matrix-doc/proposals/3821-update-redaction-rules-again.md

MSC3821: Update redaction rules, again

MSC2176 aimed to fix inconsistencies with the redaction algorithm where server-side event auth required properties to exist that were being removed. While MSC2176 fixed a number of cases, one was unfortunately missed.

This MSC aims to fix that missing case, originally identified long ago by synapse#1831.

Proposal

Note: It is recommended to read MSC2176 before this proposal as MSC2176 contains a lot of backing context.

In a future room version, the following changes are made to the redaction algorithm. Note that this requires a new room version because changing the redaction algorithm changes how event IDs are calculated, as they are reference hashes which redact the event during calculation.

• m.room.member events preserve a portion of third_party_invite under content, if present. Those properties being:

  • signed: the block is required for the server to validate the event, however excess adjacent properties such as display_name are not important.

Clients should note that because display_name is not preserved during redaction they might need to change how that event is rendered/presented to users. For example, when rendering such a redacted event the client might show it as "Bob accepted a third party invite".

Unstable prefix

Implementations looking to test these changes before adopted into a stable room version should use org.matrix.msc3821.opt1 as the room version, using v9 as a base and treating it as unstable.

History: This MSC was originally written before room version 10 existed. Implementations wishing to use v10 as a base instead of v9 should use org.matrix.msc3821.opt1.v10 as the room version instead.