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+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 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]]
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