Implementation of MSC3967: Don't require UIA for initial upload of cross signing keys (#15077)

3 files changed, 40 insertions, 9 deletions

diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index bc38fae0b6..7c81f055b6 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -194,3 +194,6 @@ class ExperimentalConfig(Config):
         self.msc3966_exact_event_property_contains = experimental.get(
             "msc3966_exact_event_property_contains", False
         )
+
+        # MSC3967: Do not require UIA when first uploading cross signing keys
+        self.msc3967_enabled = experimental.get("msc3967_enabled", False)
diff --git a/synapse/handlers/e2e_keys.py b/synapse/handlers/e2e_keys.py
index 43cbece21b..4e9c8d8db0 100644
--- a/synapse/handlers/e2e_keys.py
+++ b/synapse/handlers/e2e_keys.py
@@ -1301,6 +1301,20 @@ class E2eKeysHandler:
 
         return desired_key_data
 
+    async def is_cross_signing_set_up_for_user(self, user_id: str) -> bool:
+        """Checks if the user has cross-signing set up
+
+        Args:
+            user_id: The user to check
+
+        Returns:
+            True if the user has cross-signing set up, False otherwise
+        """
+        existing_master_key = await self.store.get_e2e_cross_signing_key(
+            user_id, "master"
+        )
+        return existing_master_key is not None
+
 
 def _check_cross_signing_key(
     key: JsonDict, user_id: str, key_type: str, signing_key: Optional[VerifyKey] = None
diff --git a/synapse/rest/client/keys.py b/synapse/rest/client/keys.py
index 7873b363c0..32bb8b9a91 100644
--- a/synapse/rest/client/keys.py
+++ b/synapse/rest/client/keys.py
@@ -312,15 +312,29 @@ class SigningKeyUploadServlet(RestServlet):
         user_id = requester.user.to_string()
         body = parse_json_object_from_request(request)
 
-        await self.auth_handler.validate_user_via_ui_auth(
-            requester,
-            request,
-            body,
-            "add a device signing key to your account",
-            # Allow skipping of UI auth since this is frequently called directly
-            # after login and it is silly to ask users to re-auth immediately.
-            can_skip_ui_auth=True,
-        )
+        if self.hs.config.experimental.msc3967_enabled:
+            if await self.e2e_keys_handler.is_cross_signing_set_up_for_user(user_id):
+                # If we already have a master key then cross signing is set up and we require UIA to reset
+                await self.auth_handler.validate_user_via_ui_auth(
+                    requester,
+                    request,
+                    body,
+                    "reset the device signing key on your account",
+                    # Do not allow skipping of UIA auth.
+                    can_skip_ui_auth=False,
+                )
+            # Otherwise we don't require UIA since we are setting up cross signing for first time
+        else:
+            # Previous behaviour is to always require UIA but allow it to be skipped
+            await self.auth_handler.validate_user_via_ui_auth(
+                requester,
+                request,
+                body,
+                "add a device signing key to your account",
+                # Allow skipping of UI auth since this is frequently called directly
+                # after login and it is silly to ask users to re-auth immediately.
+                can_skip_ui_auth=True,
+            )
 
         result = await self.e2e_keys_handler.upload_signing_keys_for_user(user_id, body)
         return 200, result