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haven't i already moved you to matrix-doc twice? :/
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Matthew Hodgson
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216d5f6b52
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Signing JSON
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============
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JSON is signed by encoding the JSON object without ``signatures`` or ``unsigned``
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keys using a canonical encoding. The JSON bytes are then signed using the
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signature algorithm and the signature encoded using base64 with the padding
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stripped. 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 server 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 version*. The *signing algorithm* identifies the algorithm used to
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sign the JSON. The currently support value for *signing algorithm* is
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``ed25519`` as implemented by NACL (http://nacl.cr.yp.to/). The *key version*
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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 servers can add unsigneddata such as time stamps
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and additional signatures.
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::
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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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"retrieved_ts_ms": 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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::
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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(sigature_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 a server does the following
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1. Checks if the ``signatures`` object contains an entry with the name of the
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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. Checks the signature bytes using the *verification key*. If this fails then
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the check fails. Otherwise the check succeeds.
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Canonical JSON
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--------------
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The canonical JSON encoding for a value is the shortest UTF-8 JSON encoding
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with dictionary keys lexicographically sorted by unicode codepoint. Numbers in
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the JSON value must be integers in the range [-(2**53)+1, (2**53)-1].
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::
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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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ensure_ascii=False,
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separators=(',',':'),
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sort_keys=True,
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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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::
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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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Signing Events
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==============
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Signing events is a more complicated process since servers can choose to redact
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non-essential event contents. Before signing the event it is encoded as
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Canonical JSON and hashed using SHA-256. The resulting hash is then stored
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in the event JSON in a ``hash`` object under a ``sha256`` key. Then all
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non-essential keys are stripped from the event object, and the resulting object
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which included the ``hash`` key is signed using the JSON signing algorithm.
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Servers can then transmit the entire event or the event with the non-essential
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keys removed. Receiving servers can then check the entire event if it is
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present by computing the SHA-256 of the event excluding the ``hash`` object, or
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by using the ``hash`` object included in the event if keys have been redacted.
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New hash functions can be introduced by adding additional keys to the ``hash``
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object. Since the ``hash`` object cannot be redacted a server shouldn't allow
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too many hashes to be listed, otherwise a server might embed illict data within
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the ``hash`` object. For similar reasons a server shouldn't allow hash values
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that are too long.
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[[TODO(markjh): We might want to specify a maximum number of keys for the
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``hash`` and we might want to specify the maximum output size of a hash]]
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[[TODO(markjh) We might want to allow the server to omit the output of well
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known hash functions like SHA-256 when none of the keys have been redacted]]
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