Use PyJWK algorithm when encoding without explicit algorithm

CHANGELOG.rst

@@ -17,6 +17,7 @@ Fixed
 - Close ``HTTPError`` response to prevent ``ResourceWarning`` on Python 3.14 by @veeceey in `#1133 <https://github.com/jpadilla/pyjwt/pull/1133>`__
 - Do not keep ``algorithms`` dict in PyJWK instances by @akx in `#1143 <https://github.com/jpadilla/pyjwt/pull/1143>`__
 - Validate the crit (Critical) Header Parameter defined in RFC 7515 §4.1.11. by @dmbs335 in `GHSA-752w-5fwx-jx9f <https://github.com/jpadilla/pyjwt/security/advisories/GHSA-752w-5fwx-jx9f>`__
+- Use PyJWK algorithm when encoding without explicit algorithm in `#1148 <https://github.com/jpadilla/pyjwt/pull/1148>`__
 
 Added
 ~~~~~

jwt/api_jws.py

@@ -27,6 +27,8 @@
     from .algorithms import AllowedPrivateKeys, AllowedPublicKeys
     from .types import SigOptions
 
+_ALGORITHM_UNSET = object()
+
 
 class PyJWS:
     header_typ = "JWT"
@@ -119,7 +121,7 @@ def encode(
         self,
         payload: bytes,
         key: AllowedPrivateKeys | PyJWK | str | bytes,
-        algorithm: str | None = "HS256",
+        algorithm: str | None = _ALGORITHM_UNSET,  # type: ignore[assignment]
         headers: dict[str, Any] | None = None,
         json_encoder: type[json.JSONEncoder] | None = None,
         is_payload_detached: bool = False,
@@ -128,7 +130,12 @@ def encode(
         segments: list[bytes] = []
 
         # declare a new var to narrow the type for type checkers
-        if algorithm is None:
+        if algorithm is _ALGORITHM_UNSET:
+            if isinstance(key, PyJWK):
+                algorithm_ = key.algorithm_name
+            else:
+                algorithm_ = "HS256"
+        elif algorithm is None:
             if isinstance(key, PyJWK):
                 algorithm_ = key.algorithm_name
             else:

jwt/api_jwt.py

@@ -8,7 +8,7 @@
 from datetime import datetime, timedelta, timezone
 from typing import TYPE_CHECKING, Any, Union, cast
 
-from .api_jws import PyJWS, _jws_global_obj
+from .api_jws import PyJWS, _ALGORITHM_UNSET, _jws_global_obj
 from .exceptions import (
     DecodeError,
     ExpiredSignatureError,
@@ -91,7 +91,7 @@ def encode(
         self,
         payload: dict[str, Any],
         key: AllowedPrivateKeyTypes,
-        algorithm: str | None = "HS256",
+        algorithm: str | None = _ALGORITHM_UNSET,  # type: ignore[assignment]
         headers: dict[str, Any] | None = None,
         json_encoder: type[json.JSONEncoder] | None = None,
         sort_headers: bool = True,

tests/test_api_jws.py

@@ -261,6 +261,27 @@ def test_encode_with_jwk(self, jws: PyJWS, payload: bytes) -> None:
             ),
         }
 
+    def test_encode_with_jwk_uses_key_algorithm(
+        self, jws: PyJWS, payload: bytes
+    ) -> None:
+        """Test that encoding with a PyJWK key uses the key's algorithm
+        when no algorithm is explicitly specified. Regression test for #1147."""
+        jwk = PyJWK(
+            {
+                "kty": "oct",
+                "alg": "HS384",
+                "k": "c2VjcmV0",  # "secret"
+            }
+        )
+        # Should use HS384 from the key, not default to HS256
+        msg = jws.encode(payload, key=jwk)
+        header = jws.get_unverified_header(msg)
+        assert header["alg"] == "HS384"
+
+        # Should also be decodable with the same key
+        decoded = jws.decode(msg, key=jwk)
+        assert decoded == payload
+
     def test_decode_algorithm_param_should_be_case_sensitive(self, jws: PyJWS) -> None:
         example_jws = (
             "eyJhbGciOiJoczI1NiIsInR5cCI6IkpXVCJ9"  # alg = hs256

tests/test_api_jwt.py

@@ -7,6 +7,7 @@
 import pytest
 
 from jwt.types import Options
+from jwt.api_jwk import PyJWK
 from jwt.api_jwt import PyJWT
 from jwt.exceptions import (
     DecodeError,
@@ -45,6 +46,22 @@ def test_jwt_with_options(self) -> None:
         # assert that verify_signature is respected unless verify_exp is overridden
         assert jwt.options["verify_exp"] is False
 
+    def test_encode_with_jwk_uses_key_algorithm(self, jwt: PyJWT) -> None:
+        """Test that encoding with a PyJWK key uses the key's algorithm
+        when no algorithm is explicitly specified. Regression test for #1147."""
+        jwk = PyJWK(
+            {
+                "kty": "oct",
+                "alg": "HS384",
+                "k": "c2VjcmV0",  # "secret"
+            }
+        )
+        payload = {"hello": "world"}
+        # Should use HS384 from the key, not default to HS256
+        token = jwt.encode(payload, jwk)
+        header = jwt.decode_complete(token, jwk, algorithms=["HS384"])["header"]
+        assert header["alg"] == "HS384"
+
     def test_decodes_valid_jwt(self, jwt: PyJWT) -> None:
         example_payload = {"hello": "world"}
         example_secret = "secret"