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

# MSC3173: Expose stripped state events to any potential joiner

The current design of Matrix somtimes allows for inspecting part of the room state
without being joined to the room:

* If the room has `history_visibility: world_readable`, then anyone can inspect
  it (by calling `/state` on it).
* Rooms in the [room directory](https://matrix.org/docs/spec/client_server/latest#get-matrix-client-r0-publicrooms)
  expose some of their state publicly.
* [Invited users](https://matrix.org/docs/spec/server_server/r0.1.4#put-matrix-federation-v2-invite-roomid-eventid)
  and [knocking users](https://github.com/matrix-org/matrix-doc/pull/2403)
  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.

## Background

When creating an invite it is [currently recommended](https://matrix.org/docs/spec/server_server/r0.1.4#put-matrix-federation-v2-invite-roomid-eventid)
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](https://spec.matrix.org/unstable/client-server-api/#get_matrixclientr0sync)
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](https://spec.matrix.org/unstable/client-server-api/#mroommember)
containing the invite.

[MSC2403: Add "knock" feature](https://github.com/matrix-org/matrix-doc/pull/2403)
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](https://github.com/matrix-org/matrix-doc/pull/1772) 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 two 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.

### 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. No changes are proposed to the mechanics of how the
users may get those state events.

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](https://github.com/matrix-org/matrix-doc/pull/3083)
  proposes allowing users to join a room based on their membership in a space (as defined in
  [MSC1772](https://github.com/matrix-org/matrix-doc/pull/1772)).

### 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
required<sup id="a0">[0](#f0)</sup>) that homeserver implementations include the
following as stripped state events:

* Create event (`m.room.create`)<sup id="a1">[1](#f1)</sup>
* 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`)<sup id="a2">[2](#f2)</sup>
* Room topic (`m.room.topic`)<sup id="a3">[3](#f3)</sup>

## 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 APIs

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.

### Revisions to the room directory

A future MSC could include additional information from the stripped state events
in the [room directory](https://matrix.org/docs/spec/client_server/latest#get-matrix-client-r0-publicrooms).
This seems to mostly be the encryption information, but there may also be other
pieces of information to include.

### Additional ways to join a room

[MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083) 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

<a id="f0"/>[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. [↩](#a0)

<a id="f1"/>[1]: As updated in [MSC1772](https://github.com/matrix-org/matrix-doc/pull/1772). [↩](#a1)

<a id="f2"/>[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.
[↩](#a2)

<a id="f3"/>[3]: The room topic (`m.room.topic`) is included as part of the
[room directory](https://matrix.org/docs/spec/client_server/latest#get-matrix-client-r0-publicrooms)
response for public rooms. It is also planned to be included as part of [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946)
in the spaces summary response. [↩](#a3)