Room History Visibility

This module adds support for controlling the visibility of previous events in a room.

In all cases except world_readable, a user needs to join a room to view events in that room. Once they have joined a room, they will gain access to a subset of events in the room. How this subset is chosen is controlled by the m.room.history_visibility event outlined below. After a user has left a room, they may see any events which they were allowed to see before they left the room, but no events received after they left.

The four options for the m.room.history_visibility event are:

world_readable - All events while this is the m.room.history_visibility value may be shared by any participating homeserver with anyone, regardless of whether they have ever joined the room.
shared - Previous events are always accessible to newly joined members. All events in the room are accessible, even those sent when the member was not a part of the room.
invited - Events are accessible to newly joined members from the point they were invited onwards. Events stop being accessible when the member’s state changes to something other than invite or join.
joined - Events are accessible to newly joined members from the point they joined the room onwards. Events stop being accessible when the member’s state changes to something other than join.

These options are applied at the point an event is sent. Checks are performed with the state of the m.room.history_visibility event when the event in question is added to the DAG. This means clients cannot retrospectively choose to show or hide history to new users if the setting at that time was more restrictive.

Events

`m.room.history_visibility`

This event controls whether a user can see the events that happened in a room from before they joined.

Event type:	State event
State key	A zero-length string.

Content

Name	Type	Description
`history_visibility`	`enum`	Required: Who can see the room history. One of: `[invited, joined, shared, world_readable]`.

Examples

{
  "content": {
    "history_visibility": "shared"
  },
  "event_id": "$143273582443PhrSn:example.org",
  "origin_server_ts": 1432735824653,
  "room_id": "!jEsUZKDJdhlrceRyVU:example.org",
  "sender": "@example:example.org",
  "state_key": "",
  "type": "m.room.history_visibility",
  "unsigned": {
    "age": 1234
  }
}

Client behaviour

Clients that implement this module MUST present to the user the possible options for setting history visibility when creating a room.

Clients may want to display a notice that their events may be read by non-joined people if the value is set to world_readable.

Server behaviour

By default if no history_visibility is set, or if the value is not understood, the visibility is assumed to be shared. The rules governing whether a user is allowed to see an event depend on the state of the room at that event.

If the history_visibility was set to world_readable, allow.
If the user’s membership was join, allow.
If history_visibility was set to shared, and the user joined the room at any point after the event was sent, allow.
If the user’s membership was invite, and the history_visibility was set to invited, allow.
Otherwise, deny.

For m.room.history_visibility events themselves, the user should be allowed to see the event if the history_visibility before or after the event would allow them to see it. (For example, a user should be able to see m.room.history_visibility events which change the history_visibility from world_readable to joined or from joined to world_readable, even if that user was not a member of the room.)

Likewise, for the user’s own m.room.member events, the user should be allowed to see the event if their membership before or after the event would allow them to see it. (For example, a user can always see m.room.member events which set their membership to join, or which change their membership from join to any other value, even if history_visibility is joined.)

Security considerations

The default value for history_visibility is shared for backwards-compatibility reasons. Clients need to be aware that by not setting this event they are exposing all of their room history to anyone in the room.