3 files changed, 53 insertions, 5 deletions
diff --git a/synapse/rest/admin/__init__.py b/synapse/rest/admin/__init__.py
index 9bd0d764f8..91edfd45d7 100644
--- a/synapse/rest/admin/__init__.py
+++ b/synapse/rest/admin/__init__.py
@@ -88,6 +88,7 @@ from synapse.rest.admin.users import (
     UserByThreePid,
     UserMembershipRestServlet,
     UserRegisterServlet,
+    UserReplaceMasterCrossSigningKeyRestServlet,
     UserRestServletV2,
     UsersRestServletV2,
     UserTokenRestServlet,
@@ -292,6 +293,7 @@ def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     ListDestinationsRestServlet(hs).register(http_server)
     RoomMessagesRestServlet(hs).register(http_server)
     RoomTimestampToEventRestServlet(hs).register(http_server)
+    UserReplaceMasterCrossSigningKeyRestServlet(hs).register(http_server)
     UserByExternalId(hs).register(http_server)
     UserByThreePid(hs).register(http_server)
 
diff --git a/synapse/rest/admin/users.py b/synapse/rest/admin/users.py
index 73878dd99d..9900498fbe 100644
--- a/synapse/rest/admin/users.py
+++ b/synapse/rest/admin/users.py
@@ -1270,6 +1270,46 @@ class AccountDataRestServlet(RestServlet):
         }
 
 
+class UserReplaceMasterCrossSigningKeyRestServlet(RestServlet):
+    """Allow a given user to replace their master cross-signing key without UIA.
+
+    This replacement is permitted for a limited period (currently 10 minutes).
+
+    While this is exposed via the admin API, this is intended for use by the
+    Matrix Authentication Service rather than server admins.
+    """
+
+    PATTERNS = admin_patterns(
+        "/users/(?P<user_id>[^/]*)/_allow_cross_signing_replacement_without_uia"
+    )
+    REPLACEMENT_PERIOD_MS = 10 * 60 * 1000  # 10 minutes
+
+    def __init__(self, hs: "HomeServer"):
+        self._auth = hs.get_auth()
+        self._store = hs.get_datastores().main
+
+    async def on_POST(
+        self,
+        request: SynapseRequest,
+        user_id: str,
+    ) -> Tuple[int, JsonDict]:
+        await assert_requester_is_admin(self._auth, request)
+
+        if user_id is None:
+            raise NotFoundError("User not found")
+
+        timestamp = (
+            await self._store.allow_master_cross_signing_key_replacement_without_uia(
+                user_id, self.REPLACEMENT_PERIOD_MS
+            )
+        )
+
+        if timestamp is None:
+            raise NotFoundError("User has no master cross-signing key")
+
+        return HTTPStatus.OK, {"updatable_without_uia_before_ms": timestamp}
+
+
 class UserByExternalId(RestServlet):
     """Find a user based on an external ID from an auth provider"""
 
diff --git a/synapse/rest/client/keys.py b/synapse/rest/client/keys.py
index 70b8be1aa2..add8045439 100644
--- a/synapse/rest/client/keys.py
+++ b/synapse/rest/client/keys.py
@@ -376,9 +376,10 @@ class SigningKeyUploadServlet(RestServlet):
         user_id = requester.user.to_string()
         body = parse_json_object_from_request(request)
 
-        is_cross_signing_setup = (
-            await self.e2e_keys_handler.is_cross_signing_set_up_for_user(user_id)
-        )
+        (
+            is_cross_signing_setup,
+            master_key_updatable_without_uia,
+        ) = await self.e2e_keys_handler.check_cross_signing_setup(user_id)
 
         # Before MSC3967 we required UIA both when setting up cross signing for the
         # first time and when resetting the device signing key. With MSC3967 we only
@@ -386,9 +387,14 @@ class SigningKeyUploadServlet(RestServlet):
         # time. Because there is no UIA in MSC3861, for now we throw an error if the
         # user tries to reset the device signing key when MSC3861 is enabled, but allow
         # first-time setup.
+        #
+        # XXX: We now have a get-out clause by which MAS can temporarily mark the master
+        # key as replaceable. It should do its own equivalent of user interactive auth
+        # before doing so.
         if self.hs.config.experimental.msc3861.enabled:
-            # There is no way to reset the device signing key with MSC3861
-            if is_cross_signing_setup:
+            # The auth service has to explicitly mark the master key as replaceable
+            # without UIA to reset the device signing key with MSC3861.
+            if is_cross_signing_setup and not master_key_updatable_without_uia:
                 raise SynapseError(
                     HTTPStatus.NOT_IMPLEMENTED,
                     "Resetting cross signing keys is not yet supported with MSC3861",