Add detached payload support for JWS encoding and decoding

Specifications allow to have JWS with unencoded detached payloads.
This changeset adds detached payload support for encoding and decoding
functions.

For encoding, detached payload can be enabled by setting the
"is_payload_detached" arg or having the "b64=False" inside the headers.

For decoding, the detached payload content (bytes) has to be provided
with the "detached_payload" arg and "b64=False" has to be found inside
the decoded headers.

Functionnally, when this feature is used, the signature will be computed
over the raw data bytes of the payload, without being base64 encoded and
obviously, the payload will not be provided inside the generated JWS.
So, the generated JWS will look like:
base64url(header)..base64url(signature)

Relevant specifications:
RFC 7515: "JSON Web Signature (JWS)". (Annexe F)
RFC 7797: "JSON Web Signature (JWS) Unencoded Payload Option".

jwt/api_jws.py

@@ -80,34 +80,54 @@ def encode(
         algorithm: Optional[str] = "HS256",
         headers: Optional[Dict] = None,
         json_encoder: Optional[Type[json.JSONEncoder]] = None,
+        is_payload_detached: bool = False,
     ) -> str:
         segments = []
 
         if algorithm is None:
             algorithm = "none"
 
-        # Prefer headers["alg"] if present to algorithm parameter.
-        if headers and "alg" in headers and headers["alg"]:
-            algorithm = headers["alg"]
+        # Prefer headers values if present to function parameters.
+        if headers:
+            headers_alg = headers.get("alg")
+            if headers_alg:
+                algorithm = headers["alg"]
+
+            headers_b64 = headers.get("b64")
+            if headers_b64 is False:
+                is_payload_detached = True
 
         # Header
-        header = {"typ": self.header_typ, "alg": algorithm}
+        header = {"typ": self.header_typ, "alg": algorithm}  # type: Dict[str, Any]
 
         if headers:
             self._validate_headers(headers)
             header.update(headers)
-            if not header["typ"]:
-                del header["typ"]
+
+        if not header["typ"]:
+            del header["typ"]
+
+        if is_payload_detached:
+            header["b64"] = False
+        elif "b64" in header:
+            # True is the standard value for b64, so no need for it
+            del header["b64"]
 
         json_header = json.dumps(
             header, separators=(",", ":"), cls=json_encoder
         ).encode()
 
         segments.append(base64url_encode(json_header))
-        segments.append(base64url_encode(payload))
+
+        if is_payload_detached:
+            msg_payload = payload
+        else:
+            msg_payload = base64url_encode(payload)
+        segments.append(msg_payload)
 
         # Segments
         signing_input = b".".join(segments)
+
         try:
             alg_obj = self._algorithms[algorithm]
             key = alg_obj.prepare_key(key)
@@ -119,11 +139,13 @@ def encode(
                     "Algorithm '%s' could not be found. Do you have cryptography "
                     "installed?" % algorithm
                 ) from e
-            else:
-                raise NotImplementedError("Algorithm not supported") from e
+            raise NotImplementedError("Algorithm not supported") from e
 
         segments.append(base64url_encode(signature))
 
+        # Don't put the payload content inside the encoded token when detached
+        if is_payload_detached:
+            segments[1] = b""
         encoded_string = b".".join(segments)
 
         return encoded_string.decode("utf-8")
@@ -134,6 +156,7 @@ def decode_complete(
         key: str = "",
         algorithms: Optional[List[str]] = None,
         options: Optional[Dict] = None,
+        detached_payload: Optional[bytes] = None,
         **kwargs,
     ) -> Dict[str, Any]:
         if options is None:
@@ -148,6 +171,10 @@ def decode_complete(
 
         payload, signing_input, header, signature = self._load(jwt)
 
+        if detached_payload is not None and header.get("b64", True) is False:
+            payload = detached_payload
+            signing_input = b".".join([signing_input.rsplit(b".", 1)[0], payload])
+
         if verify_signature:
             self._verify_signature(signing_input, header, signature, key, algorithms)