matrix-spec-proposals/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 allowing 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 events to include as stripped state for potential joiners and provides a way to query for the stripped state directly.

This will allow for improved future use cases, such as:

• Improved user experience for more complicated access controls (e.g. MSC3083).
• Showing a room preview on platforms when peeking fails (using for clients as well as matrix.to).
• Joining by alias (e.g. as in Element) could show a room preview.

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

This proposal includes a few aspects which are dealt with separately:

1. Generalizing when a user is allowed to view the stripped state of a room.
2. A consistent recommendation for which events to include in the stripped state.
3. Providing a dedicated API for accessing the stripped state of the room.

Accessing the stripped state of a room

Any user who is able to join a room shall be allowed to have access the stripped state events of that room. Additionally, any user who could access the state of a room may access the stripped state of a room, as it is a strict subset of information.

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).

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).

Events to include in the stripped state

It is also proposed to create a single definition for what stripped state events should be provided to be potential joiners. Thus, it is recommended (although not required⁰) 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

Stripped state API

GET /_matrix/client/r0/rooms/{roomId}/stripped_state

A dedicated API is provided to query for the stripped state of a room. As described above, any potential joiner may access the stripped state of a room (and in the case of a room with history_visibility: world_readable -- anyone may access the stripped state, as it is a strict subset of the state).

This API is rate-limited and does not require authentication.

The request format follows the /state endpoint.

The response body includes an array of StrippedState, as described in the /sync response.

Example request:

GET /_matrix/client/r0/rooms/%21636q39766251%3Aexample.com/stripped_state HTTP/1.1

Responses:

Status code 200:

The current stripped state of the room

[
  {
    "content": {
      "join_rule": "public"
    },
    "type": "m.room.join_rules",
    "sender": "@example:example.org",
    "state_key": ""
  },
  {
    "content": {
      "creator": "@example:example.org",
      "room_version": "1",
      "m.federate": true,
      "predecessor": {
        "event_id": "$something:example.org",
        "room_id": "!oldroom:example.org"
      }
    },
    "type": "m.room.create",
    "sender": "@example:example.org",
    "state_key": ""
  }
]

Note that this is the same example as the /state endpoint, but limited to what would be returned as stripped state.

Status code 403:

You are not a member of the room, a potential joiner, and the room is not publicly viewable.

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 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. 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. Additionally, this information is already provided by the room directory (if the room is listed there).

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 be weakening 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 join a room

MSC3083 leverages this to expose the information available in stripped state events via the spaces summary for potential joiners due to membership in a space.

Unstable prefix

N/A

Footnotes

[0]: 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. ↩

[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 also planned to be included as part of MSC2946 in the spaces summary response. ↩