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+# MSC4358: Out-of-room server discovery
+
+Currently in Matrix, it is possible for the servers which a new joiner
+trusts during the join process to place the joiner into a parallel
+branch that is not communicated to any other server.
+
+Consider the following scenario:
+
+![](./images/4358-out-of-room-server-discovery.png)
+
+A room has several members, Blue, Green, Yellow, and Bronze. They have
+been participating for some time.
+
+Then, the Blue decides as a room creator to create a concurrent branch
+in the room that is only going to be communicated to new joiners.
+Blue is also in control of the room alias, and other external links to
+the room. This prevents new joiners from seeing the existing members.
+
+## Proposal
+
+When a server joins a room for the first time, it creates a
+`room_participation_tag` for itself which is stored permanently. This
+tag is derived from a hmac-sha256 of the server name and the
+room_id: `hmac-sha256(server_name, room_id)`.
+
+### The `POST /_matrix/federation/v1/discover_common_rooms` endpoint
+
+A new endpoint is specified to allow servers to discover common rooms
+without either party revealing which rooms are being queried unless
+both parties have provable knowledge of the room.
+
+For the set of rooms the requesting server is interested in,
+the server creates a _room participation tag_ for each room
+by taking the requested server's name and deriving a hmac-sha256 of
+the _server name_ and the `room_id`: `hmac-sha256(server_name, room_id)`.
+
+#### Request body
+
+- The `room_participation_tags` are the tags for which the requesting
+  server is asking if the requested acknowledges. This is an array of
+  strings.
+
+#### Response body
+
+The responding server simply responds with the token tags for
+which it has a match stored:
+
+- The `recognised_tags` are any of the queried tags which the
+  requested server acknowledges. This property is an array of strings.
+
+## Potential issues
+
+### Left/Banned members can track new members when queried
+
+If members which have left or are banned from a room are queried about
+a key from a server, then they can potentially infer that the querying
+server is a member of the room.
+
+### Useless if a victim server hasn't built up knowledge of common peers through other rooms
+
+A server will not know which servers to query if it hasn't interacted
+with any other server before through membership in other rooms.
+
+### Not clear when it is appropriate to query common rooms
+
+How often should rooms check for unexpected results?
+
+When joining a new room is a good start. But how frequently otherwise?
+
+## Alternatives
+
+### OPRF key in the create event
+
+An OPRF[^oprf] key could be placed into the create event which is used
+instead via `OPRF(room_key, server_name)`. However, it's unclear
+if there are any situations where the `room_id` is known and not
+the create event.
+
+[^oprf]: https://datatracker.ietf.org/doc/rfc9497/
+
+## Security considerations
+
+- None considered.
+
+## Unstable prefix
+
+- The endpoint `/_matrix/federation/v1/discover_common`_rooms is `/_matrix/federation/v1/org.matrix.msc4358/discover_common_rooms`
+
+## Dependencies
+
+- None.
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