1 files changed, 29 insertions, 3 deletions
diff --git a/synapse/rest/client/sync.py b/synapse/rest/client/sync.py
index 1d8cbfdf00..93fe1d439e 100644
--- a/synapse/rest/client/sync.py
+++ b/synapse/rest/client/sync.py
@@ -1081,15 +1081,41 @@ class SlidingSyncRestServlet(RestServlet):
     async def encode_extensions(
         self, requester: Requester, extensions: SlidingSyncResult.Extensions
     ) -> JsonDict:
-        result = {}
+        serialized_extensions: JsonDict = {}
 
         if extensions.to_device is not None:
-            result["to_device"] = {
+            serialized_extensions["to_device"] = {
                 "next_batch": extensions.to_device.next_batch,
                 "events": extensions.to_device.events,
             }
 
-        return result
+        if extensions.e2ee is not None:
+            serialized_extensions["e2ee"] = {
+                # We always include this because
+                # https://github.com/vector-im/element-android/issues/3725. The spec
+                # isn't terribly clear on when this can be omitted and how a client
+                # would tell the difference between "no keys present" and "nothing
+                # changed" in terms of whole field absent / individual key type entry
+                # absent Corresponding synapse issue:
+                # https://github.com/matrix-org/synapse/issues/10456
+                "device_one_time_keys_count": extensions.e2ee.device_one_time_keys_count,
+                # https://github.com/matrix-org/matrix-doc/blob/54255851f642f84a4f1aaf7bc063eebe3d76752b/proposals/2732-olm-fallback-keys.md
+                # states that this field should always be included, as long as the
+                # server supports the feature.
+                "device_unused_fallback_key_types": extensions.e2ee.device_unused_fallback_key_types,
+            }
+
+            if extensions.e2ee.device_list_updates is not None:
+                serialized_extensions["e2ee"]["device_lists"] = {}
+
+                serialized_extensions["e2ee"]["device_lists"]["changed"] = list(
+                    extensions.e2ee.device_list_updates.changed
+                )
+                serialized_extensions["e2ee"]["device_lists"]["left"] = list(
+                    extensions.e2ee.device_list_updates.left
+                )
+
+        return serialized_extensions
 
 
 def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None: