pyjwt: ES256 algorithm support for PyJWT.

Add optional support for ES256 JWT signing/verifying to PyJWT using
@dmazzella's cryptography port.

Signed-off-by: Jonah Bron <hi@jonah.id>

python-ecosys/pyjwt/jwt.py

@@ -4,6 +4,17 @@ import hmac
 import json
 from time import time
 
+# Optionally depend on https://github.com/dmazzella/ucryptography
+try:
+    # Try importing from ucryptography port.
+    import cryptography
+    from cryptography import hashes, ec, serialization, utils
+
+    _ec_supported = True
+except ImportError:
+    # No cryptography library available, no EC256 support.
+    _ec_supported = False
+
 
 def _to_b64url(data):
     return (
@@ -19,6 +30,28 @@ def _from_b64url(data):
     return binascii.a2b_base64(data.replace(b"-", b"+").replace(b"_", b"/") + b"===")
 
 
+def _sig_der_to_jws(signed):
+    """Accept a DER signature and convert to JSON Web Signature bytes.
+
+    `cryptography` produces signatures encoded in DER ASN.1 binary format.
+    JSON Web Algorithm instead encodes the signature as the point coordinates
+    as bigendian byte strings concatenated.
+
+    See https://datatracker.ietf.org/doc/html/rfc7518#section-3.4
+    """
+    r, s = utils.decode_dss_signature(signed)
+    return r.to_bytes(32, "big") + s.to_bytes(32, "big")
+
+
+def _sig_jws_to_der(signed):
+    """Accept a JSON Web Signature and convert to a DER signature.
+
+    See `_sig_der_to_jws()`
+    """
+    r, s = int.from_bytes(signed[0:32], "big"), int.from_bytes(signed[32:], "big")
+    return utils.encode_dss_signature(r, s)
+
+
 class exceptions:
     class PyJWTError(Exception):
         pass
@@ -37,19 +70,32 @@ class exceptions:
 
 
 def encode(payload, key, algorithm="HS256"):
-    if algorithm != "HS256":
+    if algorithm != "HS256" and algorithm != "ES256":
         raise exceptions.InvalidAlgorithmError
 
-    if isinstance(key, str):
-        key = key.encode()
     header = _to_b64url(json.dumps({"typ": "JWT", "alg": algorithm}).encode())
     payload = _to_b64url(json.dumps(payload).encode())
-    signature = _to_b64url(hmac.new(key, header + b"." + payload, hashlib.sha256).digest())
+
+    if algorithm == "HS256":
+        if isinstance(key, str):
+            key = key.encode()
+        signature = _to_b64url(hmac.new(key, header + b"." + payload, hashlib.sha256).digest())
+    elif algorithm == "ES256":
+        if not _ec_supported:
+            raise exceptions.InvalidAlgorithmError(
+                "Required dependencies for ES256 are not available"
+            )
+        if isinstance(key, int):
+            key = ec.derive_private_key(key, ec.SECP256R1())
+        signature = _to_b64url(
+            _sig_der_to_jws(key.sign(header + b"." + payload, ec.ECDSA(hashes.SHA256())))
+        )
+
     return (header + b"." + payload + b"." + signature).decode()
 
 
-def decode(token, key, algorithms=["HS256"]):
+def decode(token, key, algorithms=["HS256", "ES256"]):
-    if "HS256" not in algorithms:
+    if "HS256" not in algorithms and "ES256" not in algorithms:
         raise exceptions.InvalidAlgorithmError
 
     parts = token.encode().split(b".")
@@ -63,14 +109,31 @@ def decode(token, key, algorithms=["HS256"]):
     except Exception:
         raise exceptions.InvalidTokenError
 
-    if header["alg"] not in algorithms or header["alg"] != "HS256":
+    if header["alg"] not in algorithms or (header["alg"] != "HS256" and header["alg"] != "ES256"):
         raise exceptions.InvalidAlgorithmError
 
-    if isinstance(key, str):
+    if header["alg"] == "HS256":
-        key = key.encode()
+        if isinstance(key, str):
-    calculated_signature = hmac.new(key, parts[0] + b"." + parts[1], hashlib.sha256).digest()
+            key = key.encode()
-    if signature != calculated_signature:
+        calculated_signature = hmac.new(key, parts[0] + b"." + parts[1], hashlib.sha256).digest()
-        raise exceptions.InvalidSignatureError
+        if signature != calculated_signature:
+            raise exceptions.InvalidSignatureError
+    elif header["alg"] == "ES256":
+        if not _ec_supported:
+            raise exceptions.InvalidAlgorithmError(
+                "Required dependencies for ES256 are not available"
+            )
+
+        if isinstance(key, bytes):
+            key = ec.EllipticCurvePublicKey.from_encoded_point(key, ec.SECP256R1())
+        try:
+            key.verify(
+                _sig_jws_to_der(signature),
+                parts[0] + b"." + parts[1],
+                ec.ECDSA(hashes.SHA256()),
+            )
+        except cryptography.exceptions.InvalidSignature:
+            raise exceptions.InvalidSignatureError
 
     if "exp" in payload:
         if time() > payload["exp"]:

python-ecosys/pyjwt/test_jwt.py

@@ -1,28 +1,71 @@
 import jwt
 from time import time
 
+"""
+Run tests by executing:
+
+```
+mpremote fs cp jwt.py :lib/jwt.py + run test_jwt.py
+```
+
+Only the full test suite can be run if
+[ucryptography](https://github.com/dmazzella/ucryptography) is present in the
+firmware.
+"""
+
+# Indentation
+I = "    "
+
+print("Testing HS256")
 secret_key = "top-secret!"
 
 token = jwt.encode({"user": "joe"}, secret_key, algorithm="HS256")
-print(token)
 decoded = jwt.decode(token, secret_key, algorithms=["HS256"])
 if decoded != {"user": "joe"}:
     raise Exception("Invalid decoded JWT")
 else:
-    print("Encode/decode test: OK")
+    print(I, "Encode/decode test: OK")
 
 try:
     decoded = jwt.decode(token, "wrong-secret", algorithms=["HS256"])
 except jwt.exceptions.InvalidSignatureError:
-    print("Invalid signature test: OK")
+    print(I, "Invalid signature test: OK")
 else:
     raise Exception("Invalid JWT should have failed decoding")
 
 token = jwt.encode({"user": "joe", "exp": time() - 1}, secret_key)
-print(token)
 try:
     decoded = jwt.decode(token, secret_key, algorithms=["HS256"])
 except jwt.exceptions.ExpiredSignatureError:
-    print("Expired token test: OK")
+    print(I, "Expired token test: OK")
 else:
     raise Exception("Expired JWT should have failed decoding")
+
+
+print("Testing ES256")
+try:
+    from cryptography import ec
+except ImportError:
+    raise Exception("No cryptography lib present, can't test ES256")
+
+private_key = ec.derive_private_key(
+    0xEB6DFB26C7A3C23D33C60F7C7BA61B6893451F2643E0737B20759E457825EE75, ec.SECP256R1()
+)
+wrong_private_key = ec.derive_private_key(
+    0x25D91A0DA38F69283A0CE32B87D82817CA4E134A1693BE6083C2292BF562A451, ec.SECP256R1()
+)
+
+token = jwt.encode({"user": "joe"}, private_key, algorithm="ES256")
+decoded = jwt.decode(token, private_key.public_key(), algorithms=["ES256"])
+if decoded != {"user": "joe"}:
+    raise Exception("Invalid decoded JWT")
+else:
+    print(I, "Encode/decode test: OK")
+
+token = jwt.encode({"user": "joe"}, private_key, algorithm="ES256")
+try:
+    decoded = jwt.decode(token + "a", wrong_private_key.public_key(), algorithms=["ES256"])
+except jwt.exceptions.InvalidSignatureError:
+    print(I, "Invalid signature test: OK")
+else:
+    raise Exception("Invalid JWT should have fialed decoding")