diff --git a/synapse/api/auth/msc3861_delegated.py b/synapse/api/auth/msc3861_delegated.py
new file mode 100644
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+++ b/synapse/api/auth/msc3861_delegated.py
@@ -0,0 +1,280 @@
+# Copyright 2023 The Matrix.org Foundation.
+#
+# 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, Any, Dict, List, Optional
+from urllib.parse import urlencode
+
+from authlib.oauth2 import ClientAuth
+from authlib.oauth2.auth import encode_client_secret_basic, encode_client_secret_post
+from authlib.oauth2.rfc7523 import ClientSecretJWT, PrivateKeyJWT, private_key_jwt_sign
+from authlib.oauth2.rfc7662 import IntrospectionToken
+from authlib.oidc.discovery import OpenIDProviderMetadata, get_well_known_url
+
+from twisted.web.client import readBody
+from twisted.web.http_headers import Headers
+
+from synapse.api.auth.base import BaseAuth
+from synapse.api.errors import (
+ AuthError,
+ InvalidClientTokenError,
+ OAuthInsufficientScopeError,
+ StoreError,
+)
+from synapse.http.site import SynapseRequest
+from synapse.logging.context import make_deferred_yieldable
+from synapse.types import Requester, UserID, create_requester
+from synapse.util import json_decoder
+from synapse.util.caches.cached_call import RetryOnExceptionCachedCall
+
+if TYPE_CHECKING:
+ from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+def scope_to_list(scope: str) -> List[str]:
+ """Convert a scope string to a list of scope tokens"""
+ return scope.strip().split(" ")
+
+
+class PrivateKeyJWTWithKid(PrivateKeyJWT):
+ """An implementation of the private_key_jwt client auth method that includes a kid header.
+
+ This is needed because some providers (Keycloak) require the kid header to figure
+ out which key to use to verify the signature.
+ """
+
+ def sign(self, auth: Any, token_endpoint: str) -> bytes:
+ return private_key_jwt_sign(
+ auth.client_secret,
+ client_id=auth.client_id,
+ token_endpoint=token_endpoint,
+ claims=self.claims,
+ header={"kid": auth.client_secret["kid"]},
+ )
+
+
+class MSC3861DelegatedAuth(BaseAuth):
+ AUTH_METHODS = {
+ "client_secret_post": encode_client_secret_post,
+ "client_secret_basic": encode_client_secret_basic,
+ "client_secret_jwt": ClientSecretJWT(),
+ "private_key_jwt": PrivateKeyJWTWithKid(),
+ }
+
+ EXTERNAL_ID_PROVIDER = "oauth-delegated"
+
+ def __init__(self, hs: "HomeServer"):
+ super().__init__(hs)
+
+ self._config = hs.config.experimental.msc3861
+ auth_method = MSC3861DelegatedAuth.AUTH_METHODS.get(
+ self._config.client_auth_method.value, None
+ )
+ # Those assertions are already checked when parsing the config
+ assert self._config.enabled, "OAuth delegation is not enabled"
+ assert self._config.issuer, "No issuer provided"
+ assert self._config.client_id, "No client_id provided"
+ assert auth_method is not None, "Invalid client_auth_method provided"
+
+ self._http_client = hs.get_proxied_http_client()
+ self._hostname = hs.hostname
+
+ self._issuer_metadata = RetryOnExceptionCachedCall(self._load_metadata)
+
+ if isinstance(auth_method, PrivateKeyJWTWithKid):
+ # Use the JWK as the client secret when using the private_key_jwt method
+ assert self._config.jwk, "No JWK provided"
+ self._client_auth = ClientAuth(
+ self._config.client_id, self._config.jwk, auth_method
+ )
+ else:
+ # Else use the client secret
+ assert self._config.client_secret, "No client_secret provided"
+ self._client_auth = ClientAuth(
+ self._config.client_id, self._config.client_secret, auth_method
+ )
+
+ async def _load_metadata(self) -> OpenIDProviderMetadata:
+ if self._config.issuer_metadata is not None:
+ return OpenIDProviderMetadata(**self._config.issuer_metadata)
+ url = get_well_known_url(self._config.issuer, external=True)
+ response = await self._http_client.get_json(url)
+ metadata = OpenIDProviderMetadata(**response)
+ # metadata.validate_introspection_endpoint()
+ return metadata
+
+ async def _introspect_token(self, token: str) -> IntrospectionToken:
+ metadata = await self._issuer_metadata.get()
+ introspection_endpoint = metadata.get("introspection_endpoint")
+ raw_headers: Dict[str, str] = {
+ "Content-Type": "application/x-www-form-urlencoded",
+ "User-Agent": str(self._http_client.user_agent, "utf-8"),
+ "Accept": "application/json",
+ }
+
+ args = {"token": token, "token_type_hint": "access_token"}
+ body = urlencode(args, True)
+
+ # Fill the body/headers with credentials
+ uri, raw_headers, body = self._client_auth.prepare(
+ method="POST", uri=introspection_endpoint, headers=raw_headers, body=body
+ )
+ headers = Headers({k: [v] for (k, v) in raw_headers.items()})
+
+ # Do the actual request
+ # We're not using the SimpleHttpClient util methods as we don't want to
+ # check the HTTP status code and we do the body encoding ourself.
+ response = await self._http_client.request(
+ method="POST",
+ uri=uri,
+ data=body.encode("utf-8"),
+ headers=headers,
+ )
+
+ resp_body = await make_deferred_yieldable(readBody(response))
+ # TODO: Let's not worry about 5xx errors & co. for now and just try
+ # decoding that as JSON. We should also do some validation of the
+ # response
+ resp = json_decoder.decode(resp_body.decode("utf-8"))
+ return IntrospectionToken(**resp)
+
+ async def is_server_admin(self, requester: Requester) -> bool:
+ return "urn:synapse:admin:*" in requester.scope
+
+ async def get_user_by_req(
+ self,
+ request: SynapseRequest,
+ allow_guest: bool = False,
+ allow_expired: bool = False,
+ ) -> Requester:
+ access_token = self.get_access_token_from_request(request)
+
+ # TODO: we probably want to assert the allow_guest inside this call so that we don't provision the user if they don't have enough permission:
+ requester = await self.get_user_by_access_token(access_token, allow_expired)
+
+ if not allow_guest and requester.is_guest:
+ raise OAuthInsufficientScopeError(
+ ["urn:matrix:org.matrix.msc2967.client:api:*"]
+ )
+
+ return requester
+
+ async def get_user_by_access_token(
+ self,
+ token: str,
+ allow_expired: bool = False,
+ ) -> Requester:
+ introspection_result = await self._introspect_token(token)
+
+ logger.info(f"Introspection result: {introspection_result!r}")
+
+ # TODO: introspection verification should be more extensive, especially:
+ # - verify the audience
+ if not introspection_result.get("active"):
+ raise InvalidClientTokenError("Token is not active")
+
+ # Let's look at the scope
+ scope: List[str] = scope_to_list(introspection_result.get("scope", ""))
+
+ # Determine type of user based on presence of particular scopes
+ has_admin_scope = "urn:synapse:admin:*" in scope
+ has_user_scope = "urn:matrix:org.matrix.msc2967.client:api:*" in scope
+ has_guest_scope = "urn:matrix:org.matrix.msc2967.client:api:guest" in scope
+ is_user = has_user_scope or has_admin_scope
+ is_guest = has_guest_scope and not is_user
+
+ if not is_user and not is_guest:
+ raise InvalidClientTokenError("No scope in token granting user rights")
+
+ # Match via the sub claim
+ sub: Optional[str] = introspection_result.get("sub")
+ if sub is None:
+ raise InvalidClientTokenError(
+ "Invalid sub claim in the introspection result"
+ )
+
+ user_id_str = await self.store.get_user_by_external_id(
+ MSC3861DelegatedAuth.EXTERNAL_ID_PROVIDER, sub
+ )
+ if user_id_str is None:
+ # If we could not find a user via the external_id, it either does not exist,
+ # or the external_id was never recorded
+
+ # TODO: claim mapping should be configurable
+ username: Optional[str] = introspection_result.get("username")
+ if username is None or not isinstance(username, str):
+ raise AuthError(
+ 500,
+ "Invalid username claim in the introspection result",
+ )
+ user_id = UserID(username, self._hostname)
+
+ # First try to find a user from the username claim
+ user_info = await self.store.get_userinfo_by_id(user_id=user_id.to_string())
+ if user_info is None:
+ # If the user does not exist, we should create it on the fly
+ # TODO: we could use SCIM to provision users ahead of time and listen
+ # for SCIM SET events if those ever become standard:
+ # https://datatracker.ietf.org/doc/html/draft-hunt-scim-notify-00
+
+ # TODO: claim mapping should be configurable
+ # If present, use the name claim as the displayname
+ name: Optional[str] = introspection_result.get("name")
+
+ await self.store.register_user(
+ user_id=user_id.to_string(), create_profile_with_displayname=name
+ )
+
+ # And record the sub as external_id
+ await self.store.record_user_external_id(
+ MSC3861DelegatedAuth.EXTERNAL_ID_PROVIDER, sub, user_id.to_string()
+ )
+ else:
+ user_id = UserID.from_string(user_id_str)
+
+ # Find device_id in scope
+ device_id = None
+ for tok in scope:
+ if tok.startswith("urn:matrix:org.matrix.msc2967.client:device:"):
+ parts = tok.split(":")
+ if len(parts) == 5:
+ device_id = parts[4]
+
+ if device_id:
+ # Create the device on the fly if it does not exist
+ try:
+ await self.store.get_device(
+ user_id=user_id.to_string(), device_id=device_id
+ )
+ except StoreError:
+ await self.store.store_device(
+ user_id=user_id.to_string(),
+ device_id=device_id,
+ initial_device_display_name="OIDC-native client",
+ )
+
+ # TODO: there is a few things missing in the requester here, which still need
+ # to be figured out, like:
+ # - impersonation, with the `authenticated_entity`, which is used for
+ # rate-limiting, MAU limits, etc.
+ # - shadow-banning, with the `shadow_banned` flag
+ # - a proper solution for appservices, which still needs to be figured out in
+ # the context of MSC3861
+ return create_requester(
+ user_id=user_id,
+ device_id=device_id,
+ scope=scope,
+ is_guest=is_guest,
+ )
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