naiveproxy/net/tools/transport_security_state_generator

Transport Security State Generator

This directory contains the code for the transport security state generator, a tool that generates a C++ file based on preload data in transport_security_state_static.json. This JSON file contains the domain security policy configurations for all preloaded domains.

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Domain Security Policies

Website owners can set a number of security policies for their domains, usually by sending configuration in a HTTP header. Chromium supports preloading for some of these security policies so that users benefit from these policies regardless of their browsing history. Website owners can request preloading for their domains. Chromium supports preloading for the following domain security policies:

• HTTP Strict Transport Security (HSTS)
• Public Key Pinning Extension for HTTP
• Expect-CT Extension for HTTP

Chromium and most other browsers ship the preloaded configurations inside their binary. Chromium uses a custom data structure for this.

I want to preload a website

Please follow the instructions at hstspreload.org.

I want to use the preload list for another project

Please contact the list maintainers before you do.

The Preload Generator

The transport security state generator is executed during the build process (it may execute multiple times depending on the targets you're building) and generates data structures that are compiled into the binary. You can find the generated output in [build-folder]/gen/net/http/transport_security_state_static*.h.

Usage

Make sure you have build the transport_security_state_generator target.

transport_security_state_generator <json-file> <pins-file> <template-file> <output-file> [--v=1]

• json-file: JSON file containing all preload configurations (e.g. net/http/transport_security_state_static.json)
• pins-file: file containing the public key information for the pinsets referenced from json-file (e.g. net/http/transport_security_state_static.pins)
• template-file: contains the global structure of the header file with placeholder for the generated data (e.g. net/http/transport_security_state_static.template)
• output-file: file to write the output to
• --v: verbosity level

The Preload Format

The preload data is stored in the Chromium binary as a trie encoded in a byte array (net::TransportSecurityStateSource::preloaded_data). The hostnames are stored in their canonicalized form and compressed using a Huffman coding. The generic decoder for preloaded Huffman encoded trie data is PreloadDecoder and lives in net/extras/preload_data/decoder.cc. The HSTS specific implementation is DecodeHSTSPreload and lives in net/http/transport_security_state.cc.

Huffman Coding

A Huffman coding is calculated for all characters used in the trie (characters in hostnames and the end of table and terminal values). The Huffman tree can be rebuild from the net::TransportSecurityStateSource::huffman_tree array.

The (internal) nodes of the tree are encoded as pairs of uint8s. The last node in the array is the root of the tree. Each node is two uint8_t values, the first is "left" and the second is "right". If a uint8_t value has the MSB set it is a leaf value and the 7 least significant bits represent a ASCII character (from the range 0-127, the tree does not support extended ASCII). If the MSB is not set it is a pointer to the n'th node in the array.

For example, the following uint8_t array

0xE1, 0xE2, 0xE3, 0x0, 0xE4, 0xE5, 0x1, 0x2

represents 9 elements:

• the implicit root node (node 3)
• 3 internal nodes: 0x0 (node 0), 0x1 (node 1), and 0x2 (node 2)
• 5 leaf values: 0xE1, 0xE2, 0xE3, 0xE4, and 0xE5 (which all have the most significant bit set)

When decoded this results in the following Huffman tree:

             root (node 3)
            /             \
     node 1                 node 2
   /       \               /      \
0xE3 (c)    node 0     0xE4 (d)    0xE5 (e)
           /      \
       0xE1 (a)    0xE2 (b)

The Trie Encoding

The byte array containing the trie is made up of a set of nodes represented by dispatch tables. Each dispatch table contains a (possibly empty) shared prefix, a value, and zero or more pointers to child dispatch tables. The node value is an encoded entry and the associated hostname can be found by going up the trie.

The trie contains the hostnames in reverse and the hostnames are terminated by a terminal value.

The dispatch table for the root node starts at bit position net::TransportSecurityStateSource::root_position.

The binary format for the trie is defined by the following ABNF.

trie               = 1*dispatch-table

dispatch-table     = prefix-part         ; a common prefix for the node and its children
                     1*value-part        ; 1 or more values or pointers to children
                     end-of-table-value  ; signals the end of the table

prefix-part        = *%b1               ; 0 or more 1 bits indicating the prefix length
                     %b0                ; 0 bit to indicate the end of the length encoding
                     prefix-characters  ; the actual prefix characters
value-part         = huffman-character node-value
                     ; table with the node value and pointers to children

node-value         = node-entry    ; preload entry for the hostname at this node
                   / node-pointer  ; a bit offset pointing to another dispatch
                                   ; table

node-entry         = preloaded-entry  ; encoded preload configuration for one
                                      ; hostname (see section below)
node-pointer       = long-bit-offset
                   / short-bit-offset

long-bit-offset    = %b1      ; 1 bit indicates long form will follow
                     4BIT     ; 4 bit number indicating bit length of the offset
                     8*22BIT  ; offset encoded as an n bit number (see above)
                              ; where n is the offset length (see above) + 8
short-bit-offset   = %b0      ; 0 bit indicates short form will follow
                     7BIT     ; offset as a 7 bit number

terminal-value     = huffman-character  ; ASCII value 0x00 encoded using Huffman
end-of-table-value = huffman-character  ; ASCII value 0x7F encoded using Huffman

prefix-characters  = *huffman-character
huffman-character  = 1*BIT

The Preloaded Entry Encoding

The entries are encoded using a variable length encoding. Each entry is made up of 4 parts, one for each supported policy. The length of these parts depends on the actual configuration, some field will be omitted in some cases.

The binary format for an entry is defined by the following ABNF.

preloaded-entry    = BIT                   ; simple entry flag
                     [hsts-part hpkp-part expect-ct-part]
                                           ; policy specific parts are only
                                           ; present when the simple entry flag
                                           ; is set to 0 and omitted otherwise

hsts-part          = include-subdomains    ; HSTS includeSubdomains flag
                     BIT                   ; whether to force HTTPS

hpkp-part          = BIT                   ; whether to enable pinning
                     [pinset-id]           ; only present when pinning is enabled
                     [include-subdomains]  ; HPKP includeSubdomains flag, only
                                           ; present when pinning is enabled and
                                           ; HSTS includeSubdomains is not used

hpkp-pinset-id     = array-index

expect-ct-part     = BIT              ; whether to enable Expect-CT
                     [report-uri-id]  ; only present when Expect-CT is enabled

report-uri-id      = array-index
include-subdomains = BIT
array-index        = 4BIT             ; a 4 bit number

The array-index values are indices in the associated arrays:

• net::TransportSecurityStateSource::pinsets for pinset-id
• net::TransportSecurityStateSource::expect_ct_report_uris for Expect-CT's report-uri-id

Simple entries

The majority of entries on the preload list are submitted through hstspreload.org and share the same policy configuration (HSTS + includeSubdomains only). To safe space, these entries (called simple entries) use a shorter encoding where the first bit (simple entry flag) is set to 1 and the rest of the configuration is omitted.

Tests

The generator code has its own unittests in the net/tools/transport_security_state_generator folder.

The encoder and decoder for the preload format life in different places and are tested by end-to-end tests (TransportSecurityStateTest.DecodePreload*) in net/http/transport_security_state_unittest.cc. The tests use their own preload lists, the data structures for these lists are generated in the same way as for the official Chromium list.

All these tests are part of the net_unittests target.

Writing tests that depend on static transport security state

Tests in net_unittests (except for TransportSecurityStateStaticTest) should not depend on the real preload list. If you are writing tests that require a static transport security state use transport_security_state_static_unittest_default.json instead. Tests can override the active preload list by calling SetTransportSecurityStateSourceForTesting.

See also

• https://hstspreload.org/
• https://www.chromium.org/hsts