
+
+// typeTags returns the possible type bytes for the given reflect.Type, which
+// should be a struct. The possible values are separated by a '|' character.
+func typeTags(structType reflect.Type) (tags []byte) {
+	tagStr := structType.Field(0).Tag.Get("sshtype")
+
+	for _, tag := range strings.Split(tagStr, "|") {
+		i, err := strconv.Atoi(tag)
+		if err == nil {
+			tags = append(tags, byte(i))
+		}
+	}
+
+	return tags
+}
+
+func fieldError(t reflect.Type, field int, problem string) error {
+	if problem != "" {
+		problem = ": " + problem
+	}
+	return fmt.Errorf("ssh: unmarshal error for field %s of type %s%s", t.Field(field).Name, t.Name(), problem)
+}
+
+var errShortRead = errors.New("ssh: short read")
+
+// Unmarshal parses data in SSH wire format into a structure. The out
+// argument should be a pointer to struct. If the first member of the
+// struct has the "sshtype" tag set to a '|'-separated set of numbers
+// in decimal, the packet must start with one of those numbers. In
+// case of error, Unmarshal returns a ParseError or
+// UnexpectedMessageError.
+func Unmarshal(data []byte, out interface{}) error {
+	v := reflect.ValueOf(out).Elem()
+	structType := v.Type()
+	expectedTypes := typeTags(structType)
+
+	var expectedType byte
+	if len(expectedTypes) > 0 {
+		expectedType = expectedTypes[0]
+	}
+
+	if len(data) == 0 {
+		return parseError(expectedType)
+	}
+
+	if len(expectedTypes) > 0 {
+		goodType := false
+		for _, e := range expectedTypes {
+			if e > 0 && data[0] == e {
+				goodType = true
+				break
+			}
+		}
+		if !goodType {
+			return fmt.Errorf("ssh: unexpected message type %d (expected one of %v)", data[0], expectedTypes)
+		}
+		data = data[1:]
+	}
+
+	var ok bool
+	for i := 0; i < v.NumField(); i++ {
+		field := v.Field(i)
+		t := field.Type()
+		switch t.Kind() {
+		case reflect.Bool:
+			if len(data) < 1 {
+				return errShortRead
+			}
+			field.SetBool(data[0] != 0)
+			data = data[1:]
+		case reflect.Array:
+			if t.Elem().Kind() != reflect.Uint8 {
+				return fieldError(structType, i, "array of unsupported type")
+			}
+			if len(data) < t.Len() {
+				return errShortRead
+			}
+			for j, n := 0, t.Len(); j < n; j++ {
+				field.Index(j).Set(reflect.ValueOf(data[j]))
+			}
+			data = data[t.Len():]
+		case reflect.Uint64:
+			var u64 uint64
+			if u64, data, ok = parseUint64(data); !ok {
+				return errShortRead
+			}
+			field.SetUint(u64)
+		case reflect.Uint32:
+			var u32 uint32
+			if u32, data, ok = parseUint32(data); !ok {
+				return errShortRead
+			}
+			field.SetUint(uint64(u32))
+		case reflect.Uint8:
+			if len(data) < 1 {
+				return errShortRead
+			}
+			field.SetUint(uint64(data[0]))
+			data = data[1:]
+		case reflect.String:
+			var s []byte
+			if s, data, ok = parseString(data); !ok {
+				return fieldError(structType, i, "")
+			}
+			field.SetString(string(s))
+		case reflect.Slice:
+			switch t.Elem().Kind() {
+			case reflect.Uint8:
+				if structType.Field(i).Tag.Get("ssh") == "rest" {
+					field.Set(reflect.ValueOf(data))
+					data = nil
+				} else {
+					var s []byte
+					if s, data, ok = parseString(data); !ok {
+						return errShortRead
+					}
+					field.Set(reflect.ValueOf(s))
+				}
+			case reflect.String:
+				var nl []string
+				if nl, data, ok = parseNameList(data); !ok {
+					return errShortRead
+				}
+				field.Set(reflect.ValueOf(nl))
+			default:
+				return fieldError(structType, i, "slice of unsupported type")
+			}
+		case reflect.Ptr:
+			if t == bigIntType {
+				var n *big.Int
+				if n, data, ok = parseInt(data); !ok {
+					return errShortRead
+				}
+				field.Set(reflect.ValueOf(n))
+			} else {
+				return fieldError(structType, i, "pointer to unsupported type")
+			}
+		default:
+			return fieldError(structType, i, fmt.Sprintf("unsupported type: %v", t))
+		}
+	}
+
+	if len(data) != 0 {
+		return parseError(expectedType)
+	}
+
+	return nil
+}
+
+// Marshal serializes the message in msg to SSH wire format.  The msg
+// argument should be a struct or pointer to struct. If the first
+// member has the "sshtype" tag set to a number in decimal, that
+// number is prepended to the result. If the last of member has the
+// "ssh" tag set to "rest", its contents are appended to the output.
+func Marshal(msg interface{}) []byte {
+	out := make([]byte, 0, 64)
+	return marshalStruct(out, msg)
+}
+
+func marshalStruct(out []byte, msg interface{}) []byte {
+	v := reflect.Indirect(reflect.ValueOf(msg))
+	msgTypes := typeTags(v.Type())
+	if len(msgTypes) > 0 {
+		out = append(out, msgTypes[0])
+	}
+
+	for i, n := 0, v.NumField(); i < n; i++ {
+		field := v.Field(i)
+		switch t := field.Type(); t.Kind() {
+		case reflect.Bool:
+			var v uint8
+			if field.Bool() {
+				v = 1
+			}
+			out = append(out, v)
+		case reflect.Array:
+			if t.Elem().Kind() != reflect.Uint8 {
+				panic(fmt.Sprintf("array of non-uint8 in field %d: %T", i, field.Interface()))
+			}
+			for j, l := 0, t.Len(); j < l; j++ {
+				out = append(out, uint8(field.Index(j).Uint()))
+			}
+		case reflect.Uint32:
+			out = appendU32(out, uint32(field.Uint()))
+		case reflect.Uint64:
+			out = appendU64(out, uint64(field.Uint()))
+		case reflect.Uint8:
+			out = append(out, uint8(field.Uint()))
+		case reflect.String:
+			s := field.String()
+			out = appendInt(out, len(s))
+			out = append(out, s...)
+		case reflect.Slice:
+			switch t.Elem().Kind() {
+			case reflect.Uint8:
+				if v.Type().Field(i).Tag.Get("ssh") != "rest" {
+					out = appendInt(out, field.Len())
+				}
+				out = append(out, field.Bytes()...)
+			case reflect.String:
+				offset := len(out)
+				out = appendU32(out, 0)
+				if n := field.Len(); n > 0 {
+					for j := 0; j < n; j++ {
+						f := field.Index(j)
+						if j != 0 {
+							out = append(out, ',')
+						}
+						out = append(out, f.String()...)
+					}
+					// overwrite length value
+					binary.BigEndian.PutUint32(out[offset:], uint32(len(out)-offset-4))
+				}
+			default:
+				panic(fmt.Sprintf("slice of unknown type in field %d: %T", i, field.Interface()))
+			}
+		case reflect.Ptr:
+			if t == bigIntType {
+				var n *big.Int
+				nValue := reflect.ValueOf(&n)
+				nValue.Elem().Set(field)
+				needed := intLength(n)
+				oldLength := len(out)
+
+				if cap(out)-len(out) < needed {
+					newOut := make([]byte, len(out), 2*(len(out)+needed))
+					copy(newOut, out)
+					out = newOut
+				}
+				out = out[:oldLength+needed]
+				marshalInt(out[oldLength:], n)
+			} else {
+				panic(fmt.Sprintf("pointer to unknown type in field %d: %T", i, field.Interface()))
+			}
+		}
+	}
+
+	return out
+}
+
+var bigOne = big.NewInt(1)
+
+func parseString(in []byte) (out, rest []byte, ok bool) {
+	if len(in) < 4 {
+		return
+	}
+	length := binary.BigEndian.Uint32(in)
+	in = in[4:]
+	if uint32(len(in)) < length {
+		return
+	}
+	out = in[:length]
+	rest = in[length:]
+	ok = true
+	return
+}
+
+var (
+	comma         = []byte{','}
+	emptyNameList = []string{}
+)
+
+func parseNameList(in []byte) (out []string, rest []byte, ok bool) {
+	contents, rest, ok := parseString(in)
+	if !ok {
+		return
+	}
+	if len(contents) == 0 {
+		out = emptyNameList
+		return
+	}
+	parts := bytes.Split(contents, comma)
+	out = make([]string, len(parts))
+	for i, part := range parts {
+		out[i] = string(part)
+	}
+	return
+}
+
+func parseInt(in []byte) (out *big.Int, rest []byte, ok bool) {
+	contents, rest, ok := parseString(in)
+	if !ok {
+		return
+	}
+	out = new(big.Int)
+
+	if len(contents) > 0 && contents[0]&0x80 == 0x80 {
+		// This is a negative number
+		notBytes := make([]byte, len(contents))
+		for i := range notBytes {
+			notBytes[i] = ^contents[i]
+		}
+		out.SetBytes(notBytes)
+		out.Add(out, bigOne)
+		out.Neg(out)
+	} else {
+		// Positive number
+		out.SetBytes(contents)
+	}
+	ok = true
+	return
+}
+
+func parseUint32(in []byte) (uint32, []byte, bool) {
+	if len(in) < 4 {
+		return 0, nil, false
+	}
+	return binary.BigEndian.Uint32(in), in[4:], true
+}
+
+func parseUint64(in []byte) (uint64, []byte, bool) {
+	if len(in) < 8 {
+		return 0, nil, false
+	}
+	return binary.BigEndian.Uint64(in), in[8:], true
+}
+
+func intLength(n *big.Int) int {
+	length := 4 /* length bytes */
+	if n.Sign() < 0 {
+		nMinus1 := new(big.Int).Neg(n)
+		nMinus1.Sub(nMinus1, bigOne)
+		bitLen := nMinus1.BitLen()
+		if bitLen%8 == 0 {
+			// The number will need 0xff padding
+			length++
+		}
+		length += (bitLen + 7) / 8
+	} else if n.Sign() == 0 {
+		// A zero is the zero length string
+	} else {
+		bitLen := n.BitLen()
+		if bitLen%8 == 0 {
+			// The number will need 0x00 padding
+			length++
+		}
+		length += (bitLen + 7) / 8
+	}
+
+	return length
+}
+
+func marshalUint32(to []byte, n uint32) []byte {
+	binary.BigEndian.PutUint32(to, n)
+	return to[4:]
+}
+
+func marshalUint64(to []byte, n uint64) []byte {
+	binary.BigEndian.PutUint64(to, n)
+	return to[8:]
+}
+
+func marshalInt(to []byte, n *big.Int) []byte {
+	lengthBytes := to
+	to = to[4:]
+	length := 0
+
+	if n.Sign() < 0 {
+		// A negative number has to be converted to two's-complement
+		// form. So we'll subtract 1 and invert. If the
+		// most-significant-bit isn't set then we'll need to pad the
+		// beginning with 0xff in order to keep the number negative.
+		nMinus1 := new(big.Int).Neg(n)
+		nMinus1.Sub(nMinus1, bigOne)
+		bytes := nMinus1.Bytes()
+		for i := range bytes {
+			bytes[i] ^= 0xff
+		}
+		if len(bytes) == 0 || bytes[0]&0x80 == 0 {
+			to[0] = 0xff
+			to = to[1:]
+			length++
+		}
+		nBytes := copy(to, bytes)
+		to = to[nBytes:]
+		length += nBytes
+	} else if n.Sign() == 0 {
+		// A zero is the zero length string
+	} else {
+		bytes := n.Bytes()
+		if len(bytes) > 0 && bytes[0]&0x80 != 0 {
+			// We'll have to pad this with a 0x00 in order to
+			// stop it looking like a negative number.
+			to[0] = 0
+			to = to[1:]
+			length++
+		}
+		nBytes := copy(to, bytes)
+		to = to[nBytes:]
+		length += nBytes
+	}
+
+	lengthBytes[0] = byte(length >> 24)
+	lengthBytes[1] = byte(length >> 16)
+	lengthBytes[2] = byte(length >> 8)
+	lengthBytes[3] = byte(length)
+	return to
+}
+
+func writeInt(w io.Writer, n *big.Int) {
+	length := intLength(n)
+	buf := make([]byte, length)
+	marshalInt(buf, n)
+	w.Write(buf)
+}
+
+func writeString(w io.Writer, s []byte) {
+	var lengthBytes [4]byte
+	lengthBytes[0] = byte(len(s) >> 24)
+	lengthBytes[1] = byte(len(s) >> 16)
+	lengthBytes[2] = byte(len(s) >> 8)
+	lengthBytes[3] = byte(len(s))
+	w.Write(lengthBytes[:])
+	w.Write(s)
+}
+
+func stringLength(n int) int {
+	return 4 + n
+}
+
+func marshalString(to []byte, s []byte) []byte {
+	to[0] = byte(len(s) >> 24)
+	to[1] = byte(len(s) >> 16)
+	to[2] = byte(len(s) >> 8)
+	to[3] = byte(len(s))
+	to = to[4:]
+	copy(to, s)
+	return to[len(s):]
+}
+
+var bigIntType = reflect.TypeOf((*big.Int)(nil))
+
+// Decode a packet into its corresponding message.
+func decode(packet []byte) (interface{}, error) {
+	var msg interface{}
+	switch packet[0] {
+	case msgDisconnect:
+		msg = new(disconnectMsg)
+	case msgServiceRequest:
+		msg = new(serviceRequestMsg)
+	case msgServiceAccept:
+		msg = new(serviceAcceptMsg)
+	case msgExtInfo:
+		msg = new(extInfoMsg)
+	case msgKexInit:
+		msg = new(kexInitMsg)
+	case msgKexDHInit:
+		msg = new(kexDHInitMsg)
+	case msgKexDHReply:
+		msg = new(kexDHReplyMsg)
+	case msgUserAuthRequest:
+		msg = new(userAuthRequestMsg)
+	case msgUserAuthSuccess:
+		return new(userAuthSuccessMsg), nil
+	case msgUserAuthFailure:
+		msg = new(userAuthFailureMsg)
+	case msgUserAuthPubKeyOk:
+		msg = new(userAuthPubKeyOkMsg)
+	case msgGlobalRequest:
+		msg = new(globalRequestMsg)
+	case msgRequestSuccess:
+		msg = new(globalRequestSuccessMsg)
+	case msgRequestFailure:
+		msg = new(globalRequestFailureMsg)
+	case msgChannelOpen:
+		msg = new(channelOpenMsg)
+	case msgChannelData:
+		msg = new(channelDataMsg)
+	case msgChannelOpenConfirm:
+		msg = new(channelOpenConfirmMsg)
+	case msgChannelOpenFailure:
+		msg = new(channelOpenFailureMsg)
+	case msgChannelWindowAdjust:
+		msg = new(windowAdjustMsg)
+	case msgChannelEOF:
+		msg = new(channelEOFMsg)
+	case msgChannelClose:
+		msg = new(channelCloseMsg)
+	case msgChannelRequest:
+		msg = new(channelRequestMsg)
+	case msgChannelSuccess:
+		msg = new(channelRequestSuccessMsg)
+	case msgChannelFailure:
+		msg = new(channelRequestFailureMsg)
+	case msgUserAuthGSSAPIToken:
+		msg = new(userAuthGSSAPIToken)
+	case msgUserAuthGSSAPIMIC:
+		msg = new(userAuthGSSAPIMIC)
+	case msgUserAuthGSSAPIErrTok:
+		msg = new(userAuthGSSAPIErrTok)
+	case msgUserAuthGSSAPIError:
+		msg = new(userAuthGSSAPIError)
+	default:
+		return nil, unexpectedMessageError(0, packet[0])
+	}
+	if err := Unmarshal(packet, msg); err != nil {
+		return nil, err
+	}
+	return msg, nil
+}
+
+var packetTypeNames = map[byte]string{
+	msgDisconnect:          "disconnectMsg",
+	msgServiceRequest:      "serviceRequestMsg",
+	msgServiceAccept:       "serviceAcceptMsg",
+	msgExtInfo:             "extInfoMsg",
+	msgKexInit:             "kexInitMsg",
+	msgKexDHInit:           "kexDHInitMsg",
+	msgKexDHReply:          "kexDHReplyMsg",
+	msgUserAuthRequest:     "userAuthRequestMsg",
+	msgUserAuthSuccess:     "userAuthSuccessMsg",
+	msgUserAuthFailure:     "userAuthFailureMsg",
+	msgUserAuthPubKeyOk:    "userAuthPubKeyOkMsg",
+	msgGlobalRequest:       "globalRequestMsg",
+	msgRequestSuccess:      "globalRequestSuccessMsg",
+	msgRequestFailure:      "globalRequestFailureMsg",
+	msgChannelOpen:         "channelOpenMsg",
+	msgChannelData:         "channelDataMsg",
+	msgChannelOpenConfirm:  "channelOpenConfirmMsg",
+	msgChannelOpenFailure:  "channelOpenFailureMsg",
+	msgChannelWindowAdjust: "windowAdjustMsg",
+	msgChannelEOF:          "channelEOFMsg",
+	msgChannelClose:        "channelCloseMsg",
+	msgChannelRequest:      "channelRequestMsg",
+	msgChannelSuccess:      "channelRequestSuccessMsg",
+	msgChannelFailure:      "channelRequestFailureMsg",
+}

pkg/proto/ssh/mux.go

@@ -0,0 +1,357 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+	"log"
+	"sync"
+	"sync/atomic"
+)
+
+// debugMux, if set, causes messages in the connection protocol to be
+// logged.
+const debugMux = false
+
+// chanList is a thread safe channel list.
+type chanList struct {
+	// protects concurrent access to chans
+	sync.Mutex
+
+	// chans are indexed by the local id of the channel, which the
+	// other side should send in the PeersId field.
+	chans []*channel
+
+	// This is a debugging aid: it offsets all IDs by this
+	// amount. This helps distinguish otherwise identical
+	// server/client muxes
+	offset uint32
+}
+
+// Assigns a channel ID to the given channel.
+func (c *chanList) add(ch *channel) uint32 {
+	c.Lock()
+	defer c.Unlock()
+	for i := range c.chans {
+		if c.chans[i] == nil {
+			c.chans[i] = ch
+			return uint32(i) + c.offset
+		}
+	}
+	c.chans = append(c.chans, ch)
+	return uint32(len(c.chans)-1) + c.offset
+}
+
+// getChan returns the channel for the given ID.
+func (c *chanList) getChan(id uint32) *channel {
+	id -= c.offset
+
+	c.Lock()
+	defer c.Unlock()
+	if id < uint32(len(c.chans)) {
+		return c.chans[id]
+	}
+	return nil
+}
+
+func (c *chanList) remove(id uint32) {
+	id -= c.offset
+	c.Lock()
+	if id < uint32(len(c.chans)) {
+		c.chans[id] = nil
+	}
+	c.Unlock()
+}
+
+// dropAll forgets all channels it knows, returning them in a slice.
+func (c *chanList) dropAll() []*channel {
+	c.Lock()
+	defer c.Unlock()
+	var r []*channel
+
+	for _, ch := range c.chans {
+		if ch == nil {
+			continue
+		}
+		r = append(r, ch)
+	}
+	c.chans = nil
+	return r
+}
+
+// mux represents the state for the SSH connection protocol, which
+// multiplexes many channels onto a single packet transport.
+type mux struct {
+	conn     packetConn
+	chanList chanList
+
+	incomingChannels chan NewChannel
+
+	globalSentMu     sync.Mutex
+	globalResponses  chan interface{}
+	incomingRequests chan *Request
+
+	errCond *sync.Cond
+	err     error
+}
+
+// When debugging, each new chanList instantiation has a different
+// offset.
+var globalOff uint32
+
+func (m *mux) Wait() error {
+	m.errCond.L.Lock()
+	defer m.errCond.L.Unlock()
+	for m.err == nil {
+		m.errCond.Wait()
+	}
+	return m.err
+}
+
+// newMux returns a mux that runs over the given connection.
+func newMux(p packetConn) *mux {
+	m := &mux{
+		conn:             p,
+		incomingChannels: make(chan NewChannel, chanSize),
+		globalResponses:  make(chan interface{}, 1),
+		incomingRequests: make(chan *Request, chanSize),
+		errCond:          newCond(),
+	}
+	if debugMux {
+		m.chanList.offset = atomic.AddUint32(&globalOff, 1)
+	}
+
+	go m.loop()
+	return m
+}
+
+func (m *mux) sendMessage(msg interface{}) error {
+	p := Marshal(msg)
+	if debugMux {
+		log.Printf("send global(%d): %#v", m.chanList.offset, msg)
+	}
+	return m.conn.writePacket(p)
+}
+
+func (m *mux) SendRequest(name string, wantReply bool, payload []byte) (bool, []byte, error) {
+	if wantReply {
+		m.globalSentMu.Lock()
+		defer m.globalSentMu.Unlock()
+	}
+
+	if err := m.sendMessage(globalRequestMsg{
+		Type:      name,
+		WantReply: wantReply,
+		Data:      payload,
+	}); err != nil {
+		return false, nil, err
+	}
+
+	if !wantReply {
+		return false, nil, nil
+	}
+
+	msg, ok := <-m.globalResponses
+	if !ok {
+		return false, nil, io.EOF
+	}
+	switch msg := msg.(type) {
+	case *globalRequestFailureMsg:
+		return false, msg.Data, nil
+	case *globalRequestSuccessMsg:
+		return true, msg.Data, nil
+	default:
+		return false, nil, fmt.Errorf("ssh: unexpected response to request: %#v", msg)
+	}
+}
+
+// ackRequest must be called after processing a global request that
+// has WantReply set.
+func (m *mux) ackRequest(ok bool, data []byte) error {
+	if ok {
+		return m.sendMessage(globalRequestSuccessMsg{Data: data})
+	}
+	return m.sendMessage(globalRequestFailureMsg{Data: data})
+}
+
+func (m *mux) Close() error {
+	return m.conn.Close()
+}
+
+// loop runs the connection machine. It will process packets until an
+// error is encountered. To synchronize on loop exit, use mux.Wait.
+func (m *mux) loop() {
+	var err error
+	for err == nil {
+		err = m.onePacket()
+	}
+
+	for _, ch := range m.chanList.dropAll() {
+		ch.close()
+	}
+
+	close(m.incomingChannels)
+	close(m.incomingRequests)
+	close(m.globalResponses)
+
+	m.conn.Close()
+
+	m.errCond.L.Lock()
+	m.err = err
+	m.errCond.Broadcast()
+	m.errCond.L.Unlock()
+
+	if debugMux {
+		log.Println("loop exit", err)
+	}
+}
+
+// onePacket reads and processes one packet.
+func (m *mux) onePacket() error {
+	packet, err := m.conn.readPacket()
+	if err != nil {
+		return err
+	}
+
+	if debugMux {
+		if packet[0] == msgChannelData || packet[0] == msgChannelExtendedData {
+			log.Printf("decoding(%d): data packet - %d bytes", m.chanList.offset, len(packet))
+		} else {
+			p, _ := decode(packet)
+			log.Printf("decoding(%d): %d %#v - %d bytes", m.chanList.offset, packet[0], p, len(packet))
+		}
+	}
+
+	switch packet[0] {
+	case msgChannelOpen:
+		return m.handleChannelOpen(packet)
+	case msgGlobalRequest, msgRequestSuccess, msgRequestFailure:
+		return m.handleGlobalPacket(packet)
+	case msgPing:
+		var msg pingMsg
+		if err := Unmarshal(packet, &msg); err != nil {
+			return fmt.Errorf("failed to unmarshal ping@openssh.com message: %w", err)
+		}
+		return m.sendMessage(pongMsg(msg))
+	}
+
+	// assume a channel packet.
+	if len(packet) < 5 {
+		return parseError(packet[0])
+	}
+	id := binary.BigEndian.Uint32(packet[1:])
+	ch := m.chanList.getChan(id)
+	if ch == nil {
+		return m.handleUnknownChannelPacket(id, packet)
+	}
+
+	return ch.handlePacket(packet)
+}
+
+func (m *mux) handleGlobalPacket(packet []byte) error {
+	msg, err := decode(packet)
+	if err != nil {
+		return err
+	}
+
+	switch msg := msg.(type) {
+	case *globalRequestMsg:
+		m.incomingRequests <- &Request{
+			Type:      msg.Type,
+			WantReply: msg.WantReply,
+			Payload:   msg.Data,
+			mux:       m,
+		}
+	case *globalRequestSuccessMsg, *globalRequestFailureMsg:
+		m.globalResponses <- msg
+	default:
+		panic(fmt.Sprintf("not a global message %#v", msg))
+	}
+
+	return nil
+}
+
+// handleChannelOpen schedules a channel to be Accept()ed.
+func (m *mux) handleChannelOpen(packet []byte) error {
+	var msg channelOpenMsg
+	if err := Unmarshal(packet, &msg); err != nil {
+		return err
+	}
+
+	if msg.MaxPacketSize < minPacketLength || msg.MaxPacketSize > 1<<31 {
+		failMsg := channelOpenFailureMsg{
+			PeersID:  msg.PeersID,
+			Reason:   ConnectionFailed,
+			Message:  "invalid request",
+			Language: "en_US.UTF-8",
+		}
+		return m.sendMessage(failMsg)
+	}
+
+	c := m.newChannel(msg.ChanType, channelInbound, msg.TypeSpecificData)
+	c.remoteId = msg.PeersID
+	c.maxRemotePayload = msg.MaxPacketSize
+	c.remoteWin.add(msg.PeersWindow)
+	m.incomingChannels <- c
+	return nil
+}
+
+func (m *mux) OpenChannel(chanType string, extra []byte) (Channel, <-chan *Request, error) {
+	ch, err := m.openChannel(chanType, extra)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return ch, ch.incomingRequests, nil
+}
+
+func (m *mux) openChannel(chanType string, extra []byte) (*channel, error) {
+	ch := m.newChannel(chanType, channelOutbound, extra)
+
+	ch.maxIncomingPayload = channelMaxPacket
+
+	open := channelOpenMsg{
+		ChanType:         chanType,
+		PeersWindow:      ch.myWindow,
+		MaxPacketSize:    ch.maxIncomingPayload,
+		TypeSpecificData: extra,
+		PeersID:          ch.localId,
+	}
+	if err := m.sendMessage(open); err != nil {
+		return nil, err
+	}
+
+	switch msg := (<-ch.msg).(type) {
+	case *channelOpenConfirmMsg:
+		return ch, nil
+	case *channelOpenFailureMsg:
+		return nil, &OpenChannelError{msg.Reason, msg.Message}
+	default:
+		return nil, fmt.Errorf("ssh: unexpected packet in response to channel open: %T", msg)
+	}
+}
+
+func (m *mux) handleUnknownChannelPacket(id uint32, packet []byte) error {
+	msg, err := decode(packet)
+	if err != nil {
+		return err
+	}
+
+	switch msg := msg.(type) {
+	// RFC 4254 section 5.4 says unrecognized channel requests should
+	// receive a failure response.
+	case *channelRequestMsg:
+		if msg.WantReply {
+			return m.sendMessage(channelRequestFailureMsg{
+				PeersID: msg.PeersID,
+			})
+		}
+		return nil
+	default:
+		return fmt.Errorf("ssh: invalid channel %d", id)
+	}
+}

pkg/proto/ssh/server.go

@@ -0,0 +1,800 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"strings"
+)
+
+// The Permissions type holds fine-grained permissions that are
+// specific to a user or a specific authentication method for a user.
+// The Permissions value for a successful authentication attempt is
+// available in ServerConn, so it can be used to pass information from
+// the user-authentication phase to the application layer.
+type Permissions struct {
+	// CriticalOptions indicate restrictions to the default
+	// permissions, and are typically used in conjunction with
+	// user certificates. The standard for SSH certificates
+	// defines "force-command" (only allow the given command to
+	// execute) and "source-address" (only allow connections from
+	// the given address). The SSH package currently only enforces
+	// the "source-address" critical option. It is up to server
+	// implementations to enforce other critical options, such as
+	// "force-command", by checking them after the SSH handshake
+	// is successful. In general, SSH servers should reject
+	// connections that specify critical options that are unknown
+	// or not supported.
+	CriticalOptions map[string]string
+
+	// Extensions are extra functionality that the server may
+	// offer on authenticated connections. Lack of support for an
+	// extension does not preclude authenticating a user. Common
+	// extensions are "permit-agent-forwarding",
+	// "permit-X11-forwarding". The Go SSH library currently does
+	// not act on any extension, and it is up to server
+	// implementations to honor them. Extensions can be used to
+	// pass data from the authentication callbacks to the server
+	// application layer.
+	Extensions map[string]string
+}
+
+type GSSAPIWithMICConfig struct {
+	// AllowLogin, must be set, is called when gssapi-with-mic
+	// authentication is selected (RFC 4462 section 3). The srcName is from the
+	// results of the GSS-API authentication. The format is username@DOMAIN.
+	// GSSAPI just guarantees to the server who the user is, but not if they can log in, and with what permissions.
+	// This callback is called after the user identity is established with GSSAPI to decide if the user can login with
+	// which permissions. If the user is allowed to login, it should return a nil error.
+	AllowLogin func(conn ConnMetadata, srcName string) (*Permissions, error)
+
+	// Server must be set. It's the implementation
+	// of the GSSAPIServer interface. See GSSAPIServer interface for details.
+	Server GSSAPIServer
+}
+
+// ServerConfig holds server specific configuration data.
+type ServerConfig struct {
+	// Config contains configuration shared between client and server.
+	Config
+
+	// PublicKeyAuthAlgorithms specifies the supported client public key
+	// authentication algorithms. Note that this should not include certificate
+	// types since those use the underlying algorithm. This list is sent to the
+	// client if it supports the server-sig-algs extension. Order is irrelevant.
+	// If unspecified then a default set of algorithms is used.
+	PublicKeyAuthAlgorithms []string
+
+	hostKeys []Signer
+
+	// NoClientAuth is true if clients are allowed to connect without
+	// authenticating.
+	// To determine NoClientAuth at runtime, set NoClientAuth to true
+	// and the optional NoClientAuthCallback to a non-nil value.
+	NoClientAuth bool
+
+	// NoClientAuthCallback, if non-nil, is called when a user
+	// attempts to authenticate with auth method "none".
+	// NoClientAuth must also be set to true for this be used, or
+	// this func is unused.
+	NoClientAuthCallback func(ConnMetadata) (*Permissions, error)
+
+	// MaxAuthTries specifies the maximum number of authentication attempts
+	// permitted per connection. If set to a negative number, the number of
+	// attempts are unlimited. If set to zero, the number of attempts are limited
+	// to 6.
+	MaxAuthTries int
+
+	// PasswordCallback, if non-nil, is called when a user
+	// attempts to authenticate using a password.
+	PasswordCallback func(conn ConnMetadata, password []byte) (*Permissions, error)
+
+	// PublicKeyCallback, if non-nil, is called when a client
+	// offers a public key for authentication. It must return a nil error
+	// if the given public key can be used to authenticate the
+	// given user. For example, see CertChecker.Authenticate. A
+	// call to this function does not guarantee that the key
+	// offered is in fact used to authenticate. To record any data
+	// depending on the public key, store it inside a
+	// Permissions.Extensions entry.
+	PublicKeyCallback func(conn ConnMetadata, key PublicKey) (*Permissions, error)
+
+	// KeyboardInteractiveCallback, if non-nil, is called when
+	// keyboard-interactive authentication is selected (RFC
+	// 4256). The client object's Challenge function should be
+	// used to query the user. The callback may offer multiple
+	// Challenge rounds. To avoid information leaks, the client
+	// should be presented a challenge even if the user is
+	// unknown.
+	KeyboardInteractiveCallback func(conn ConnMetadata, client KeyboardInteractiveChallenge) (*Permissions, error)
+
+	// AuthLogCallback, if non-nil, is called to log all authentication
+	// attempts.
+	AuthLogCallback func(conn ConnMetadata, method string, err error)
+
+	// ServerVersion is the version identification string to announce in
+	// the public handshake.
+	// If empty, a reasonable default is used.
+	// Note that RFC 4253 section 4.2 requires that this string start with
+	// "SSH-2.0-".
+	ServerVersion string
+
+	// BannerCallback, if present, is called and the return string is sent to
+	// the client after key exchange completed but before authentication.
+	BannerCallback func(conn ConnMetadata) string
+
+	// GSSAPIWithMICConfig includes gssapi server and callback, which if both non-nil, is used
+	// when gssapi-with-mic authentication is selected (RFC 4462 section 3).
+	GSSAPIWithMICConfig *GSSAPIWithMICConfig
+}
+
+// AddHostKey adds a private key as a host key. If an existing host
+// key exists with the same public key format, it is replaced. Each server
+// config must have at least one host key.
+func (s *ServerConfig) AddHostKey(key Signer) {
+	for i, k := range s.hostKeys {
+		if k.PublicKey().Type() == key.PublicKey().Type() {
+			s.hostKeys[i] = key
+			return
+		}
+	}
+
+	s.hostKeys = append(s.hostKeys, key)
+}
+
+// cachedPubKey contains the results of querying whether a public key is
+// acceptable for a user.
+type cachedPubKey struct {
+	user       string
+	pubKeyData []byte
+	result     error
+	perms      *Permissions
+}
+
+const maxCachedPubKeys = 16
+
+// pubKeyCache caches tests for public keys.  Since SSH clients
+// will query whether a public key is acceptable before attempting to
+// authenticate with it, we end up with duplicate queries for public
+// key validity.  The cache only applies to a single ServerConn.
+type pubKeyCache struct {
+	keys []cachedPubKey
+}
+
+// get returns the result for a given user/algo/key tuple.
+func (c *pubKeyCache) get(user string, pubKeyData []byte) (cachedPubKey, bool) {
+	for _, k := range c.keys {
+		if k.user == user && bytes.Equal(k.pubKeyData, pubKeyData) {
+			return k, true
+		}
+	}
+	return cachedPubKey{}, false
+}
+
+// add adds the given tuple to the cache.
+func (c *pubKeyCache) add(candidate cachedPubKey) {
+	if len(c.keys) < maxCachedPubKeys {
+		c.keys = append(c.keys, candidate)
+	}
+}
+
+// ServerConn is an authenticated SSH connection, as seen from the
+// server
+type ServerConn struct {
+	Conn
+
+	// If the succeeding authentication callback returned a
+	// non-nil Permissions pointer, it is stored here.
+	Permissions *Permissions
+}
+
+// NewServerConn starts a new SSH server with c as the underlying
+// transport.  It starts with a handshake and, if the handshake is
+// unsuccessful, it closes the connection and returns an error.  The
+// Request and NewChannel channels must be serviced, or the connection
+// will hang.
+//
+// The returned error may be of type *ServerAuthError for
+// authentication errors.
+func NewServerConn(c net.Conn, config *ServerConfig) (*ServerConn, <-chan NewChannel, <-chan *Request, error) {
+	fullConf := *config
+	fullConf.SetDefaults()
+	if fullConf.MaxAuthTries == 0 {
+		fullConf.MaxAuthTries = 6
+	}
+	if len(fullConf.PublicKeyAuthAlgorithms) == 0 {
+		fullConf.PublicKeyAuthAlgorithms = supportedPubKeyAuthAlgos
+	} else {
+		for _, algo := range fullConf.PublicKeyAuthAlgorithms {
+			if !contains(supportedPubKeyAuthAlgos, algo) {
+				return nil, nil, nil, fmt.Errorf("ssh: unsupported public key authentication algorithm %s", algo)
+			}
+		}
+	}
+	// Check if the config contains any unsupported key exchanges
+	for _, kex := range fullConf.KeyExchanges {
+		if _, ok := serverForbiddenKexAlgos[kex]; ok {
+			return nil, nil, nil, fmt.Errorf("ssh: unsupported key exchange %s for server", kex)
+		}
+	}
+
+	s := &connection{
+		sshConn: sshConn{conn: c},
+	}
+	perms, err := s.serverHandshake(&fullConf)
+	if err != nil {
+		c.Close()
+		return nil, nil, nil, err
+	}
+	return &ServerConn{s, perms}, s.mux.incomingChannels, s.mux.incomingRequests, nil
+}
+
+// signAndMarshal signs the data with the appropriate algorithm,
+// and serializes the result in SSH wire format. algo is the negotiate
+// algorithm and may be a certificate type.
+func signAndMarshal(k AlgorithmSigner, rand io.Reader, data []byte, algo string) ([]byte, error) {
+	sig, err := k.SignWithAlgorithm(rand, data, underlyingAlgo(algo))
+	if err != nil {
+		return nil, err
+	}
+
+	return Marshal(sig), nil
+}
+
+// handshake performs key exchange and user authentication.
+func (s *connection) serverHandshake(config *ServerConfig) (*Permissions, error) {
+	if len(config.hostKeys) == 0 {
+		return nil, errors.New("ssh: server has no host keys")
+	}
+
+	if !config.NoClientAuth && config.PasswordCallback == nil && config.PublicKeyCallback == nil &&
+		config.KeyboardInteractiveCallback == nil && (config.GSSAPIWithMICConfig == nil ||
+		config.GSSAPIWithMICConfig.AllowLogin == nil || config.GSSAPIWithMICConfig.Server == nil) {
+		return nil, errors.New("ssh: no authentication methods configured but NoClientAuth is also false")
+	}
+
+	if config.ServerVersion != "" {
+		s.serverVersion = []byte(config.ServerVersion)
+	} else {
+		s.serverVersion = []byte(packageVersion)
+	}
+	var err error
+	s.clientVersion, err = exchangeVersions(s.sshConn.conn, s.serverVersion)
+	if err != nil {
+		return nil, err
+	}
+
+	tr := newTransport(s.sshConn.conn, config.Rand, false /* not client */)
+	s.transport = newServerTransport(tr, s.clientVersion, s.serverVersion, config)
+
+	if err := s.transport.waitSession(); err != nil {
+		return nil, err
+	}
+
+	// We just did the key change, so the session ID is established.
+	s.sessionID = s.transport.getSessionID()
+
+	var packet []byte
+	if packet, err = s.transport.readPacket(); err != nil {
+		return nil, err
+	}
+
+	var serviceRequest serviceRequestMsg
+	if err = Unmarshal(packet, &serviceRequest); err != nil {
+		return nil, err
+	}
+	if serviceRequest.Service != serviceUserAuth {
+		return nil, errors.New("ssh: requested service '" + serviceRequest.Service + "' before authenticating")
+	}
+	serviceAccept := serviceAcceptMsg{
+		Service: serviceUserAuth,
+	}
+	if err := s.transport.writePacket(Marshal(&serviceAccept)); err != nil {
+		return nil, err
+	}
+
+	perms, err := s.serverAuthenticate(config)
+	if err != nil {
+		return nil, err
+	}
+	s.mux = newMux(s.transport)
+	return perms, err
+}
+
+func checkSourceAddress(addr net.Addr, sourceAddrs string) error {
+	if addr == nil {
+		return errors.New("ssh: no address known for client, but source-address match required")
+	}
+
+	tcpAddr, ok := addr.(*net.TCPAddr)
+	if !ok {
+		return fmt.Errorf("ssh: remote address %v is not an TCP address when checking source-address match", addr)
+	}
+
+	for _, sourceAddr := range strings.Split(sourceAddrs, ",") {
+		if allowedIP := net.ParseIP(sourceAddr); allowedIP != nil {
+			if allowedIP.Equal(tcpAddr.IP) {
+				return nil
+			}
+		} else {
+			_, ipNet, err := net.ParseCIDR(sourceAddr)
+			if err != nil {
+				return fmt.Errorf("ssh: error parsing source-address restriction %q: %v", sourceAddr, err)
+			}
+
+			if ipNet.Contains(tcpAddr.IP) {
+				return nil
+			}
+		}
+	}
+
+	return fmt.Errorf("ssh: remote address %v is not allowed because of source-address restriction", addr)
+}
+
+func gssExchangeToken(gssapiConfig *GSSAPIWithMICConfig, token []byte, s *connection,
+	sessionID []byte, userAuthReq userAuthRequestMsg) (authErr error, perms *Permissions, err error) {
+	gssAPIServer := gssapiConfig.Server
+	defer gssAPIServer.DeleteSecContext()
+	var srcName string
+	for {
+		var (
+			outToken     []byte
+			needContinue bool
+		)
+		outToken, srcName, needContinue, err = gssAPIServer.AcceptSecContext(token)
+		if err != nil {
+			return err, nil, nil
+		}
+		if len(outToken) != 0 {
+			if err := s.transport.writePacket(Marshal(&userAuthGSSAPIToken{
+				Token: outToken,
+			})); err != nil {
+				return nil, nil, err
+			}
+		}
+		if !needContinue {
+			break
+		}
+		packet, err := s.transport.readPacket()
+		if err != nil {
+			return nil, nil, err
+		}
+		userAuthGSSAPITokenReq := &userAuthGSSAPIToken{}
+		if err := Unmarshal(packet, userAuthGSSAPITokenReq); err != nil {
+			return nil, nil, err
+		}
+		token = userAuthGSSAPITokenReq.Token
+	}
+	packet, err := s.transport.readPacket()
+	if err != nil {
+		return nil, nil, err
+	}
+	userAuthGSSAPIMICReq := &userAuthGSSAPIMIC{}
+	if err := Unmarshal(packet, userAuthGSSAPIMICReq); err != nil {
+		return nil, nil, err
+	}
+	mic := buildMIC(string(sessionID), userAuthReq.User, userAuthReq.Service, userAuthReq.Method)
+	if err := gssAPIServer.VerifyMIC(mic, userAuthGSSAPIMICReq.MIC); err != nil {
+		return err, nil, nil
+	}
+	perms, authErr = gssapiConfig.AllowLogin(s, srcName)
+	return authErr, perms, nil
+}
+
+// isAlgoCompatible checks if the signature format is compatible with the
+// selected algorithm taking into account edge cases that occur with old
+// clients.
+func isAlgoCompatible(algo, sigFormat string) bool {
+	// Compatibility for old clients.
+	//
+	// For certificate authentication with OpenSSH 7.2-7.7 signature format can
+	// be rsa-sha2-256 or rsa-sha2-512 for the algorithm
+	// ssh-rsa-cert-v01@openssh.com.
+	//
+	// With gpg-agent < 2.2.6 the algorithm can be rsa-sha2-256 or rsa-sha2-512
+	// for signature format ssh-rsa.
+	if isRSA(algo) && isRSA(sigFormat) {
+		return true
+	}
+	// Standard case: the underlying algorithm must match the signature format.
+	return underlyingAlgo(algo) == sigFormat
+}
+
+// ServerAuthError represents server authentication errors and is
+// sometimes returned by NewServerConn. It appends any authentication
+// errors that may occur, and is returned if all of the authentication
+// methods provided by the user failed to authenticate.
+type ServerAuthError struct {
+	// Errors contains authentication errors returned by the authentication
+	// callback methods. The first entry is typically ErrNoAuth.
+	Errors []error
+}
+
+func (l ServerAuthError) Error() string {
+	var errs []string
+	for _, err := range l.Errors {
+		errs = append(errs, err.Error())
+	}
+	return "[" + strings.Join(errs, ", ") + "]"
+}
+
+// ErrNoAuth is the error value returned if no
+// authentication method has been passed yet. This happens as a normal
+// part of the authentication loop, since the client first tries
+// 'none' authentication to discover available methods.
+// It is returned in ServerAuthError.Errors from NewServerConn.
+var ErrNoAuth = errors.New("ssh: no auth passed yet")
+
+func (s *connection) serverAuthenticate(config *ServerConfig) (*Permissions, error) {
+	sessionID := s.transport.getSessionID()
+	var cache pubKeyCache
+	var perms *Permissions
+
+	authFailures := 0
+	var authErrs []error
+	var displayedBanner bool
+
+userAuthLoop:
+	for {
+		if authFailures >= config.MaxAuthTries && config.MaxAuthTries > 0 {
+			discMsg := &disconnectMsg{
+				Reason:  2,
+				Message: "too many authentication failures",
+			}
+
+			if err := s.transport.writePacket(Marshal(discMsg)); err != nil {
+				return nil, err
+			}
+
+			return nil, discMsg
+		}
+
+		var userAuthReq userAuthRequestMsg
+		if packet, err := s.transport.readPacket(); err != nil {
+			if err == io.EOF {
+				return nil, &ServerAuthError{Errors: authErrs}
+			}
+			return nil, err
+		} else if err = Unmarshal(packet, &userAuthReq); err != nil {
+			return nil, err
+		}
+
+		if userAuthReq.Service != serviceSSH {
+			return nil, errors.New("ssh: client attempted to negotiate for unknown service: " + userAuthReq.Service)
+		}
+
+		s.user = userAuthReq.User
+
+		if !displayedBanner && config.BannerCallback != nil {
+			displayedBanner = true
+			msg := config.BannerCallback(s)
+			if msg != "" {
+				bannerMsg := &userAuthBannerMsg{
+					Message: msg,
+				}
+				if err := s.transport.writePacket(Marshal(bannerMsg)); err != nil {
+					return nil, err
+				}
+			}
+		}
+
+		perms = nil
+		authErr := ErrNoAuth
+
+		switch userAuthReq.Method {
+		case "none":
+			if config.NoClientAuth {
+				if config.NoClientAuthCallback != nil {
+					perms, authErr = config.NoClientAuthCallback(s)
+				} else {
+					authErr = nil
+				}
+			}
+
+			// allow initial attempt of 'none' without penalty
+			if authFailures == 0 {
+				authFailures--
+			}
+		case "password":
+			if config.PasswordCallback == nil {
+				authErr = errors.New("ssh: password auth not configured")
+				break
+			}
+			payload := userAuthReq.Payload
+			if len(payload) < 1 || payload[0] != 0 {
+				return nil, parseError(msgUserAuthRequest)
+			}
+			payload = payload[1:]
+			password, payload, ok := parseString(payload)
+			if !ok || len(payload) > 0 {
+				return nil, parseError(msgUserAuthRequest)
+			}
+
+			perms, authErr = config.PasswordCallback(s, password)
+		case "keyboard-interactive":
+			if config.KeyboardInteractiveCallback == nil {
+				authErr = errors.New("ssh: keyboard-interactive auth not configured")
+				break
+			}
+
+			prompter := &sshClientKeyboardInteractive{s}
+			perms, authErr = config.KeyboardInteractiveCallback(s, prompter.Challenge)
+		case "publickey":
+			if config.PublicKeyCallback == nil {
+				authErr = errors.New("ssh: publickey auth not configured")
+				break
+			}
+			payload := userAuthReq.Payload
+			if len(payload) < 1 {
+				return nil, parseError(msgUserAuthRequest)
+			}
+			isQuery := payload[0] == 0
+			payload = payload[1:]
+			algoBytes, payload, ok := parseString(payload)
+			if !ok {
+				return nil, parseError(msgUserAuthRequest)
+			}
+			algo := string(algoBytes)
+			if !contains(config.PublicKeyAuthAlgorithms, underlyingAlgo(algo)) {
+				authErr = fmt.Errorf("ssh: algorithm %q not accepted", algo)
+				break
+			}
+
+			pubKeyData, payload, ok := parseString(payload)
+			if !ok {
+				return nil, parseError(msgUserAuthRequest)
+			}
+
+			pubKey, err := ParsePublicKey(pubKeyData)
+			if err != nil {
+				return nil, err
+			}
+
+			candidate, ok := cache.get(s.user, pubKeyData)
+			if !ok {
+				candidate.user = s.user
+				candidate.pubKeyData = pubKeyData
+				candidate.perms, candidate.result = config.PublicKeyCallback(s, pubKey)
+				if candidate.result == nil && candidate.perms != nil && candidate.perms.CriticalOptions != nil && candidate.perms.CriticalOptions[sourceAddressCriticalOption] != "" {
+					candidate.result = checkSourceAddress(
+						s.RemoteAddr(),
+						candidate.perms.CriticalOptions[sourceAddressCriticalOption])
+				}
+				cache.add(candidate)
+			}
+
+			if isQuery {
+				// The client can query if the given public key
+				// would be okay.
+
+				if len(payload) > 0 {
+					return nil, parseError(msgUserAuthRequest)
+				}
+
+				if candidate.result == nil {
+					okMsg := userAuthPubKeyOkMsg{
+						Algo:   algo,
+						PubKey: pubKeyData,
+					}
+					if err = s.transport.writePacket(Marshal(&okMsg)); err != nil {
+						return nil, err
+					}
+					continue userAuthLoop
+				}
+				authErr = candidate.result
+			} else {
+				sig, payload, ok := parseSignature(payload)
+				if !ok || len(payload) > 0 {
+					return nil, parseError(msgUserAuthRequest)
+				}
+				// Ensure the declared public key algo is compatible with the
+				// decoded one. This check will ensure we don't accept e.g.
+				// ssh-rsa-cert-v01@openssh.com algorithm with ssh-rsa public
+				// key type. The algorithm and public key type must be
+				// consistent: both must be certificate algorithms, or neither.
+				if !contains(algorithmsForKeyFormat(pubKey.Type()), algo) {
+					authErr = fmt.Errorf("ssh: public key type %q not compatible with selected algorithm %q",
+						pubKey.Type(), algo)
+					break
+				}
+				// Ensure the public key algo and signature algo
+				// are supported.  Compare the private key
+				// algorithm name that corresponds to algo with
+				// sig.Format.  This is usually the same, but
+				// for certs, the names differ.
+				if !contains(config.PublicKeyAuthAlgorithms, sig.Format) {
+					authErr = fmt.Errorf("ssh: algorithm %q not accepted", sig.Format)
+					break
+				}
+				if !isAlgoCompatible(algo, sig.Format) {
+					authErr = fmt.Errorf("ssh: signature %q not compatible with selected algorithm %q", sig.Format, algo)
+					break
+				}
+
+				signedData := buildDataSignedForAuth(sessionID, userAuthReq, algo, pubKeyData)
+
+				if err := pubKey.Verify(signedData, sig); err != nil {
+					return nil, err
+				}
+
+				authErr = candidate.result
+				perms = candidate.perms
+			}
+		case "gssapi-with-mic":
+			if config.GSSAPIWithMICConfig == nil {
+				authErr = errors.New("ssh: gssapi-with-mic auth not configured")
+				break
+			}
+			gssapiConfig := config.GSSAPIWithMICConfig
+			userAuthRequestGSSAPI, err := parseGSSAPIPayload(userAuthReq.Payload)
+			if err != nil {
+				return nil, parseError(msgUserAuthRequest)
+			}
+			// OpenSSH supports Kerberos V5 mechanism only for GSS-API authentication.
+			if userAuthRequestGSSAPI.N == 0 {
+				authErr = fmt.Errorf("ssh: Mechanism negotiation is not supported")
+				break
+			}
+			var i uint32
+			present := false
+			for i = 0; i < userAuthRequestGSSAPI.N; i++ {
+				if userAuthRequestGSSAPI.OIDS[i].Equal(krb5Mesh) {
+					present = true
+					break
+				}
+			}
+			if !present {
+				authErr = fmt.Errorf("ssh: GSSAPI authentication must use the Kerberos V5 mechanism")
+				break
+			}
+			// Initial server response, see RFC 4462 section 3.3.
+			if err := s.transport.writePacket(Marshal(&userAuthGSSAPIResponse{
+				SupportMech: krb5OID,
+			})); err != nil {
+				return nil, err
+			}
+			// Exchange token, see RFC 4462 section 3.4.
+			packet, err := s.transport.readPacket()
+			if err != nil {
+				return nil, err
+			}
+			userAuthGSSAPITokenReq := &userAuthGSSAPIToken{}
+			if err := Unmarshal(packet, userAuthGSSAPITokenReq); err != nil {
+				return nil, err
+			}
+			authErr, perms, err = gssExchangeToken(gssapiConfig, userAuthGSSAPITokenReq.Token, s, sessionID,
+				userAuthReq)
+			if err != nil {
+				return nil, err
+			}
+		default:
+			authErr = fmt.Errorf("ssh: unknown method %q", userAuthReq.Method)
+		}
+
+		authErrs = append(authErrs, authErr)
+
+		if config.AuthLogCallback != nil {
+			config.AuthLogCallback(s, userAuthReq.Method, authErr)
+		}
+
+		if authErr == nil {
+			break userAuthLoop
+		}
+
+		authFailures++
+		if config.MaxAuthTries > 0 && authFailures >= config.MaxAuthTries {
+			// If we have hit the max attempts, don't bother sending the
+			// final SSH_MSG_USERAUTH_FAILURE message, since there are
+			// no more authentication methods which can be attempted,
+			// and this message may cause the client to re-attempt
+			// authentication while we send the disconnect message.
+			// Continue, and trigger the disconnect at the start of
+			// the loop.
+			//
+			// The SSH specification is somewhat confusing about this,
+			// RFC 4252 Section 5.1 requires each authentication failure
+			// be responded to with a respective SSH_MSG_USERAUTH_FAILURE
+			// message, but Section 4 says the server should disconnect
+			// after some number of attempts, but it isn't explicit which
+			// message should take precedence (i.e. should there be a failure
+			// message than a disconnect message, or if we are going to
+			// disconnect, should we only send that message.)
+			//
+			// Either way, OpenSSH disconnects immediately after the last
+			// failed authnetication attempt, and given they are typically
+			// considered the golden implementation it seems reasonable
+			// to match that behavior.
+			continue
+		}
+
+		var failureMsg userAuthFailureMsg
+		if config.PasswordCallback != nil {
+			failureMsg.Methods = append(failureMsg.Methods, "password")
+		}
+		if config.PublicKeyCallback != nil {
+			failureMsg.Methods = append(failureMsg.Methods, "publickey")
+		}
+		if config.KeyboardInteractiveCallback != nil {
+			failureMsg.Methods = append(failureMsg.Methods, "keyboard-interactive")
+		}
+		if config.GSSAPIWithMICConfig != nil && config.GSSAPIWithMICConfig.Server != nil &&
+			config.GSSAPIWithMICConfig.AllowLogin != nil {
+			failureMsg.Methods = append(failureMsg.Methods, "gssapi-with-mic")
+		}
+
+		if len(failureMsg.Methods) == 0 {
+			return nil, errors.New("ssh: no authentication methods configured but NoClientAuth is also false")
+		}
+
+		if err := s.transport.writePacket(Marshal(&failureMsg)); err != nil {
+			return nil, err
+		}
+	}
+
+	if err := s.transport.writePacket([]byte{msgUserAuthSuccess}); err != nil {
+		return nil, err
+	}
+	return perms, nil
+}
+
+// sshClientKeyboardInteractive implements a ClientKeyboardInteractive by
+// asking the client on the other side of a ServerConn.
+type sshClientKeyboardInteractive struct {
+	*connection
+}
+
+func (c *sshClientKeyboardInteractive) Challenge(name, instruction string, questions []string, echos []bool) (answers []string, err error) {
+	if len(questions) != len(echos) {
+		return nil, errors.New("ssh: echos and questions must have equal length")
+	}
+
+	var prompts []byte
+	for i := range questions {
+		prompts = appendString(prompts, questions[i])
+		prompts = appendBool(prompts, echos[i])
+	}
+
+	if err := c.transport.writePacket(Marshal(&userAuthInfoRequestMsg{
+		Name:        name,
+		Instruction: instruction,
+		NumPrompts:  uint32(len(questions)),
+		Prompts:     prompts,
+	})); err != nil {
+		return nil, err
+	}
+
+	packet, err := c.transport.readPacket()
+	if err != nil {
+		return nil, err
+	}
+	if packet[0] != msgUserAuthInfoResponse {
+		return nil, unexpectedMessageError(msgUserAuthInfoResponse, packet[0])
+	}
+	packet = packet[1:]
+
+	n, packet, ok := parseUint32(packet)
+	if !ok || int(n) != len(questions) {
+		return nil, parseError(msgUserAuthInfoResponse)
+	}
+
+	for i := uint32(0); i < n; i++ {
+		ans, rest, ok := parseString(packet)
+		if !ok {
+			return nil, parseError(msgUserAuthInfoResponse)
+		}
+
+		answers = append(answers, string(ans))
+		packet = rest
+	}
+	if len(packet) != 0 {
+		return nil, errors.New("ssh: junk at end of message")
+	}
+
+	return answers, nil
+}

pkg/proto/ssh/session.go

@@ -0,0 +1,647 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+// Session implements an interactive session described in
+// "RFC 4254, section 6".
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"sync"
+)
+
+type Signal string
+
+// POSIX signals as listed in RFC 4254 Section 6.10.
+const (
+	SIGABRT Signal = "ABRT"
+	SIGALRM Signal = "ALRM"
+	SIGFPE  Signal = "FPE"
+	SIGHUP  Signal = "HUP"
+	SIGILL  Signal = "ILL"
+	SIGINT  Signal = "INT"
+	SIGKILL Signal = "KILL"
+	SIGPIPE Signal = "PIPE"
+	SIGQUIT Signal = "QUIT"
+	SIGSEGV Signal = "SEGV"
+	SIGTERM Signal = "TERM"
+	SIGUSR1 Signal = "USR1"
+	SIGUSR2 Signal = "USR2"
+)
+
+var signals = map[Signal]int{
+	SIGABRT: 6,
+	SIGALRM: 14,
+	SIGFPE:  8,
+	SIGHUP:  1,
+	SIGILL:  4,
+	SIGINT:  2,
+	SIGKILL: 9,
+	SIGPIPE: 13,
+	SIGQUIT: 3,
+	SIGSEGV: 11,
+	SIGTERM: 15,
+}
+
+type TerminalModes map[uint8]uint32
+
+// POSIX terminal mode flags as listed in RFC 4254 Section 8.
+const (
+	tty_OP_END    = 0
+	VINTR         = 1
+	VQUIT         = 2
+	VERASE        = 3
+	VKILL         = 4
+	VEOF          = 5
+	VEOL          = 6
+	VEOL2         = 7
+	VSTART        = 8
+	VSTOP         = 9
+	VSUSP         = 10
+	VDSUSP        = 11
+	VREPRINT      = 12
+	VWERASE       = 13
+	VLNEXT        = 14
+	VFLUSH        = 15
+	VSWTCH        = 16
+	VSTATUS       = 17
+	VDISCARD      = 18
+	IGNPAR        = 30
+	PARMRK        = 31
+	INPCK         = 32
+	ISTRIP        = 33
+	INLCR         = 34
+	IGNCR         = 35
+	ICRNL         = 36
+	IUCLC         = 37
+	IXON          = 38
+	IXANY         = 39
+	IXOFF         = 40
+	IMAXBEL       = 41
+	IUTF8         = 42 // RFC 8160
+	ISIG          = 50
+	ICANON        = 51
+	XCASE         = 52
+	ECHO          = 53
+	ECHOE         = 54
+	ECHOK         = 55
+	ECHONL        = 56
+	NOFLSH        = 57
+	TOSTOP        = 58
+	IEXTEN        = 59
+	ECHOCTL       = 60
+	ECHOKE        = 61
+	PENDIN        = 62
+	OPOST         = 70
+	OLCUC         = 71
+	ONLCR         = 72
+	OCRNL         = 73
+	ONOCR         = 74
+	ONLRET        = 75
+	CS7           = 90
+	CS8           = 91
+	PARENB        = 92
+	PARODD        = 93
+	TTY_OP_ISPEED = 128
+	TTY_OP_OSPEED = 129
+)
+
+// A Session represents a connection to a remote command or shell.
+type Session struct {
+	// Stdin specifies the remote process's standard input.
+	// If Stdin is nil, the remote process reads from an empty
+	// bytes.Buffer.
+	Stdin io.Reader
+
+	// Stdout and Stderr specify the remote process's standard
+	// output and error.
+	//
+	// If either is nil, Run connects the corresponding file
+	// descriptor to an instance of io.Discard. There is a
+	// fixed amount of buffering that is shared for the two streams.
+	// If either blocks it may eventually cause the remote
+	// command to block.
+	Stdout io.Writer
+	Stderr io.Writer
+
+	ch        Channel // the channel backing this session
+	started   bool    // true once Start, Run or Shell is invoked.
+	copyFuncs []func() error
+	errors    chan error // one send per copyFunc
+
+	// true if pipe method is active
+	stdinpipe, stdoutpipe, stderrpipe bool
+
+	// stdinPipeWriter is non-nil if StdinPipe has not been called
+	// and Stdin was specified by the user; it is the write end of
+	// a pipe connecting Session.Stdin to the stdin channel.
+	stdinPipeWriter io.WriteCloser
+
+	exitStatus chan error
+}
+
+// SendRequest sends an out-of-band channel request on the SSH channel
+// underlying the session.
+func (s *Session) SendRequest(name string, wantReply bool, payload []byte) (bool, error) {
+	return s.ch.SendRequest(name, wantReply, payload)
+}
+
+func (s *Session) Close() error {
+	return s.ch.Close()
+}
+
+// RFC 4254 Section 6.4.
+type setenvRequest struct {
+	Name  string
+	Value string
+}
+
+// Setenv sets an environment variable that will be applied to any
+// command executed by Shell or Run.
+func (s *Session) Setenv(name, value string) error {
+	msg := setenvRequest{
+		Name:  name,
+		Value: value,
+	}
+	ok, err := s.ch.SendRequest("env", true, Marshal(&msg))
+	if err == nil && !ok {
+		err = errors.New("ssh: setenv failed")
+	}
+	return err
+}
+
+// RFC 4254 Section 6.2.
+type ptyRequestMsg struct {
+	Term     string
+	Columns  uint32
+	Rows     uint32
+	Width    uint32
+	Height   uint32
+	Modelist string
+}
+
+// RequestPty requests the association of a pty with the session on the remote host.
+func (s *Session) RequestPty(term string, h, w int, termmodes TerminalModes) error {
+	var tm []byte
+	for k, v := range termmodes {
+		kv := struct {
+			Key byte
+			Val uint32
+		}{k, v}
+
+		tm = append(tm, Marshal(&kv)...)
+	}
+	tm = append(tm, tty_OP_END)
+	req := ptyRequestMsg{
+		Term:     term,
+		Columns:  uint32(w),
+		Rows:     uint32(h),
+		Width:    uint32(w * 8),
+		Height:   uint32(h * 8),
+		Modelist: string(tm),
+	}
+	ok, err := s.ch.SendRequest("pty-req", true, Marshal(&req))
+	if err == nil && !ok {
+		err = errors.New("ssh: pty-req failed")
+	}
+	return err
+}
+
+// RFC 4254 Section 6.5.
+type subsystemRequestMsg struct {
+	Subsystem string
+}
+
+// RequestSubsystem requests the association of a subsystem with the session on the remote host.
+// A subsystem is a predefined command that runs in the background when the ssh session is initiated
+func (s *Session) RequestSubsystem(subsystem string) error {
+	msg := subsystemRequestMsg{
+		Subsystem: subsystem,
+	}
+	ok, err := s.ch.SendRequest("subsystem", true, Marshal(&msg))
+	if err == nil && !ok {
+		err = errors.New("ssh: subsystem request failed")
+	}
+	return err
+}
+
+// RFC 4254 Section 6.7.
+type ptyWindowChangeMsg struct {
+	Columns uint32
+	Rows    uint32
+	Width   uint32
+	Height  uint32
+}
+
+// WindowChange informs the remote host about a terminal window dimension change to h rows and w columns.
+func (s *Session) WindowChange(h, w int) error {
+	req := ptyWindowChangeMsg{
+		Columns: uint32(w),
+		Rows:    uint32(h),
+		Width:   uint32(w * 8),
+		Height:  uint32(h * 8),
+	}
+	_, err := s.ch.SendRequest("window-change", false, Marshal(&req))
+	return err
+}
+
+// RFC 4254 Section 6.9.
+type signalMsg struct {
+	Signal string
+}
+
+// Signal sends the given signal to the remote process.
+// sig is one of the SIG* constants.
+func (s *Session) Signal(sig Signal) error {
+	msg := signalMsg{
+		Signal: string(sig),
+	}
+
+	_, err := s.ch.SendRequest("signal", false, Marshal(&msg))
+	return err
+}
+
+// RFC 4254 Section 6.5.
+type execMsg struct {
+	Command string
+}
+
+// Start runs cmd on the remote host. Typically, the remote
+// server passes cmd to the shell for interpretation.
+// A Session only accepts one call to Run, Start or Shell.
+func (s *Session) Start(cmd string) error {
+	if s.started {
+		return errors.New("ssh: session already started")
+	}
+	req := execMsg{
+		Command: cmd,
+	}
+
+	ok, err := s.ch.SendRequest("exec", true, Marshal(&req))
+	if err == nil && !ok {
+		err = fmt.Errorf("ssh: command %v failed", cmd)
+	}
+	if err != nil {
+		return err
+	}
+	return s.start()
+}
+
+// Run runs cmd on the remote host. Typically, the remote
+// server passes cmd to the shell for interpretation.
+// A Session only accepts one call to Run, Start, Shell, Output,
+// or CombinedOutput.
+//
+// The returned error is nil if the command runs, has no problems
+// copying stdin, stdout, and stderr, and exits with a zero exit
+// status.
+//
+// If the remote server does not send an exit status, an error of type
+// *ExitMissingError is returned. If the command completes
+// unsuccessfully or is interrupted by a signal, the error is of type
+// *ExitError. Other error types may be returned for I/O problems.
+func (s *Session) Run(cmd string) error {
+	err := s.Start(cmd)
+	if err != nil {
+		return err
+	}
+	return s.Wait()
+}
+
+// Output runs cmd on the remote host and returns its standard output.
+func (s *Session) Output(cmd string) ([]byte, error) {
+	if s.Stdout != nil {
+		return nil, errors.New("ssh: Stdout already set")
+	}
+	var b bytes.Buffer
+	s.Stdout = &b
+	err := s.Run(cmd)
+	return b.Bytes(), err
+}
+
+type singleWriter struct {
+	b  bytes.Buffer
+	mu sync.Mutex
+}
+
+func (w *singleWriter) Write(p []byte) (int, error) {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	return w.b.Write(p)
+}
+
+// CombinedOutput runs cmd on the remote host and returns its combined
+// standard output and standard error.
+func (s *Session) CombinedOutput(cmd string) ([]byte, error) {
+	if s.Stdout != nil {
+		return nil, errors.New("ssh: Stdout already set")
+	}
+	if s.Stderr != nil {
+		return nil, errors.New("ssh: Stderr already set")
+	}
+	var b singleWriter
+	s.Stdout = &b
+	s.Stderr = &b
+	err := s.Run(cmd)
+	return b.b.Bytes(), err
+}
+
+// Shell starts a login shell on the remote host. A Session only
+// accepts one call to Run, Start, Shell, Output, or CombinedOutput.
+func (s *Session) Shell() error {
+	if s.started {
+		return errors.New("ssh: session already started")
+	}
+
+	ok, err := s.ch.SendRequest("shell", true, nil)
+	if err == nil && !ok {
+		return errors.New("ssh: could not start shell")
+	}
+	if err != nil {
+		return err
+	}
+	return s.start()
+}
+
+func (s *Session) start() error {
+	s.started = true
+
+	type F func(*Session)
+	for _, setupFd := range []F{(*Session).stdin, (*Session).stdout, (*Session).stderr} {
+		setupFd(s)
+	}
+
+	s.errors = make(chan error, len(s.copyFuncs))
+	for _, fn := range s.copyFuncs {
+		go func(fn func() error) {
+			s.errors <- fn()
+		}(fn)
+	}
+	return nil
+}
+
+// Wait waits for the remote command to exit.
+//
+// The returned error is nil if the command runs, has no problems
+// copying stdin, stdout, and stderr, and exits with a zero exit
+// status.
+//
+// If the remote server does not send an exit status, an error of type
+// *ExitMissingError is returned. If the command completes
+// unsuccessfully or is interrupted by a signal, the error is of type
+// *ExitError. Other error types may be returned for I/O problems.
+func (s *Session) Wait() error {
+	if !s.started {
+		return errors.New("ssh: session not started")
+	}
+	waitErr := <-s.exitStatus
+
+	if s.stdinPipeWriter != nil {
+		s.stdinPipeWriter.Close()
+	}
+	var copyError error
+	for range s.copyFuncs {
+		if err := <-s.errors; err != nil && copyError == nil {
+			copyError = err
+		}
+	}
+	if waitErr != nil {
+		return waitErr
+	}
+	return copyError
+}
+
+func (s *Session) wait(reqs <-chan *Request) error {
+	wm := Waitmsg{status: -1}
+	// Wait for msg channel to be closed before returning.
+	for msg := range reqs {
+		switch msg.Type {
+		case "exit-status":
+			wm.status = int(binary.BigEndian.Uint32(msg.Payload))
+		case "exit-signal":
+			var sigval struct {
+				Signal     string
+				CoreDumped bool
+				Error      string
+				Lang       string
+			}
+			if err := Unmarshal(msg.Payload, &sigval); err != nil {
+				return err
+			}
+
+			// Must sanitize strings?
+			wm.signal = sigval.Signal
+			wm.msg = sigval.Error
+			wm.lang = sigval.Lang
+		default:
+			// This handles keepalives and matches
+			// OpenSSH's behaviour.
+			if msg.WantReply {
+				msg.Reply(false, nil)
+			}
+		}
+	}
+	if wm.status == 0 {
+		return nil
+	}
+	if wm.status == -1 {
+		// exit-status was never sent from server
+		if wm.signal == "" {
+			// signal was not sent either.  RFC 4254
+			// section 6.10 recommends against this
+			// behavior, but it is allowed, so we let
+			// clients handle it.
+			return &ExitMissingError{}
+		}
+		wm.status = 128
+		if _, ok := signals[Signal(wm.signal)]; ok {
+			wm.status += signals[Signal(wm.signal)]
+		}
+	}
+
+	return &ExitError{wm}
+}
+
+// ExitMissingError is returned if a session is torn down cleanly, but
+// the server sends no confirmation of the exit status.
+type ExitMissingError struct{}
+
+func (e *ExitMissingError) Error() string {
+	return "wait: remote command exited without exit status or exit signal"
+}
+
+func (s *Session) stdin() {
+	if s.stdinpipe {
+		return
+	}
+	var stdin io.Reader
+	if s.Stdin == nil {
+		stdin = new(bytes.Buffer)
+	} else {
+		r, w := io.Pipe()
+		go func() {
+			_, err := io.Copy(w, s.Stdin)
+			w.CloseWithError(err)
+		}()
+		stdin, s.stdinPipeWriter = r, w
+	}
+	s.copyFuncs = append(s.copyFuncs, func() error {
+		_, err := io.Copy(s.ch, stdin)
+		if err1 := s.ch.CloseWrite(); err == nil && err1 != io.EOF {
+			err = err1
+		}
+		return err
+	})
+}
+
+func (s *Session) stdout() {
+	if s.stdoutpipe {
+		return
+	}
+	if s.Stdout == nil {
+		s.Stdout = io.Discard
+	}
+	s.copyFuncs = append(s.copyFuncs, func() error {
+		_, err := io.Copy(s.Stdout, s.ch)
+		return err
+	})
+}
+
+func (s *Session) stderr() {
+	if s.stderrpipe {
+		return
+	}
+	if s.Stderr == nil {
+		s.Stderr = io.Discard
+	}
+	s.copyFuncs = append(s.copyFuncs, func() error {
+		_, err := io.Copy(s.Stderr, s.ch.Stderr())
+		return err
+	})
+}
+
+// sessionStdin reroutes Close to CloseWrite.
+type sessionStdin struct {
+	io.Writer
+	ch Channel
+}
+
+func (s *sessionStdin) Close() error {
+	return s.ch.CloseWrite()
+}
+
+// StdinPipe returns a pipe that will be connected to the
+// remote command's standard input when the command starts.
+func (s *Session) StdinPipe() (io.WriteCloser, error) {
+	if s.Stdin != nil {
+		return nil, errors.New("ssh: Stdin already set")
+	}
+	if s.started {
+		return nil, errors.New("ssh: StdinPipe after process started")
+	}
+	s.stdinpipe = true
+	return &sessionStdin{s.ch, s.ch}, nil
+}
+
+// StdoutPipe returns a pipe that will be connected to the
+// remote command's standard output when the command starts.
+// There is a fixed amount of buffering that is shared between
+// stdout and stderr streams. If the StdoutPipe reader is
+// not serviced fast enough it may eventually cause the
+// remote command to block.
+func (s *Session) StdoutPipe() (io.Reader, error) {
+	if s.Stdout != nil {
+		return nil, errors.New("ssh: Stdout already set")
+	}
+	if s.started {
+		return nil, errors.New("ssh: StdoutPipe after process started")
+	}
+	s.stdoutpipe = true
+	return s.ch, nil
+}
+
+// StderrPipe returns a pipe that will be connected to the
+// remote command's standard error when the command starts.
+// There is a fixed amount of buffering that is shared between
+// stdout and stderr streams. If the StderrPipe reader is
+// not serviced fast enough it may eventually cause the
+// remote command to block.
+func (s *Session) StderrPipe() (io.Reader, error) {
+	if s.Stderr != nil {
+		return nil, errors.New("ssh: Stderr already set")
+	}
+	if s.started {
+		return nil, errors.New("ssh: StderrPipe after process started")
+	}
+	s.stderrpipe = true
+	return s.ch.Stderr(), nil
+}
+
+// newSession returns a new interactive session on the remote host.
+func newSession(ch Channel, reqs <-chan *Request) (*Session, error) {
+	s := &Session{
+		ch: ch,
+	}
+	s.exitStatus = make(chan error, 1)
+	go func() {
+		s.exitStatus <- s.wait(reqs)
+	}()
+
+	return s, nil
+}
+
+// An ExitError reports unsuccessful completion of a remote command.
+type ExitError struct {
+	Waitmsg
+}
+
+func (e *ExitError) Error() string {
+	return e.Waitmsg.String()
+}
+
+// Waitmsg stores the information about an exited remote command
+// as reported by Wait.
+type Waitmsg struct {
+	status int
+	signal string
+	msg    string
+	lang   string
+}
+
+// ExitStatus returns the exit status of the remote command.
+func (w Waitmsg) ExitStatus() int {
+	return w.status
+}
+
+// Signal returns the exit signal of the remote command if
+// it was terminated violently.
+func (w Waitmsg) Signal() string {
+	return w.signal
+}
+
+// Msg returns the exit message given by the remote command
+func (w Waitmsg) Msg() string {
+	return w.msg
+}
+
+// Lang returns the language tag. See RFC 3066
+func (w Waitmsg) Lang() string {
+	return w.lang
+}
+
+func (w Waitmsg) String() string {
+	str := fmt.Sprintf("Process exited with status %v", w.status)
+	if w.signal != "" {
+		str += fmt.Sprintf(" from signal %v", w.signal)
+	}
+	if w.msg != "" {
+		str += fmt.Sprintf(". Reason was: %v", w.msg)
+	}
+	return str
+}

pkg/proto/ssh/ssh_gss.go

@@ -0,0 +1,139 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"encoding/asn1"
+	"errors"
+)
+
+var krb5OID []byte
+
+func init() {
+	krb5OID, _ = asn1.Marshal(krb5Mesh)
+}
+
+// GSSAPIClient provides the API to plug-in GSSAPI authentication for client logins.
+type GSSAPIClient interface {
+	// InitSecContext initiates the establishment of a security context for GSS-API between the
+	// ssh client and ssh server. Initially the token parameter should be specified as nil.
+	// The routine may return a outputToken which should be transferred to
+	// the ssh server, where the ssh server will present it to
+	// AcceptSecContext. If no token need be sent, InitSecContext will indicate this by setting
+	// needContinue to false. To complete the context
+	// establishment, one or more reply tokens may be required from the ssh
+	// server;if so, InitSecContext will return a needContinue which is true.
+	// In this case, InitSecContext should be called again when the
+	// reply token is received from the ssh server, passing the reply
+	// token to InitSecContext via the token parameters.
+	// See RFC 2743 section 2.2.1 and RFC 4462 section 3.4.
+	InitSecContext(target string, token []byte, isGSSDelegCreds bool) (outputToken []byte, needContinue bool, err error)
+	// GetMIC generates a cryptographic MIC for the SSH2 message, and places
+	// the MIC in a token for transfer to the ssh server.
+	// The contents of the MIC field are obtained by calling GSS_GetMIC()
+	// over the following, using the GSS-API context that was just
+	// established:
+	//  string    session identifier
+	//  byte      SSH_MSG_USERAUTH_REQUEST
+	//  string    user name
+	//  string    service
+	//  string    "gssapi-with-mic"
+	// See RFC 2743 section 2.3.1 and RFC 4462 3.5.
+	GetMIC(micFiled []byte) ([]byte, error)
+	// Whenever possible, it should be possible for
+	// DeleteSecContext() calls to be successfully processed even
+	// if other calls cannot succeed, thereby enabling context-related
+	// resources to be released.
+	// In addition to deleting established security contexts,
+	// gss_delete_sec_context must also be able to delete "half-built"
+	// security contexts resulting from an incomplete sequence of
+	// InitSecContext()/AcceptSecContext() calls.
+	// See RFC 2743 section 2.2.3.
+	DeleteSecContext() error
+}
+
+// GSSAPIServer provides the API to plug in GSSAPI authentication for server logins.
+type GSSAPIServer interface {
+	// AcceptSecContext allows a remotely initiated security context between the application
+	// and a remote peer to be established by the ssh client. The routine may return a
+	// outputToken which should be transferred to the ssh client,
+	// where the ssh client will present it to InitSecContext.
+	// If no token need be sent, AcceptSecContext will indicate this
+	// by setting the needContinue to false. To
+	// complete the context establishment, one or more reply tokens may be
+	// required from the ssh client. if so, AcceptSecContext
+	// will return a needContinue which is true, in which case it
+	// should be called again when the reply token is received from the ssh
+	// client, passing the token to AcceptSecContext via the
+	// token parameters.
+	// The srcName return value is the authenticated username.
+	// See RFC 2743 section 2.2.2 and RFC 4462 section 3.4.
+	AcceptSecContext(token []byte) (outputToken []byte, srcName string, needContinue bool, err error)
+	// VerifyMIC verifies that a cryptographic MIC, contained in the token parameter,
+	// fits the supplied message is received from the ssh client.
+	// See RFC 2743 section 2.3.2.
+	VerifyMIC(micField []byte, micToken []byte) error
+	// Whenever possible, it should be possible for
+	// DeleteSecContext() calls to be successfully processed even
+	// if other calls cannot succeed, thereby enabling context-related
+	// resources to be released.
+	// In addition to deleting established security contexts,
+	// gss_delete_sec_context must also be able to delete "half-built"
+	// security contexts resulting from an incomplete sequence of
+	// InitSecContext()/AcceptSecContext() calls.
+	// See RFC 2743 section 2.2.3.
+	DeleteSecContext() error
+}
+
+var (
+	// OpenSSH supports Kerberos V5 mechanism only for GSS-API authentication,
+	// so we also support the krb5 mechanism only.
+	// See RFC 1964 section 1.
+	krb5Mesh = asn1.ObjectIdentifier{1, 2, 840, 113554, 1, 2, 2}
+)
+
+// The GSS-API authentication method is initiated when the client sends an SSH_MSG_USERAUTH_REQUEST
+// See RFC 4462 section 3.2.
+type userAuthRequestGSSAPI struct {
+	N    uint32
+	OIDS []asn1.ObjectIdentifier
+}
+
+func parseGSSAPIPayload(payload []byte) (*userAuthRequestGSSAPI, error) {
+	n, rest, ok := parseUint32(payload)
+	if !ok {
+		return nil, errors.New("parse uint32 failed")
+	}
+	s := &userAuthRequestGSSAPI{
+		N:    n,
+		OIDS: make([]asn1.ObjectIdentifier, n),
+	}
+	for i := 0; i < int(n); i++ {
+		var (
+			desiredMech []byte
+			err         error
+		)
+		desiredMech, rest, ok = parseString(rest)
+		if !ok {
+			return nil, errors.New("parse string failed")
+		}
+		if rest, err = asn1.Unmarshal(desiredMech, &s.OIDS[i]); err != nil {
+			return nil, err
+		}
+
+	}
+	return s, nil
+}
+
+// See RFC 4462 section 3.6.
+func buildMIC(sessionID string, username string, service string, authMethod string) []byte {
+	out := make([]byte, 0, 0)
+	out = appendString(out, sessionID)
+	out = append(out, msgUserAuthRequest)
+	out = appendString(out, username)
+	out = appendString(out, service)
+	out = appendString(out, authMethod)
+	return out
+}

pkg/proto/ssh/streamlocal.go

@@ -0,0 +1,116 @@
+package ssh
+
+import (
+	"errors"
+	"io"
+	"net"
+)
+
+// streamLocalChannelOpenDirectMsg is a struct used for SSH_MSG_CHANNEL_OPEN message
+// with "direct-streamlocal@openssh.com" string.
+//
+// See openssh-portable/PROTOCOL, section 2.4. connection: Unix domain socket forwarding
+// https://github.com/openssh/openssh-portable/blob/master/PROTOCOL#L235
+type streamLocalChannelOpenDirectMsg struct {
+	socketPath string
+	reserved0  string
+	reserved1  uint32
+}
+
+// forwardedStreamLocalPayload is a struct used for SSH_MSG_CHANNEL_OPEN message
+// with "forwarded-streamlocal@openssh.com" string.
+type forwardedStreamLocalPayload struct {
+	SocketPath string
+	Reserved0  string
+}
+
+// streamLocalChannelForwardMsg is a struct used for SSH2_MSG_GLOBAL_REQUEST message
+// with "streamlocal-forward@openssh.com"/"cancel-streamlocal-forward@openssh.com" string.
+type streamLocalChannelForwardMsg struct {
+	socketPath string
+}
+
+// ListenUnix is similar to ListenTCP but uses a Unix domain socket.
+func (c *Client) ListenUnix(socketPath string) (net.Listener, error) {
+	c.handleForwardsOnce.Do(c.handleForwards)
+	m := streamLocalChannelForwardMsg{
+		socketPath,
+	}
+	// send message
+	ok, _, err := c.SendRequest("streamlocal-forward@openssh.com", true, Marshal(&m))
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		return nil, errors.New("ssh: streamlocal-forward@openssh.com request denied by peer")
+	}
+	ch := c.forwards.add(&net.UnixAddr{Name: socketPath, Net: "unix"})
+
+	return &unixListener{socketPath, c, ch}, nil
+}
+
+func (c *Client) dialStreamLocal(socketPath string) (Channel, error) {
+	msg := streamLocalChannelOpenDirectMsg{
+		socketPath: socketPath,
+	}
+	ch, in, err := c.OpenChannel("direct-streamlocal@openssh.com", Marshal(&msg))
+	if err != nil {
+		return nil, err
+	}
+	go DiscardRequests(in)
+	return ch, err
+}
+
+type unixListener struct {
+	socketPath string
+
+	conn *Client
+	in   <-chan forward
+}
+
+// Accept waits for and returns the next connection to the listener.
+func (l *unixListener) Accept() (net.Conn, error) {
+	s, ok := <-l.in
+	if !ok {
+		return nil, io.EOF
+	}
+	ch, incoming, err := s.newCh.Accept()
+	if err != nil {
+		return nil, err
+	}
+	go DiscardRequests(incoming)
+
+	return &chanConn{
+		Channel: ch,
+		laddr: &net.UnixAddr{
+			Name: l.socketPath,
+			Net:  "unix",
+		},
+		raddr: &net.UnixAddr{
+			Name: "@",
+			Net:  "unix",
+		},
+	}, nil
+}
+
+// Close closes the listener.
+func (l *unixListener) Close() error {
+	// this also closes the listener.
+	l.conn.forwards.remove(&net.UnixAddr{Name: l.socketPath, Net: "unix"})
+	m := streamLocalChannelForwardMsg{
+		l.socketPath,
+	}
+	ok, _, err := l.conn.SendRequest("cancel-streamlocal-forward@openssh.com", true, Marshal(&m))
+	if err == nil && !ok {
+		err = errors.New("ssh: cancel-streamlocal-forward@openssh.com failed")
+	}
+	return err
+}
+
+// Addr returns the listener's network address.
+func (l *unixListener) Addr() net.Addr {
+	return &net.UnixAddr{
+		Name: l.socketPath,
+		Net:  "unix",
+	}
+}

pkg/proto/ssh/tcpip.go

@@ -0,0 +1,478 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"math/rand"
+	"net"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+)
+
+// Listen requests the remote peer open a listening socket on
+// addr. Incoming connections will be available by calling Accept on
+// the returned net.Listener. The listener must be serviced, or the
+// SSH connection may hang.
+// N must be "tcp", "tcp4", "tcp6", or "unix".
+func (c *Client) Listen(n, addr string) (net.Listener, error) {
+	switch n {
+	case "tcp", "tcp4", "tcp6":
+		laddr, err := net.ResolveTCPAddr(n, addr)
+		if err != nil {
+			return nil, err
+		}
+		return c.ListenTCP(laddr)
+	case "unix":
+		return c.ListenUnix(addr)
+	default:
+		return nil, fmt.Errorf("ssh: unsupported protocol: %s", n)
+	}
+}
+
+// Automatic port allocation is broken with OpenSSH before 6.0. See
+// also https://bugzilla.mindrot.org/show_bug.cgi?id=2017.  In
+// particular, OpenSSH 5.9 sends a channelOpenMsg with port number 0,
+// rather than the actual port number. This means you can never open
+// two different listeners with auto allocated ports. We work around
+// this by trying explicit ports until we succeed.
+
+const openSSHPrefix = "OpenSSH_"
+
+var portRandomizer = rand.New(rand.NewSource(time.Now().UnixNano()))
+
+// isBrokenOpenSSHVersion returns true if the given version string
+// specifies a version of OpenSSH that is known to have a bug in port
+// forwarding.
+func isBrokenOpenSSHVersion(versionStr string) bool {
+	i := strings.Index(versionStr, openSSHPrefix)
+	if i < 0 {
+		return false
+	}
+	i += len(openSSHPrefix)
+	j := i
+	for ; j < len(versionStr); j++ {
+		if versionStr[j] < '0' || versionStr[j] > '9' {
+			break
+		}
+	}
+	version, _ := strconv.Atoi(versionStr[i:j])
+	return version < 6
+}
+
+// autoPortListenWorkaround simulates automatic port allocation by
+// trying random ports repeatedly.
+func (c *Client) autoPortListenWorkaround(laddr *net.TCPAddr) (net.Listener, error) {
+	var sshListener net.Listener
+	var err error
+	const tries = 10
+	for i := 0; i < tries; i++ {
+		addr := *laddr
+		addr.Port = 1024 + portRandomizer.Intn(60000)
+		sshListener, err = c.ListenTCP(&addr)
+		if err == nil {
+			laddr.Port = addr.Port
+			return sshListener, err
+		}
+	}
+	return nil, fmt.Errorf("ssh: listen on random port failed after %d tries: %v", tries, err)
+}
+
+// RFC 4254 7.1
+type channelForwardMsg struct {
+	addr  string
+	rport uint32
+}
+
+// handleForwards starts goroutines handling forwarded connections.
+// It's called on first use by (*Client).ListenTCP to not launch
+// goroutines until needed.
+func (c *Client) handleForwards() {
+	go c.forwards.handleChannels(c.HandleChannelOpen("forwarded-tcpip"))
+	go c.forwards.handleChannels(c.HandleChannelOpen("forwarded-streamlocal@openssh.com"))
+}
+
+// ListenTCP requests the remote peer open a listening socket
+// on laddr. Incoming connections will be available by calling
+// Accept on the returned net.Listener.
+func (c *Client) ListenTCP(laddr *net.TCPAddr, fakeHost ...string) (net.Listener, error) {
+	c.handleForwardsOnce.Do(c.handleForwards)
+	if laddr.Port == 0 && isBrokenOpenSSHVersion(string(c.ServerVersion())) {
+		return c.autoPortListenWorkaround(laddr)
+	}
+
+	host := laddr.IP.String()
+	if len(fakeHost) > 0 {
+		host = fakeHost[0]
+	}
+	m := channelForwardMsg{
+		host,
+		uint32(laddr.Port),
+	}
+	// send message
+	ok, resp, err := c.SendRequest("tcpip-forward", true, Marshal(&m))
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		return nil, errors.New("ssh: tcpip-forward request denied by peer")
+	}
+
+	// If the original port was 0, then the remote side will
+	// supply a real port number in the response.
+	if laddr.Port == 0 {
+		var p struct {
+			Port uint32
+		}
+		if err := Unmarshal(resp, &p); err != nil {
+			return nil, err
+		}
+		laddr.Port = int(p.Port)
+	}
+
+	// Register this forward, using the port number we obtained.
+	ch := c.forwards.add(laddr)
+
+	return &tcpListener{laddr, c, ch}, nil
+}
+
+// forwardList stores a mapping between remote
+// forward requests and the tcpListeners.
+type forwardList struct {
+	sync.Mutex
+	entries []forwardEntry
+}
+
+// forwardEntry represents an established mapping of a laddr on a
+// remote ssh server to a channel connected to a tcpListener.
+type forwardEntry struct {
+	laddr net.Addr
+	c     chan forward
+}
+
+// forward represents an incoming forwarded tcpip connection. The
+// arguments to add/remove/lookup should be address as specified in
+// the original forward-request.
+type forward struct {
+	newCh NewChannel // the ssh client channel underlying this forward
+	raddr net.Addr   // the raddr of the incoming connection
+}
+
+func (l *forwardList) add(addr net.Addr) chan forward {
+	l.Lock()
+	defer l.Unlock()
+	f := forwardEntry{
+		laddr: addr,
+		c:     make(chan forward, 1),
+	}
+	l.entries = append(l.entries, f)
+	return f.c
+}
+
+// See RFC 4254, section 7.2
+type forwardedTCPPayload struct {
+	Addr       string
+	Port       uint32
+	OriginAddr string
+	OriginPort uint32
+}
+
+// parseTCPAddr parses the originating address from the remote into a *net.TCPAddr.
+func parseTCPAddr(addr string, port uint32) (*net.TCPAddr, error) {
+	if port == 0 || port > 65535 {
+		return nil, fmt.Errorf("ssh: port number out of range: %d", port)
+	}
+	ip := net.ParseIP(string(addr))
+	if ip == nil {
+		return nil, fmt.Errorf("ssh: cannot parse IP address %q", addr)
+	}
+	return &net.TCPAddr{IP: ip, Port: int(port)}, nil
+}
+
+func (l *forwardList) handleChannels(in <-chan NewChannel) {
+	for ch := range in {
+		var (
+			laddr net.Addr
+			raddr net.Addr
+			err   error
+		)
+		switch channelType := ch.ChannelType(); channelType {
+		case "forwarded-tcpip":
+			var payload forwardedTCPPayload
+			if err = Unmarshal(ch.ExtraData(), &payload); err != nil {
+				ch.Reject(ConnectionFailed, "could not parse forwarded-tcpip payload: "+err.Error())
+				continue
+			}
+
+			// RFC 4254 section 7.2 specifies that incoming
+			// addresses should list the address, in string
+			// format. It is implied that this should be an IP
+			// address, as it would be impossible to connect to it
+			// otherwise.
+			laddr, err = parseTCPAddr(payload.Addr, payload.Port)
+			if err != nil {
+				ch.Reject(ConnectionFailed, err.Error())
+				continue
+			}
+			raddr, err = parseTCPAddr(payload.OriginAddr, payload.OriginPort)
+			if err != nil {
+				ch.Reject(ConnectionFailed, err.Error())
+				continue
+			}
+
+		case "forwarded-streamlocal@openssh.com":
+			var payload forwardedStreamLocalPayload
+			if err = Unmarshal(ch.ExtraData(), &payload); err != nil {
+				ch.Reject(ConnectionFailed, "could not parse forwarded-streamlocal@openssh.com payload: "+err.Error())
+				continue
+			}
+			laddr = &net.UnixAddr{
+				Name: payload.SocketPath,
+				Net:  "unix",
+			}
+			raddr = &net.UnixAddr{
+				Name: "@",
+				Net:  "unix",
+			}
+		default:
+			panic(fmt.Errorf("ssh: unknown channel type %s", channelType))
+		}
+		if ok := l.forward(laddr, raddr, ch); !ok {
+			// Section 7.2, implementations MUST reject spurious incoming
+			// connections.
+			ch.Reject(Prohibited, "no forward for address")
+			continue
+		}
+
+	}
+}
+
+// remove removes the forward entry, and the channel feeding its
+// listener.
+func (l *forwardList) remove(addr net.Addr) {
+	l.Lock()
+	defer l.Unlock()
+	for i, f := range l.entries {
+		if addr.Network() == f.laddr.Network() && addr.String() == f.laddr.String() {
+			l.entries = append(l.entries[:i], l.entries[i+1:]...)
+			close(f.c)
+			return
+		}
+	}
+}
+
+// closeAll closes and clears all forwards.
+func (l *forwardList) closeAll() {
+	l.Lock()
+	defer l.Unlock()
+	for _, f := range l.entries {
+		close(f.c)
+	}
+	l.entries = nil
+}
+
+func (l *forwardList) forward(laddr, raddr net.Addr, ch NewChannel) bool {
+	l.Lock()
+	defer l.Unlock()
+	for _, f := range l.entries {
+		if laddr.Network() == f.laddr.Network() && laddr.String() == f.laddr.String() {
+			f.c <- forward{newCh: ch, raddr: raddr}
+			return true
+		}
+	}
+	return false
+}
+
+type tcpListener struct {
+	laddr *net.TCPAddr
+
+	conn *Client
+	in   <-chan forward
+}
+
+// Accept waits for and returns the next connection to the listener.
+func (l *tcpListener) Accept() (net.Conn, error) {
+	s, ok := <-l.in
+	if !ok {
+		return nil, io.EOF
+	}
+	ch, incoming, err := s.newCh.Accept()
+	if err != nil {
+		return nil, err
+	}
+	go DiscardRequests(incoming)
+
+	return &chanConn{
+		Channel: ch,
+		laddr:   l.laddr,
+		raddr:   s.raddr,
+	}, nil
+}
+
+// Close closes the listener.
+func (l *tcpListener) Close() error {
+	m := channelForwardMsg{
+		l.laddr.IP.String(),
+		uint32(l.laddr.Port),
+	}
+
+	// this also closes the listener.
+	l.conn.forwards.remove(l.laddr)
+	ok, _, err := l.conn.SendRequest("cancel-tcpip-forward", true, Marshal(&m))
+	if err == nil && !ok {
+		err = errors.New("ssh: cancel-tcpip-forward failed")
+	}
+	return err
+}
+
+// Addr returns the listener's network address.
+func (l *tcpListener) Addr() net.Addr {
+	return l.laddr
+}
+
+// Dial initiates a connection to the addr from the remote host.
+// The resulting connection has a zero LocalAddr() and RemoteAddr().
+func (c *Client) Dial(n, addr string) (net.Conn, error) {
+	var ch Channel
+	switch n {
+	case "tcp", "tcp4", "tcp6":
+		// Parse the address into host and numeric port.
+		host, portString, err := net.SplitHostPort(addr)
+		if err != nil {
+			return nil, err
+		}
+		port, err := strconv.ParseUint(portString, 10, 16)
+		if err != nil {
+			return nil, err
+		}
+		ch, err = c.dial(net.IPv4zero.String(), 0, host, int(port))
+		if err != nil {
+			return nil, err
+		}
+		// Use a zero address for local and remote address.
+		zeroAddr := &net.TCPAddr{
+			IP:   net.IPv4zero,
+			Port: 0,
+		}
+		return &chanConn{
+			Channel: ch,
+			laddr:   zeroAddr,
+			raddr:   zeroAddr,
+		}, nil
+	case "unix":
+		var err error
+		ch, err = c.dialStreamLocal(addr)
+		if err != nil {
+			return nil, err
+		}
+		return &chanConn{
+			Channel: ch,
+			laddr: &net.UnixAddr{
+				Name: "@",
+				Net:  "unix",
+			},
+			raddr: &net.UnixAddr{
+				Name: addr,
+				Net:  "unix",
+			},
+		}, nil
+	default:
+		return nil, fmt.Errorf("ssh: unsupported protocol: %s", n)
+	}
+}
+
+// DialTCP connects to the remote address raddr on the network net,
+// which must be "tcp", "tcp4", or "tcp6".  If laddr is not nil, it is used
+// as the local address for the connection.
+func (c *Client) DialTCP(n string, laddr, raddr *net.TCPAddr) (net.Conn, error) {
+	if laddr == nil {
+		laddr = &net.TCPAddr{
+			IP:   net.IPv4zero,
+			Port: 0,
+		}
+	}
+	ch, err := c.dial(laddr.IP.String(), laddr.Port, raddr.IP.String(), raddr.Port)
+	if err != nil {
+		return nil, err
+	}
+	return &chanConn{
+		Channel: ch,
+		laddr:   laddr,
+		raddr:   raddr,
+	}, nil
+}
+
+// RFC 4254 7.2
+type channelOpenDirectMsg struct {
+	raddr string
+	rport uint32
+	laddr string
+	lport uint32
+}
+
+func (c *Client) dial(laddr string, lport int, raddr string, rport int) (Channel, error) {
+	msg := channelOpenDirectMsg{
+		raddr: raddr,
+		rport: uint32(rport),
+		laddr: laddr,
+		lport: uint32(lport),
+	}
+	ch, in, err := c.OpenChannel("direct-tcpip", Marshal(&msg))
+	if err != nil {
+		return nil, err
+	}
+	go DiscardRequests(in)
+	return ch, err
+}
+
+type tcpChan struct {
+	Channel // the backing channel
+}
+
+// chanConn fulfills the net.Conn interface without
+// the tcpChan having to hold laddr or raddr directly.
+type chanConn struct {
+	Channel
+	laddr, raddr net.Addr
+}
+
+// LocalAddr returns the local network address.
+func (t *chanConn) LocalAddr() net.Addr {
+	return t.laddr
+}
+
+// RemoteAddr returns the remote network address.
+func (t *chanConn) RemoteAddr() net.Addr {
+	return t.raddr
+}
+
+// SetDeadline sets the read and write deadlines associated
+// with the connection.
+func (t *chanConn) SetDeadline(deadline time.Time) error {
+	if err := t.SetReadDeadline(deadline); err != nil {
+		return err
+	}
+	return t.SetWriteDeadline(deadline)
+}
+
+// SetReadDeadline sets the read deadline.
+// A zero value for t means Read will not time out.
+// After the deadline, the error from Read will implement net.Error
+// with Timeout() == true.
+func (t *chanConn) SetReadDeadline(deadline time.Time) error {
+	// for compatibility with previous version,
+	// the error message contains "tcpChan"
+	return errors.New("ssh: tcpChan: deadline not supported")
+}
+
+// SetWriteDeadline exists to satisfy the net.Conn interface
+// but is not implemented by this type.  It always returns an error.
+func (t *chanConn) SetWriteDeadline(deadline time.Time) error {
+	return errors.New("ssh: tcpChan: deadline not supported")
+}

pkg/proto/ssh/terminal/terminal.go

@@ -0,0 +1,76 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package terminal provides support functions for dealing with terminals, as
+// commonly found on UNIX systems.
+//
+// Deprecated: this package moved to golang.org/x/term.
+package terminal
+
+import (
+	"io"
+
+	"golang.org/x/term"
+)
+
+// EscapeCodes contains escape sequences that can be written to the terminal in
+// order to achieve different styles of text.
+type EscapeCodes = term.EscapeCodes
+
+// Terminal contains the state for running a VT100 terminal that is capable of
+// reading lines of input.
+type Terminal = term.Terminal
+
+// NewTerminal runs a VT100 terminal on the given ReadWriter. If the ReadWriter is
+// a local terminal, that terminal must first have been put into raw mode.
+// prompt is a string that is written at the start of each input line (i.e.
+// "> ").
+func NewTerminal(c io.ReadWriter, prompt string) *Terminal {
+	return term.NewTerminal(c, prompt)
+}
+
+// ErrPasteIndicator may be returned from ReadLine as the error, in addition
+// to valid line data. It indicates that bracketed paste mode is enabled and
+// that the returned line consists only of pasted data. Programs may wish to
+// interpret pasted data more literally than typed data.
+var ErrPasteIndicator = term.ErrPasteIndicator
+
+// State contains the state of a terminal.
+type State = term.State
+
+// IsTerminal returns whether the given file descriptor is a terminal.
+func IsTerminal(fd int) bool {
+	return term.IsTerminal(fd)
+}
+
+// ReadPassword reads a line of input from a terminal without local echo.  This
+// is commonly used for inputting passwords and other sensitive data. The slice
+// returned does not include the \n.
+func ReadPassword(fd int) ([]byte, error) {
+	return term.ReadPassword(fd)
+}
+
+// MakeRaw puts the terminal connected to the given file descriptor into raw
+// mode and returns the previous state of the terminal so that it can be
+// restored.
+func MakeRaw(fd int) (*State, error) {
+	return term.MakeRaw(fd)
+}
+
+// Restore restores the terminal connected to the given file descriptor to a
+// previous state.
+func Restore(fd int, oldState *State) error {
+	return term.Restore(fd, oldState)
+}
+
+// GetState returns the current state of a terminal which may be useful to
+// restore the terminal after a signal.
+func GetState(fd int) (*State, error) {
+	return term.GetState(fd)
+}
+
+// GetSize returns the dimensions of the given terminal.
+func GetSize(fd int) (width, height int, err error) {
+	return term.GetSize(fd)
+}

pkg/proto/ssh/transport.go

@@ -0,0 +1,358 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"io"
+	"log"
+)
+
+// debugTransport if set, will print packet types as they go over the
+// wire. No message decoding is done, to minimize the impact on timing.
+const debugTransport = false
+
+const (
+	gcm128CipherID = "aes128-gcm@openssh.com"
+	gcm256CipherID = "aes256-gcm@openssh.com"
+	aes128cbcID    = "aes128-cbc"
+	tripledescbcID = "3des-cbc"
+)
+
+// packetConn represents a transport that implements packet based
+// operations.
+type packetConn interface {
+	// Encrypt and send a packet of data to the remote peer.
+	writePacket(packet []byte) error
+
+	// Read a packet from the connection. The read is blocking,
+	// i.e. if error is nil, then the returned byte slice is
+	// always non-empty.
+	readPacket() ([]byte, error)
+
+	// Close closes the write-side of the connection.
+	Close() error
+}
+
+// transport is the keyingTransport that implements the SSH packet
+// protocol.
+type transport struct {
+	reader connectionState
+	writer connectionState
+
+	bufReader *bufio.Reader
+	bufWriter *bufio.Writer
+	rand      io.Reader
+	isClient  bool
+	io.Closer
+}
+
+// packetCipher represents a combination of SSH encryption/MAC
+// protocol.  A single instance should be used for one direction only.
+type packetCipher interface {
+	// writeCipherPacket encrypts the packet and writes it to w. The
+	// contents of the packet are generally scrambled.
+	writeCipherPacket(seqnum uint32, w io.Writer, rand io.Reader, packet []byte) error
+
+	// readCipherPacket reads and decrypts a packet of data. The
+	// returned packet may be overwritten by future calls of
+	// readPacket.
+	readCipherPacket(seqnum uint32, r io.Reader) ([]byte, error)
+}
+
+// connectionState represents one side (read or write) of the
+// connection. This is necessary because each direction has its own
+// keys, and can even have its own algorithms
+type connectionState struct {
+	packetCipher
+	seqNum           uint32
+	dir              direction
+	pendingKeyChange chan packetCipher
+}
+
+// prepareKeyChange sets up key material for a keychange. The key changes in
+// both directions are triggered by reading and writing a msgNewKey packet
+// respectively.
+func (t *transport) prepareKeyChange(algs *algorithms, kexResult *kexResult) error {
+	ciph, err := newPacketCipher(t.reader.dir, algs.r, kexResult)
+	if err != nil {
+		return err
+	}
+	t.reader.pendingKeyChange <- ciph
+
+	ciph, err = newPacketCipher(t.writer.dir, algs.w, kexResult)
+	if err != nil {
+		return err
+	}
+	t.writer.pendingKeyChange <- ciph
+
+	return nil
+}
+
+func (t *transport) printPacket(p []byte, write bool) {
+	if len(p) == 0 {
+		return
+	}
+	who := "server"
+	if t.isClient {
+		who = "client"
+	}
+	what := "read"
+	if write {
+		what = "write"
+	}
+
+	log.Println(what, who, p[0])
+}
+
+// Read and decrypt next packet.
+func (t *transport) readPacket() (p []byte, err error) {
+	for {
+		p, err = t.reader.readPacket(t.bufReader)
+		if err != nil {
+			break
+		}
+		if len(p) == 0 || (p[0] != msgIgnore && p[0] != msgDebug) {
+			break
+		}
+	}
+	if debugTransport {
+		t.printPacket(p, false)
+	}
+
+	return p, err
+}
+
+func (s *connectionState) readPacket(r *bufio.Reader) ([]byte, error) {
+	packet, err := s.packetCipher.readCipherPacket(s.seqNum, r)
+	s.seqNum++
+	if err == nil && len(packet) == 0 {
+		err = errors.New("ssh: zero length packet")
+	}
+
+	if len(packet) > 0 {
+		switch packet[0] {
+		case msgNewKeys:
+			select {
+			case cipher := <-s.pendingKeyChange:
+				s.packetCipher = cipher
+			default:
+				return nil, errors.New("ssh: got bogus newkeys message")
+			}
+
+		case msgDisconnect:
+			// Transform a disconnect message into an
+			// error. Since this is lowest level at which
+			// we interpret message types, doing it here
+			// ensures that we don't have to handle it
+			// elsewhere.
+			var msg disconnectMsg
+			if err := Unmarshal(packet, &msg); err != nil {
+				return nil, err
+			}
+			return nil, &msg
+		}
+	}
+
+	// The packet may point to an internal buffer, so copy the
+	// packet out here.
+	fresh := make([]byte, len(packet))
+	copy(fresh, packet)
+
+	return fresh, err
+}
+
+func (t *transport) writePacket(packet []byte) error {
+	if debugTransport {
+		t.printPacket(packet, true)
+	}
+	return t.writer.writePacket(t.bufWriter, t.rand, packet)
+}
+
+func (s *connectionState) writePacket(w *bufio.Writer, rand io.Reader, packet []byte) error {
+	changeKeys := len(packet) > 0 && packet[0] == msgNewKeys
+
+	err := s.packetCipher.writeCipherPacket(s.seqNum, w, rand, packet)
+	if err != nil {
+		return err
+	}
+	if err = w.Flush(); err != nil {
+		return err
+	}
+	s.seqNum++
+	if changeKeys {
+		select {
+		case cipher := <-s.pendingKeyChange:
+			s.packetCipher = cipher
+		default:
+			panic("ssh: no key material for msgNewKeys")
+		}
+	}
+	return err
+}
+
+func newTransport(rwc io.ReadWriteCloser, rand io.Reader, isClient bool) *transport {
+	t := &transport{
+		bufReader: bufio.NewReader(rwc),
+		bufWriter: bufio.NewWriter(rwc),
+		rand:      rand,
+		reader: connectionState{
+			packetCipher:     &streamPacketCipher{cipher: noneCipher{}},
+			pendingKeyChange: make(chan packetCipher, 1),
+		},
+		writer: connectionState{
+			packetCipher:     &streamPacketCipher{cipher: noneCipher{}},
+			pendingKeyChange: make(chan packetCipher, 1),
+		},
+		Closer: rwc,
+	}
+	t.isClient = isClient
+
+	if isClient {
+		t.reader.dir = serverKeys
+		t.writer.dir = clientKeys
+	} else {
+		t.reader.dir = clientKeys
+		t.writer.dir = serverKeys
+	}
+
+	return t
+}
+
+type direction struct {
+	ivTag     []byte
+	keyTag    []byte
+	macKeyTag []byte
+}
+
+var (
+	serverKeys = direction{[]byte{'B'}, []byte{'D'}, []byte{'F'}}
+	clientKeys = direction{[]byte{'A'}, []byte{'C'}, []byte{'E'}}
+)
+
+// setupKeys sets the cipher and MAC keys from kex.K, kex.H and sessionId, as
+// described in RFC 4253, section 6.4. direction should either be serverKeys
+// (to setup server->client keys) or clientKeys (for client->server keys).
+func newPacketCipher(d direction, algs directionAlgorithms, kex *kexResult) (packetCipher, error) {
+	cipherMode := cipherModes[algs.Cipher]
+
+	iv := make([]byte, cipherMode.ivSize)
+	key := make([]byte, cipherMode.keySize)
+
+	generateKeyMaterial(iv, d.ivTag, kex)
+	generateKeyMaterial(key, d.keyTag, kex)
+
+	var macKey []byte
+	if !aeadCiphers[algs.Cipher] {
+		macMode := macModes[algs.MAC]
+		macKey = make([]byte, macMode.keySize)
+		generateKeyMaterial(macKey, d.macKeyTag, kex)
+	}
+
+	return cipherModes[algs.Cipher].create(key, iv, macKey, algs)
+}
+
+// generateKeyMaterial fills out with key material generated from tag, K, H
+// and sessionId, as specified in RFC 4253, section 7.2.
+func generateKeyMaterial(out, tag []byte, r *kexResult) {
+	var digestsSoFar []byte
+
+	h := r.Hash.New()
+	for len(out) > 0 {
+		h.Reset()
+		h.Write(r.K)
+		h.Write(r.H)
+
+		if len(digestsSoFar) == 0 {
+			h.Write(tag)
+			h.Write(r.SessionID)
+		} else {
+			h.Write(digestsSoFar)
+		}
+
+		digest := h.Sum(nil)
+		n := copy(out, digest)
+		out = out[n:]
+		if len(out) > 0 {
+			digestsSoFar = append(digestsSoFar, digest...)
+		}
+	}
+}
+
+const packageVersion = "SSH-2.0-Go"
+
+// Sends and receives a version line.  The versionLine string should
+// be US ASCII, start with "SSH-2.0-", and should not include a
+// newline. exchangeVersions returns the other side's version line.
+func exchangeVersions(rw io.ReadWriter, versionLine []byte) (them []byte, err error) {
+	// Contrary to the RFC, we do not ignore lines that don't
+	// start with "SSH-2.0-" to make the library usable with
+	// nonconforming servers.
+	for _, c := range versionLine {
+		// The spec disallows non US-ASCII chars, and
+		// specifically forbids null chars.
+		if c < 32 {
+			return nil, errors.New("ssh: junk character in version line")
+		}
+	}
+	if _, err = rw.Write(append(versionLine, '\r', '\n')); err != nil {
+		return
+	}
+
+	them, err = readVersion(rw)
+	return them, err
+}
+
+// maxVersionStringBytes is the maximum number of bytes that we'll
+// accept as a version string. RFC 4253 section 4.2 limits this at 255
+// chars
+const maxVersionStringBytes = 255
+
+// Read version string as specified by RFC 4253, section 4.2.
+func readVersion(r io.Reader) ([]byte, error) {
+	versionString := make([]byte, 0, 64)
+	var ok bool
+	var buf [1]byte
+
+	for length := 0; length < maxVersionStringBytes; length++ {
+		_, err := io.ReadFull(r, buf[:])
+		if err != nil {
+			return nil, err
+		}
+		// The RFC says that the version should be terminated with \r\n
+		// but several SSH servers actually only send a \n.
+		if buf[0] == '\n' {
+			if !bytes.HasPrefix(versionString, []byte("SSH-")) {
+				// RFC 4253 says we need to ignore all version string lines
+				// except the one containing the SSH version (provided that
+				// all the lines do not exceed 255 bytes in total).
+				versionString = versionString[:0]
+				continue
+			}
+			ok = true
+			break
+		}
+
+		// non ASCII chars are disallowed, but we are lenient,
+		// since Go doesn't use null-terminated strings.
+
+		// The RFC allows a comment after a space, however,
+		// all of it (version and comments) goes into the
+		// session hash.
+		versionString = append(versionString, buf[0])
+	}
+
+	if !ok {
+		return nil, errors.New("ssh: overflow reading version string")
+	}
+
+	// There might be a '\r' on the end which we should remove.
+	if len(versionString) > 0 && versionString[len(versionString)-1] == '\r' {
+		versionString = versionString[:len(versionString)-1]
+	}
+	return versionString, nil
+}