Signing JSON ============ JSON is signed by encoding the JSON object without ``signatures`` or ``meta`` 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 ``meta`` 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 ``meta`` object and the ``signatures`` object are not covered by the signature. Therefore intermediate servers can add metadata such as time stamps and additional signatures. :: { "name": "example.org", "signing_keys": { "ed25519:1": "XSl0kuyvrXNj6A+7/tkrB9sxSbRi08Of5uRhxOqZtEQ" }, "meta": { "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", {}) meta = json_object.pop("meta", 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 meta is not None: json_object["meta"] = meta 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 algrothims 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]]