matrix-doc/proposals/3173-expose-stripped-state-events.md

MSC3173: Expose stripped state events to any potential joiner

It can be useful to view the partial state of a room before joining to allow a user to know what they're joining. For example, it improves the user experience to show the user the room name and avatar before joining.

It is already allowed to partially view the room state without being joined to the room in some situations:

• 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 to display metadata to users.

This MSC proposes formalizing that the stripped state that is currently available to invited and knocking users is available to any user who could potentially join a room. It also defines "stripped state" and consolidates the recommendation on which events to include in the stripped state.

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

The specification does not currently define what "stripped state" is or formally describe who can access it, instead it is specified that certain situations (e.g. an invite or knock) provide a potential joiner with the stripped state of a room.

This MSC clarifies what "stripped state" is and formalizes who can access the stripped state of a room in future cases.

Potential ways that a user might be able to join a room 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).

This MSC proposes a formal definition for the stripped state of a room¹:

The stripped state of a room is a list of simplified state events to help a potential joiner identify the room. These state events may only have the sender, type, state_key and content keys present.

Client behavior

These events do not replace any state that the client already has for the room, for example if the client has archived the room. Instead the client should keep two separate copies of the state: the one from the stripped state and one from the archived state. If the client joins the room then the current state will be given as a delta against the archived state not the stripped state.

Server behavior

It is recommended (although not required²) that homeserver implementations include the following events as part of the stripped state of a room:

• Create event (m.room.create)³
• 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)⁴
• Room topic (m.room.topic)⁵

Potential issues

This is a formalization of current behavior and should not introduce new issues.

Alternatives

A different approach would be to continue with what is done today for invites, knocking, the room directory: separately specify that a user is allowed to see the stripped state (and what events the stripped state should contain).

Security considerations

This would allow for invisibly accessing the stripped state of a room with public or knock join rules.

In the case of a public room, if the room has history_visibility set to world_readable then this is no change. Additionally, this information is already provided by the room directory (if the room is listed there). Otherwise, it is trivial to access the state of the room by joining, but currently users in the room would know that the join occurred.

Similarly, in the case of knocking, a user is able to trivially access the stripped state of the room by knocking, but users in the room would know that the knock occurred.

This does not seem to weaken the security expectations of either join rule.

Future extensions

Revisions to the room directory

A future MSC could include additional information from the stripped state events in the room directory. The main missing piece seems to be the encryption information, but there may also be other pieces of information to include.

Additional ways to access the stripped state

MSC2946 proposes including the stripped state in the spaces summary. Not needing to rationalize what state can be included for a potential joiner would simplify this (and future) MSCs.

Additional ways to join a room

MSC3083 proposes a new join rule due to membership in a space. This MSC would clarify that the stripped state of a room is available to those joiners.

Dedicated API for accessing the stripped state

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.

This API would allow any potential joiner to query for the stripped state. If the server does not know the room's state it would need to query other servers for it.

Unstable prefix

N/A

Footnotes

[1]: No changes are proposed to the definition of history_visibility. The state of a room which is world_readable is available to anyone. This somewhat implies that the stripped state is also available to anyone, regardless of the join rules, but having a world_readable, invite room does not seem valuable. ↩

[2]: Privacy conscious deployments may wish to limit the metadata available to users who are not in a room as the trade-off against user experience. There seems to be no reason to not allow this. ↩

[3]: As updated in MSC1772. ↩

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

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