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92 lines
3.8 KiB
Markdown
92 lines
3.8 KiB
Markdown
---
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type: module
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weight: 120
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---
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### Room History Visibility
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This module adds support for controlling the visibility of previous
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events in a room.
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In all cases except `world_readable`, a user needs to join a room to
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view events in that room. Once they have joined a room, they will gain
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access to a subset of events in the room. How this subset is chosen is
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controlled by the `m.room.history_visibility` event outlined below.
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After a user has left a room, they may see any events which they were
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allowed to see before they left the room, but no events received after
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they 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
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the room.
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- `shared` - Previous events are always accessible to newly joined
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members. All events in the room are accessible, even those sent when
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the member was not a part of the room.
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- `invited` - Events are accessible to newly joined members from the
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point they were invited onwards. Events stop being accessible when
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the member's state changes to something other than `invite` or
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`join`.
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- `joined` - Events are accessible to newly joined members from the
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point they joined the room onwards. Events stop being accessible
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when the member's state changes to something other than `join`.
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{{% boxes/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
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the 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
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setting at that time was more restrictive.
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{{% /boxes/warning %}}
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#### Events
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{{% event event="m.room.history_visibility" %}}
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#### Client behaviour
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Clients that implement this module MUST present to the user the possible
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options 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
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non-joined people if the value is set to `world_readable`.
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#### Server behaviour
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By default if no `history_visibility` is set, or if the value is not
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understood, the visibility is assumed to be `shared`. The rules
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governing whether a user is allowed to see an event depend on the state
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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
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`history_visibility` 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
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allowed to see the event if the `history_visibility` before *or* after
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the event would allow them to see it. (For example, a user should be
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able to see `m.room.history_visibility` events which change the
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`history_visibility` from `world_readable` to `joined` *or* from
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`joined` to `world_readable`, even if that user was not a member of the
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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
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event 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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The default value for `history_visibility` is `shared` for
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backwards-compatibility reasons. Clients need to be aware that by not
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setting this event they are exposing all of their room history to anyone
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in the room.
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