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317 lines
8.7 KiB
ReStructuredText
317 lines
8.7 KiB
ReStructuredText
.. Copyright 2016 OpenMarket Ltd
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..
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.. Licensed under the Apache License, Version 2.0 (the "License");
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.. you may not use this file except in compliance with the License.
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.. You may obtain a copy of the License at
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..
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.. http://www.apache.org/licenses/LICENSE-2.0
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..
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.. Unless required by applicable law or agreed to in writing, software
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.. distributed under the License is distributed on an "AS IS" BASIS,
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.. WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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.. See the License for the specific language governing permissions and
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.. limitations under the License.
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Signing JSON
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------------
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Various points in the Matrix specification require JSON objects to be
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cryptographically signed. This requires us to encode the JSON as a binary
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string. Unfortunately the same JSON can be encoded in different ways by
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changing how much white space is used or by changing the order of keys within
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objects.
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Signing an object therefore requires it to be encoded as a sequence of bytes
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using `Canonical JSON`_, computing the signature for that sequence and then
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adding the signature to the original JSON object.
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Canonical JSON
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~~~~~~~~~~~~~~
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We define the canonical JSON encoding for a value to be the shortest UTF-8 JSON
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encoding with dictionary keys lexicographically sorted by unicode codepoint.
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Numbers in the JSON must be integers in the range ``[-(2**53)+1, (2**53)-1]``.
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We pick UTF-8 as the encoding as it should be available to all platforms and
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JSON received from the network is likely to be already encoded using UTF-8.
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We sort the keys to give a consistent ordering. We force integers to be in the
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range where they can be accurately represented using IEEE double precision
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floating point numbers since a number of JSON libraries represent all numbers
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using this representation.
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.. code:: python
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import json
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def canonical_json(value):
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return json.dumps(
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value,
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# Encode code-points outside of ASCII as UTF-8 rather than \u escapes
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ensure_ascii=False,
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# Remove unnecessary white space.
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separators=(',',':'),
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# Sort the keys of dictionaries.
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sort_keys=True,
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# Encode the resulting unicode as UTF-8 bytes.
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).encode("UTF-8")
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Grammar
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+++++++
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Adapted from the grammar in http://tools.ietf.org/html/rfc7159 removing
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insignificant whitespace, fractions, exponents and redundant character escapes.
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.. code::
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value = false / null / true / object / array / number / string
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false = %x66.61.6c.73.65
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null = %x6e.75.6c.6c
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true = %x74.72.75.65
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object = %x7B [ member *( %x2C member ) ] %7D
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member = string %x3A value
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array = %x5B [ value *( %x2C value ) ] %5B
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number = [ %x2D ] int
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int = %x30 / ( %x31-39 *digit )
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digit = %x30-39
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string = %x22 *char %x22
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char = unescaped / %x5C escaped
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unescaped = %x20-21 / %x23-5B / %x5D-10FFFF
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escaped = %x22 ; " quotation mark U+0022
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/ %x5C ; \ reverse solidus U+005C
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/ %x62 ; b backspace U+0008
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/ %x66 ; f form feed U+000C
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/ %x6E ; n line feed U+000A
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/ %x72 ; r carriage return U+000D
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/ %x74 ; t tab U+0009
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/ %x75.30.30.30 (%x30-37 / %x62 / %x65-66) ; u000X
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/ %x75.30.30.31 (%x30-39 / %x61-66) ; u001X
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Examples
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++++++++
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To assist in the development of compatible implementations, the following test
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values may be useful for verifying the canonical transformation code.
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Given the following JSON object:
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.. code:: json
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{}
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The following canonical JSON should be produced:
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.. code:: json
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{}
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Given the following JSON object:
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.. code:: json
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{
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"one": 1,
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"two": "Two"
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}
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The following canonical JSON should be produced:
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.. code:: json
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{"one":1,"two":"Two"}
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Given the following JSON object:
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.. code:: json
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{
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"b": "2",
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"a": "1"
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}
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The following canonical JSON should be produced:
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.. code:: json
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{"a":"1","b":"2"}
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Given the following JSON object:
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.. code:: json
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{"b":"2","a":"1"}
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The following canonical JSON should be produced:
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.. code:: json
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{"a":"1","b":"2"}
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Given the following JSON object:
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.. code:: json
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{
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"auth": {
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"success": true,
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"mxid": "@john.doe:example.com",
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"profile": {
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"display_name": "John Doe",
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"three_pids": [
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{
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"medium": "email",
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"address": "john.doe@example.org"
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},
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{
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"medium": "msisdn",
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"address": "123456789"
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}
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]
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}
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}
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}
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The following canonical JSON should be produced:
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.. code:: json
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{"auth":{"mxid":"@john.doe:example.com","profile":{"display_name":"John Doe","three_pids":[{"address":"john.doe@example.org","medium":"email"},{"address":"123456789","medium":"msisdn"}]},"success":true}}
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Given the following JSON object:
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.. code:: json
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{
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"a": "日本語"
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}
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The following canonical JSON should be produced:
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.. code:: json
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{"a":"日本語"}
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Given the following JSON object:
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.. code:: json
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{
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"本": 2,
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"日": 1
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}
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The following canonical JSON should be produced:
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.. code:: json
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{"日":1,"本":2}
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Given the following JSON object:
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.. code:: json
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{
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"a": "\u65E5"
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}
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The following canonical JSON should be produced:
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.. code:: json
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{"a":"日"}
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Given the following JSON object:
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.. code:: json
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{
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"a": null
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}
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The following canonical JSON should be produced:
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.. code:: json
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{"a":null}
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Signing Details
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~~~~~~~~~~~~~~~
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JSON is signed by encoding the JSON object without ``signatures`` or keys grouped
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as ``unsigned``, using the canonical encoding described above. The JSON bytes are then signed using the
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signature algorithm and the signature is encoded using `unpadded Base64`_.
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The resulting base64 signature is added to an object under the
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*signing key identifier* which is added to the ``signatures`` object under the
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name of the entity signing it which is added back to the original JSON object
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along with the ``unsigned`` object.
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The *signing key identifier* is the concatenation of the *signing algorithm*
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and a *key identifier*. The *signing algorithm* identifies the algorithm used
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to sign the JSON. The currently supported value for *signing algorithm* is
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``ed25519`` as implemented by NACL (http://nacl.cr.yp.to/). The *key identifier*
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is used to distinguish between different signing keys used by the same entity.
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The ``unsigned`` object and the ``signatures`` object are not covered by the
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signature. Therefore intermediate entities can add unsigned data such as
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timestamps and additional signatures.
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.. code:: json
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{
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"name": "example.org",
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"signing_keys": {
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"ed25519:1": "XSl0kuyvrXNj6A+7/tkrB9sxSbRi08Of5uRhxOqZtEQ"
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},
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"unsigned": {
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"age_ts": 922834800000
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},
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"signatures": {
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"example.org": {
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"ed25519:1": "s76RUgajp8w172am0zQb/iPTHsRnb4SkrzGoeCOSFfcBY2V/1c8QfrmdXHpvnc2jK5BD1WiJIxiMW95fMjK7Bw"
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}
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}
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}
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.. code:: python
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def sign_json(json_object, signing_key, signing_name):
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signatures = json_object.pop("signatures", {})
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unsigned = json_object.pop("unsigned", None)
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signed = signing_key.sign(encode_canonical_json(json_object))
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signature_base64 = encode_base64(signed.signature)
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key_id = "%s:%s" % (signing_key.alg, signing_key.version)
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signatures.setdefault(signing_name, {})[key_id] = signature_base64
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json_object["signatures"] = signatures
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if unsigned is not None:
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json_object["unsigned"] = unsigned
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return json_object
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Checking for a Signature
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~~~~~~~~~~~~~~~~~~~~~~~~
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To check if an entity has signed a JSON object an implementation does the
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following:
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1. Checks if the ``signatures`` member of the object contains an entry with
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the name of the entity. If the entry is missing then the check fails.
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2. Removes any *signing key identifiers* from the entry with algorithms it
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doesn't understand. If there are no *signing key identifiers* left then the
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check fails.
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3. Looks up *verification keys* for the remaining *signing key identifiers*
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either from a local cache or by consulting a trusted key server. If it
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cannot find a *verification key* then the check fails.
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4. Decodes the base64 encoded signature bytes. If base64 decoding fails then
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the check fails.
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5. Removes the ``signatures`` and ``unsigned`` members of the object.
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6. Encodes the remainder of the JSON object using the `Canonical JSON`_
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encoding.
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7. Checks the signature bytes against the encoded object using the
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*verification key*. If this fails then the check fails. Otherwise the check
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succeeds.
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