matrix-doc/proposals/2244-mass-redactions.md

Mass redactions

Matrix, like any platform with public chat rooms, has spammers. Currently, redacting spam essentially requires spamming redaction events in a 1:1 ratio, which is not optimal¹. Most clients do not even have any mass redaction tools, likely in part due to the lack of a mass redaction API. A mass redaction API on the other hand has not been implemented as it would require sending lots of events at once. However, this problem could be solved by allowing a single redaction event to redact many events instead of sending many redaction events.

Proposal

This proposal builds upon MSC2174 and suggests making the redacts field in the content of m.room.redaction events an array of event ID strings instead of a single event ID string.

It would be easiest to do this before MSC2174 is written into the spec, as then only one migration would be needed: from an event-level redacts string to a content-level redacts array.

Backwards compatibility

There is no easy way to stay fully compatible with older clients, so the proposed solution is to not support them. In order to not break old clients completely, servers should still add a redacts string containing one of the redacted event IDs to the top level of m.room.redaction events in newer room versions when serving such events over the Client-Server API.

Like MSC2174, for improved compatibility with newer clients, servers should add a redacts array to the content of m.room.redaction events in older room versions when serving such events over the Client-Server API.

Number of redactions

Room v4+ event IDs are 44 bytes long, which means the federation event size limit would cap a single redaction event at a bit less than 1500 targets. Redactions are not intrinsically heavy, so a separate limit should not be necessary.

Due to the possible large number of redaction targets per redaction event, servers should omit the list of redaction targets from the unsigned -> redacted_because field of redacted events. If clients want to get the list of targets of a redaction event in redacted_because, they should read the event_id field of the redacted_because event and use the /rooms/{roomId}/event/{eventId} endpoint to fetch the content.

Client behavior

Clients shall apply existing m.room.redaction target behavior over an array of event ID strings.

Server behavior (auth rules)

The target events of an m.room.redaction shall no longer be considered when authorizing an m.room.redaction event. Any other existing rules remain unchanged.

After a server accepts an m.room.redaction using the modified auth rules, it evaluates individually whether each target can be redacted under the existing room v5 auth rules. Servers MUST NOT include failing and unknown entries to clients.

Servers do not know whether redaction targets are authorized at the time they receive the m.room.redaction unless they are in possession of the target event. Implementations retain entries in the original list which were not shared with clients to later evaluate the target's redaction status.

When the implementation receives a belated target from an earlier m.room.redaction, it evaluates at that point whether the redaction is authorized.

Servers should not send belated target events to clients if their redaction was found to be in effect, as clients were not made aware of the redaction. That fact is also used to simply ignore unauthorized targets and send the events to clients normally.

Tradeoffs

Potential issues

Security considerations

Server implementations should ensure that large redaction events do not become a DoS vector, e.g. by processing redactions in the background.