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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
+striped. 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(value, signing_key, signing_name):
+ signatures = value.pop("signatures", {})
+ signatures_for_name = signatures.pop(signing_name, {})
+ meta = value.pop("meta", None)
+ signature = signing_key.sign(canonical_json(value))
+ key_identifier = "%s:%s" % (signing_key.algorithm, signing_key.version)
+ signatures_for_name[key_identifier] = encode_base64(signature.signature)
+ signatures[signing_name] = signatures_for_name
+ value["signatures"] = signatures
+ if meta is not None:
+ value["meta"] = meta
+ return value
+
+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
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