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Add detached payload support for JWS encoding and decoding (#723)
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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".
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fviard authored Mar 26, 2022
1 parent 6a624f4 commit 1e79156
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Showing 2 changed files with 91 additions and 9 deletions.
49 changes: 40 additions & 9 deletions jwt/api_jws.py
Original file line number Diff line number Diff line change
Expand Up @@ -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)
Expand All @@ -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")
Expand All @@ -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:
Expand All @@ -148,6 +171,14 @@ def decode_complete(

payload, signing_input, header, signature = self._load(jwt)

if header.get("b64", True) is False:
if detached_payload is None:
raise DecodeError(
'It is required that you pass in a value for the "detached_payload" argument to decode a message having the b64 header set to 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)

Expand Down
51 changes: 51 additions & 0 deletions tests/test_api_jws.py
Original file line number Diff line number Diff line change
Expand Up @@ -719,3 +719,54 @@ def test_encode_fails_on_invalid_kid_types(self, jws, payload):
jws.encode(payload, "secret", headers={"kid": None})

assert "Key ID header parameter must be a string" == str(exc.value)

def test_encode_decode_with_detached_content(self, jws, payload):
secret = "secret"
jws_message = jws.encode(
payload, secret, algorithm="HS256", is_payload_detached=True
)

jws.decode(jws_message, secret, algorithms=["HS256"], detached_payload=payload)

def test_encode_detached_content_with_b64_header(self, jws, payload):
secret = "secret"

# Check that detached content is automatically detected when b64 is false
headers = {"b64": False}
token = jws.encode(payload, secret, "HS256", headers)

msg_header, msg_payload, _ = token.split(".")
msg_header = base64url_decode(msg_header.encode())
msg_header_obj = json.loads(msg_header)

assert "b64" in msg_header_obj
assert msg_header_obj["b64"] is False
# Check that the payload is not inside the token
assert not msg_payload

# Check that content is not detached and b64 header removed when b64 is true
headers = {"b64": True}
token = jws.encode(payload, secret, "HS256", headers)

msg_header, msg_payload, _ = token.split(".")
msg_header = base64url_decode(msg_header.encode())
msg_header_obj = json.loads(msg_header)

assert "b64" not in msg_header_obj
assert msg_payload

def test_decode_detached_content_without_proper_argument(self, jws):
example_jws = (
"eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiIsImI2NCI6ZmFsc2V9"
"."
".65yNkX_ZH4A_6pHaTL_eI84OXOHtfl4K0k5UnlXZ8f4"
)
example_secret = "secret"

with pytest.raises(DecodeError) as exc:
jws.decode(example_jws, example_secret, algorithms=["HS256"])

assert (
'It is required that you pass in a value for the "detached_payload" argument to decode a message having the b64 header set to false.'
in str(exc.value)
)

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