Room History Visibility ======================= .. _module: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``. .. WARNING:: 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_event}} 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*. 1. If the ``history_visibility`` was set to ``world_readable``, allow. 2. If the user's ``membership`` was ``join``, allow. 3. If ``history_visibility`` was set to ``shared``, and the user joined the room at any point after the event was sent, allow. 4. If the user's ``membership`` was ``invite``, and the ``history_visibility`` was set to ``invited``, allow. 5. 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 ``joineded`` 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.