MSC3173: Expose stripped state events to any potential joiner

It is currently possible to inspect the state of rooms in some circumstances:

If the room has history_visibility: world_readable, then anyone can inspect it (by calling /state on it).
Rooms in the room directory expose some of their state publicly.
Invited users (and knocking users) receive stripped state events.

This MSC proposes exposing the stripped state events that are currently available to invited and knocking users to any user who could potentially join a room. It also consolidates the recommendation on which states events are available to potential joiners.

Background

When creating an invite it is currently recommended to include stripped state events which are useful for displaying the invite to a user:

An optional list of simplified events to help the receiver of the invite identify the room. The recommended events to include are the join rules, canonical alias, avatar, and name of the room.

The invited user receives these stripped state events as part of the /sync response:

The state of a room that the user has been invited to. These state events may only have the sender, type, state_key and content keys present. These events do not replace any state that the client already has for the room, for example if the client has archived the room.

These are sent as part of the unsigned content of the m.room.member event containing the invite.

MSC2403: Add "knock" feature extends this concept to also include the stripped state events in the /sync response for knocking users:

It is proposed to add a fourth possible key to rooms, called knock. Its value is a mapping from room ID to room information. The room information is a mapping from a key knock_state to another mapping with key events being a list of StrippedStateEvent.

It is also provides an extended rationale of why this is useful:

These stripped state events contain information about the room, most notably the room's name and avatar. A client will need this information to show a nice representation of pending knocked rooms. The recommended events to include are the join rules, canonical alias, avatar, name and encryption state of the room, rather than all room state. This behaviour matches the information sent to remote homeservers when remote users are invited to a room.

MSC1772: Spaces additionally recommends including the m.room.create event as one of the stripped state events:

Join rules, invites and 3PID invites work as for a normal room, with the exception that invite_state sent along with invites should be amended to include the m.room.create event, to allow clients to discern whether an invite is to a space-room or not.

Proposal

Any user who is able to join a room can access the stripped state events of that room.

Potential ways that a user might be able to join include, but are not limited to, the following mechanisms:

A room that has join_rules set to public or knock.
A room that the user is in possession of an invite to (regardless of the join_rules).

Future MSCs might include additional mechanism for a user to join a room and should consider this MSC, for example:

MSC3083: Restricting room membership based on space membership proposes allowing users to join a room based on their membership in a space (as defined in MSC1772).

Additionally, it is recommended, but not required, that homeserver implementations include the following as stripped state events:

Create event (m.room.create)¹
Join rules (m.room.join_rules)
Canonical alias (m.room.canonical_alias)
Room avatar (m.room.avatar)
Room name (m.room.name)
Encryption information (m.room.encryption)²
Room topic (m.room.topic)³

This also implies that the above information is available to any potential joiner in the API proposed in MSC2946: Spaces summary. E.g. rooms which could be joined due to MSC3083 can expose the information available in stripped state events.

Potential issues

This is a generalization of current behavior and shouldn't introduce any new issues.

Alternatives

A different approach to this would be to separately specify each situation in which a user is allowed to see stripped state events, as we do currently for invites and knocking.

Security considerations

This would allow for invisibly accessing the stripped state of a room with knock join rules. This is already trivially accessible by knocking on the room, but currently users in the room would know that the knock occurred. This does not seem to be a major weakening of the security.

Future extensions

Dedicated client-server and server-server APIs could be added to request the stripped state events, but that is considered out-of-scope for the current proposal.

Unstable prefix

N/A

Footnotes

[1]: As updated in MSC1772. ↩

[2]: The encryption information (m.room.encryption) is already sent from Synapse and generally seems useful for a user to know before joining a room. ↩

[3]: The room topic (m.room.topic) is included as part of the room directory response for public rooms. It is additional to be included as part of MSC2946 in the spaces summary response. ↩

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