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| -rw-r--r-- | docs/server-server/signing.rst | 151 |
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diff --git a/docs/server-server/signing.rst b/docs/server-server/signing.rst deleted file mode 100644 index 60c701ca91..0000000000 --- a/docs/server-server/signing.rst +++ /dev/null @@ -1,151 +0,0 @@ -Signing JSON -============ - -JSON is signed by encoding the JSON object without ``signatures`` or ``unsigned`` -keys using a canonical encoding. The JSON bytes are then signed using the -signature algorithm and the signature encoded using base64 with the padding -stripped. The resulting base64 signature is added to an object under the -*signing key identifier* which is added to the ``signatures`` object under the -name of the server signing it which is added back to the original JSON object -along with the ``unsigned`` object. - -The *signing key identifier* is the concatenation of the *signing algorithm* -and a *key version*. The *signing algorithm* identifies the algorithm used to -sign the JSON. The currently support value for *signing algorithm* is -``ed25519`` as implemented by NACL (http://nacl.cr.yp.to/). The *key version* -is used to distinguish between different signing keys used by the same entity. - -The ``unsigned`` object and the ``signatures`` object are not covered by the -signature. Therefore intermediate servers can add unsigneddata such as time stamps -and additional signatures. - - -:: - - { - "name": "example.org", - "signing_keys": { - "ed25519:1": "XSl0kuyvrXNj6A+7/tkrB9sxSbRi08Of5uRhxOqZtEQ" - }, - "unsigned": { - "retrieved_ts_ms": 922834800000 - }, - "signatures": { - "example.org": { - "ed25519:1": "s76RUgajp8w172am0zQb/iPTHsRnb4SkrzGoeCOSFfcBY2V/1c8QfrmdXHpvnc2jK5BD1WiJIxiMW95fMjK7Bw" - } - } - } - -:: - - def sign_json(json_object, signing_key, signing_name): - signatures = json_object.pop("signatures", {}) - unsigned = json_object.pop("unsigned", None) - - signed = signing_key.sign(encode_canonical_json(json_object)) - signature_base64 = encode_base64(signed.signature) - - key_id = "%s:%s" % (signing_key.alg, signing_key.version) - signatures.setdefault(sigature_name, {})[key_id] = signature_base64 - - json_object["signatures"] = signatures - if unsigned is not None: - json_object["unsigned"] = unsigned - - return json_object - -Checking for a Signature ------------------------- - -To check if an entity has signed a JSON object a server does the following - -1. Checks if the ``signatures`` object contains an entry with the name of the - entity. If the entry is missing then the check fails. -2. Removes any *signing key identifiers* from the entry with algorithms it - doesn't understand. If there are no *signing key identifiers* left then the - check fails. -3. Looks up *verification keys* for the remaining *signing key identifiers* - either from a local cache or by consulting a trusted key server. If it - cannot find a *verification key* then the check fails. -4. Decodes the base64 encoded signature bytes. If base64 decoding fails then - the check fails. -5. Checks the signature bytes using the *verification key*. If this fails then - the check fails. Otherwise the check succeeds. - -Canonical JSON --------------- - -The canonical JSON encoding for a value is the shortest UTF-8 JSON encoding -with dictionary keys lexicographically sorted by unicode codepoint. Numbers in -the JSON value must be integers in the range [-(2**53)+1, (2**53)-1]. - -:: - - import json - - def canonical_json(value): - return json.dumps( - value, - ensure_ascii=False, - separators=(',',':'), - sort_keys=True, - ).encode("UTF-8") - -Grammar -+++++++ - -Adapted from the grammar in http://tools.ietf.org/html/rfc7159 removing -insignificant whitespace, fractions, exponents and redundant character escapes - -:: - - value = false / null / true / object / array / number / string - false = %x66.61.6c.73.65 - null = %x6e.75.6c.6c - true = %x74.72.75.65 - object = %x7B [ member *( %x2C member ) ] %7D - member = string %x3A value - array = %x5B [ value *( %x2C value ) ] %5B - number = [ %x2D ] int - int = %x30 / ( %x31-39 *digit ) - digit = %x30-39 - string = %x22 *char %x22 - char = unescaped / %x5C escaped - unescaped = %x20-21 / %x23-5B / %x5D-10FFFF - escaped = %x22 ; " quotation mark U+0022 - / %x5C ; \ reverse solidus U+005C - / %x62 ; b backspace U+0008 - / %x66 ; f form feed U+000C - / %x6E ; n line feed U+000A - / %x72 ; r carriage return U+000D - / %x74 ; t tab U+0009 - / %x75.30.30.30 (%x30-37 / %x62 / %x65-66) ; u000X - / %x75.30.30.31 (%x30-39 / %x61-66) ; u001X - -Signing Events -============== - -Signing events is a more complicated process since servers can choose to redact -non-essential event contents. Before signing the event it is encoded as -Canonical JSON and hashed using SHA-256. The resulting hash is then stored -in the event JSON in a ``hash`` object under a ``sha256`` key. Then all -non-essential keys are stripped from the event object, and the resulting object -which included the ``hash`` key is signed using the JSON signing algorithm. - -Servers can then transmit the entire event or the event with the non-essential -keys removed. Receiving servers can then check the entire event if it is -present by computing the SHA-256 of the event excluding the ``hash`` object, or -by using the ``hash`` object included in the event if keys have been redacted. - -New hash functions can be introduced by adding additional keys to the ``hash`` -object. Since the ``hash`` object cannot be redacted a server shouldn't allow -too many hashes to be listed, otherwise a server might embed illict data within -the ``hash`` object. For similar reasons a server shouldn't allow hash values -that are too long. - -[[TODO(markjh): We might want to specify a maximum number of keys for the -``hash`` and we might want to specify the maximum output size of a hash]] - -[[TODO(markjh) We might want to allow the server to omit the output of well -known hash functions like SHA-256 when none of the keys have been redacted]] |