79 lines
3.8 KiB
Markdown
79 lines
3.8 KiB
Markdown
# Mass redactions
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Matrix, like any platform with public chat rooms, has spammers. Currently,
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redacting spam essentially requires spamming redaction events in a 1:1 ratio,
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which is not optimal<sup>[1](images/2244-redaction-spam.png)</sup>. Most
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clients do not even have any mass redaction tools, likely in part due to the
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lack of a mass redaction API. A mass redaction API on the other hand has not
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been implemented as it would require sending lots of events at once. However,
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this problem could be solved by allowing a single redaction event to redact
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many events instead of sending many redaction events.
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## Proposal
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This proposal builds upon [MSC2174](https://github.com/matrix-org/matrix-doc/pull/2174)
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and suggests making the `redacts` field in the content of `m.room.redaction`
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events an array of event ID strings instead of a single event ID string.
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It would be easiest to do this before MSC2174 is written into the spec, as then
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only one migration would be needed: from an event-level redacts string to a
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content-level redacts array.
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### Backwards compatibility
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There is no easy way to stay fully compatible with *older* clients, so the
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proposed solution is to not support them. In order to not break old clients
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completely, servers should still add a `redacts` string containing one of the
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redacted event IDs to the top level of `m.room.redaction` events in *newer*
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room versions when serving such events over the Client-Server API.
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Like MSC2174, for improved compatibility with *newer* clients, servers should
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add a `redacts` array to the `content` of `m.room.redaction` events in *older*
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room versions when serving such events over the Client-Server API.
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### Number of redactions
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Room v4+ event IDs are 44 bytes long, which means the federation event size
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limit would cap a single redaction event at a bit less than 1500 targets.
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Redactions are not intrinsically heavy, so a separate limit should not be
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necessary.
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Due to the possible large number of redaction targets per redaction event,
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servers should omit the list of redaction targets from the `unsigned` ->
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`redacted_because` field of redacted events. If clients want to get the list
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of targets of a redaction event in `redacted_because`, they should read the
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`event_id` field of the `redacted_because` event and use the
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`/rooms/{roomId}/event/{eventId}` endpoint to fetch the content.
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### Client behavior
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Clients shall apply existing `m.room.redaction` target behavior over an array
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of event ID strings.
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### Server behavior (auth rules)
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The target events of an `m.room.redaction` shall no longer be considered when
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authorizing an `m.room.redaction` event. Any other existing rules remain
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unchanged.
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After a server accepts an `m.room.redaction` using the modified auth rules, it
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evaluates individually whether each target can be redacted under the existing
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room v5 auth rules. Servers MUST NOT include failing and unknown entries to
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clients.
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> Servers do not know whether redaction targets are authorized at the time they
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receive the `m.room.redaction` unless they are in possession of the target
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event. Implementations retain entries in the original list which were not
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shared with clients to later evaluate the target's redaction status.
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When the implementation receives a belated target from an earlier
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`m.room.redaction`, it evaluates at that point whether the redaction is
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authorized.
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> Servers should not send belated target events to clients if their redaction
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was found to be in effect, as clients were not made aware of the redaction.
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That fact is also used to simply ignore unauthorized targets and send the
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events to clients normally.
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## Tradeoffs
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## Potential issues
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## Security considerations
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Server implementations should ensure that large redaction events do not become
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a DoS vector, e.g. by processing redactions in the background.
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