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+# MSC3884: Sliding Sync Extension: E2EE
+
+This MSC is an extension to [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575)
+which includes support for end-to-end encrypted room fields in the `/sync` response.
+
+## Proposal
+
+MSC3575 does not include support for end-to-end encrypted rooms. This extension will add support for
+end-to-end encrypted fields, specifically one-time keys, changed devices and fallback key types.
+
+The prosposal is to introduce a new extension called `e2ee` with the following request parameters:
+```json5
+{
+    "enabled": true, // sticky
+}
+```
+
+_Note: This extension ignores the core `lists` and `rooms` parameters to extensions, because none of the
+data returned by this extension is scoped to a particular room._
+
+If `enabled` is `true`, then the sliding sync response MAY include the following response fields in
+the `e2ee` extension response:
+```json
+{
+    "device_one_time_keys_count": {
+        "signed_curve25519": 50
+    },
+    "device_lists": {
+        "changed": ["@alice:example.com"],
+        "left": ["@bob:example.com"]
+    },
+    "device_unused_fallback_key_types": [
+        "signed_curve25519"
+    ]
+}
+```
+
+All keys are optional and clients MUST check if they exist prior to use. If there are zero changes to
+every field then the server MAY omit sending back an `e2ee` extension entirely.
+
+The semantics of these fields is exactly the same as the current `/sync` implementation, as implemented
+in v1.3 of the Client-Server Specification. `device_lists` may be omitted if there are no users who
+have changed/left.
+
+For sliding sync, an initial response will include all fields. When there are updates, only the
+_changed_ fields are returned. That is to say, if `device_one_time_keys_count` has not changed between
+requests, it will be omitted which means to use the previous value. This deviates from the current
+`/sync` implementation which always includes this field. Likewise for `device_unused_fallback_key_types`.
+
+Particular care must be taken when a fallback key is used, as this will cause the response to be:
+```json
+"device_unused_fallback_key_types": []
+```
+which is not the same as the field being omitted/null. The empty array means the key was used. Omitted
+fields means no changes.
+
+## Potential issues
+
+It's unclear if `device_unused_fallback_key_types` and `device_one_time_keys_count` should always be
+included or not, as this extension deviates from the logic as v1.3 of the Specification which is not
+clear on this. If it is always included, this adds extra bytes and therefore consumes needless
+bandwidth. In practice, Synapse _always_ includes these fields, when this is probably not needed.
+Changing this behaviour may break clients which expect these fields to always exist.
+
+### Concurrent connections
+
+The sliding sync protocol allows multiple concurrent connections disambiguated
+by a `conn_id`: see "Concurrent connections" of [MSC3575](
+    https://github.com/matrix-org/matrix-spec-proposals/pull/3575
+). Clients using multiple concurrent connections MUST enable the E2EE
+extension on at most one connection. Using the E2EE extension on multiple
+concurrent connections is not supported; doing so risks data loss and E2EE
+messaging failures.
+
+## Alternatives
+
+The alternative is to not include this extension at all, making it impossible to include reliable
+E2EE support in Sliding Sync. As this extension is opt-in, as all Sliding Sync extensions are, it
+feels undesirable to not provide this extension.
+
+## Security considerations
+
+No additional security considerations beyond what the current `/sync` implementation provides.
+
+## Unstable prefix
+
+No unstable prefix as Sliding Sync is still in review. To enable this extension, just add this to
+your request JSON:
+```json5
+{
+    "extensions": {
+        "e2ee": {
+            "enabled": true,
+        }
+    }
+}
+```
+
+## Dependencies
+
+This MSC builds on MSC3575, which at the time of writing has not yet been accepted into the spec.