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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 seen 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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- ``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`` - Previous 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`` - Previous 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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- ``world_readable`` - All events while this is the ``m.room.history_visibility`` value
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may be shared by any participating homeserver with anyone, regardless of whether
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they have ever joined the room.
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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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solely on the state of the room *at that event*:
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1. If the user was joined, allow.
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2. If the user was invited and the ``history_visibility`` was set to
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``invited`` or ``shared``, allow.
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3. If the user was neither invited nor joined but the ``history_visibility``
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was set to ``shared``, allow.
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4. Otherwise, deny.
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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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