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88 lines
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ReStructuredText
88 lines
3.9 KiB
ReStructuredText
Room History Visibility
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=======================
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.. _module:history-visibility:
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This module adds support for controlling the visibility of previous events in a
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room.
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In all cases except ``world_readable``, a user needs to join a room to view events in that room. Once they
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have joined a room, they will gain access to a subset of events in the room. How
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this subset is chosen is controlled by the ``m.room.history_visibility`` event
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outlined below. After a user has left a room, they may see any events which they
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were allowed to see before they left the room, but no events received after they
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left.
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The four options for the ``m.room.history_visibility`` event are:
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- ``world_readable`` - All events while this is the
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``m.room.history_visibility`` value may be shared by any participating
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homeserver with anyone, regardless of whether they have ever joined the room.
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- ``shared`` - Previous events are always accessible to newly joined members. All
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events in the room are accessible, even those sent when the member was not a part
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of the room.
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- ``invited`` - Events are accessible to newly joined members from the point
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they were invited onwards. Events stop being accessible when the member's state
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changes to something other than ``invite`` or ``join``.
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- ``joined`` - Events are accessible to newly joined members from the point
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they joined the room onwards. Events stop being accessible when the member's state
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changes to something other than ``join``.
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.. WARNING::
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These options are applied at the point an event is *sent*. Checks are
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performed with the state of the ``m.room.history_visibility`` event when the
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event in question is added to the DAG. This means clients cannot
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retrospectively choose to show or hide history to new users if the setting at
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that time was more restrictive.
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Events
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------
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{{m_room_history_visibility_event}}
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Client behaviour
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----------------
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Clients that implement this module MUST present to the user the possible options
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for setting history visibility when creating a room.
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Clients may want to display a notice that their events may be read by non-joined
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people if the value is set to ``world_readable``.
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Server behaviour
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----------------
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By default if no ``history_visibility`` is set, or if the value is not understood, the visibility is assumed to be
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``shared``. The rules governing whether a user is allowed to see an event depend
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on the state of the room *at that event*.
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1. If the ``history_visibility`` was set to ``world_readable``, allow.
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2. If the user's ``membership`` was ``join``, allow.
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3. If ``history_visibility`` was set to ``shared``, and the user joined the
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room at any point after the event was sent, allow.
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4. If the user's ``membership`` was ``invite``, and the ``history_visibility``
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was set to ``invited``, allow.
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5. Otherwise, deny.
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For ``m.room.history_visibility`` events themselves, the user should be allowed
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to see the event if the ``history_visibility`` before *or* after the event
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would allow them to see it. (For example, a user should be able to see
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``m.room.history_visibility`` events which change the ``history_visibility``
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from ``world_readable`` to ``joined`` *or* from ``joineded`` to
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``world_readable``, even if that user was not a member of the room.)
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Likewise, for the user's own ``m.room.member`` events, the user should be
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allowed to see the event if their ``membership`` before *or* after the event
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would allow them to see it. (For example, a user can always see
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``m.room.member`` events which set their membership to ``join``, or which
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change their membership from ``join`` to any other value, even if
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``history_visibility`` is ``joined``.)
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Security considerations
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-----------------------
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The default value for ``history_visibility`` is ``shared`` for
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backwards-compatibility reasons. Clients need to be aware that by not setting
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this event they are exposing all of their room history to anyone in the room.
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