Merge pull request #2689 from Awesome-Technologies/fix_guest_e2ee

MSC2689: Fix E2EE for guests

proposals/2689-fix-e2ee-for-guests.md

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+# MSC2689: Allow guests to operate in encrypted rooms
+
+[#751](https://github.com/matrix-org/matrix-doc/pull/751) granted guest users access to several endpoints in order to allow them to use E2EE.
+I found that guests are able to join encrypted rooms and read messages from other members. But when the
+guest wants to send an event into the room the client receives a "guest access not allowed" error
+for the `/rooms/{room_id}/members` endpoint. I assume the client tries to read the list of room members
+to prepare the encryption of the event for the present members. Tests with a patched Synapse showed that
+allowing guests to use this endpoint results in a normal behaviour and enables guests to communicate in
+encrypted rooms.
+
+
+## Proposal
+
+Allow guests to use the `GET /_matrix/client/r0/rooms/{room_id}/members` endpoint to enable them to
+operate properly in encrypted rooms.
+
+
+## Alternatives
+
+The list of room members could also be read from the sync. However that would not work with Lazy Loading.