Expose the public keys used for client authentication on an endpoint

synapse/rest/synapse/client/__init__.py

@@ -46,6 +46,12 @@ def build_synapse_client_resource_tree(hs: "HomeServer") -> Mapping[str, Resourc
         "/_synapse/client/unsubscribe": UnsubscribeResource(hs),
     }
 
+    # Expose the JWKS endpoint if OAuth2 delegation is enabled
+    if hs.config.auth.oauth_delegation_enabled:
+        from synapse.rest.synapse.client.jwks import JwksResource
+
+        resources["/_synapse/jwks"] = JwksResource(hs)
+
     # provider-specific SSO bits. Only load these if they are enabled, since they
     # rely on optional dependencies.
     if hs.config.oidc.oidc_enabled:

synapse/rest/synapse/client/jwks.py

@@ -0,0 +1,72 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import logging
+from typing import TYPE_CHECKING, Tuple
+
+from synapse.http.server import DirectServeJsonResource
+from synapse.http.site import SynapseRequest
+from synapse.types import JsonDict
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class JwksResource(DirectServeJsonResource):
+    def __init__(self, hs: "HomeServer"):
+        from authlib.jose.rfc7517 import Key
+
+        super().__init__(extract_context=True)
+
+        # Parameters that are allowed to be exposed in the public key.
+        # This is done manually, because authlib's private to public key conversion
+        # is unreliable depending on the version. Instead, we just serialize the private
+        # key and only keep the public parameters.
+        # List from https://www.iana.org/assignments/jose/jose.xhtml#web-key-parameters
+        public_parameters = {
+            "kty",
+            "use",
+            "key_ops",
+            "alg",
+            "kid",
+            "x5u",
+            "x5c",
+            "x5t",
+            "x5t#S256",
+            "crv",
+            "x",
+            "y",
+            "n",
+            "e",
+            "ext",
+        }
+
+        secret = hs.config.auth.oauth_delegation_client_secret
+
+        if isinstance(secret, Key):
+            private_key = secret.as_dict()
+            public_key = {
+                k: v for k, v in private_key.items() if k in public_parameters
+            }
+            keys = [public_key]
+        else:
+            keys = []
+
+        self.res = {
+            "keys": keys,
+        }
+
+    async def _async_render_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
+        return 200, self.res