MSC2176: Update the redaction rules

[richvdh]

Rendered.

Implementation: matrix-org/synapse#8984

[ara4n]

looks plausible to me. sounds like we should get it out there in a room v6 sooner than later

[richvdh]

We probably need to get agreement on #1996 and matrix-org/matrix-spec#357 as a prerequisite here. If people have opinions on them, perhaps they could weigh in on the relevant bugs.

[bwindels]

For completeness, there is also some ongoing discussion as part of MSC1849/2154 about preserving content/m.relates_to/rel_type on redaction for at least edits, if not all relations.

[turt2live]

lgtm - I'm more torn on whether notifications on power levels should be kept though.

[richvdh]

A quick note on some of my thinking here.

As I see it, there is a scale between properties which we absolutely must preserve on redaction, for fear of breaking the protocol (mostly those involved in event auth), and those which have very little effect on the protocol at all so can safely be redacted. Somewhere in the middle we have things like predecessor, which probably isn't really a thing you want to redact, but actually the protocol will mostly keep working.

I'm trying quite hard to limit the list of things that get preserved to things which completely break the protocol, rather than just things that are a bit annoying/surprising when you redact them, mostly to keep the server-server protocol as simple as possible. The fact that we have to bump the room version to change the redaction rules also means that it's really not great to have to keep adding new rules. Rather, I see it as the job of either clients (or, possibly, the C-S API) to protect you from foot-guns.

All that said, there is room for debate on where different keys sit on that scale, and where we draw the line, but I'd like each candidate to be retained to be argued on its own merits.

[turt2live]

m.room.encryption's algorithm should be preserved.

[richvdh]

I disagree. It doesn't change the behaviour of the room at the server-server level.

[KitsuneRal]

But if we eventually have two algorithms rather than one how should a client treat m.room.encryption with redacted algorithm?

[richvdh]

But if we eventually have two algorithms rather than one

I don't think there is a way to end up with two encryption algorithms in the room state?

how should a client treat m.room.encryption with redacted algorithm?

The same way that they should treat a new m.room.encryption event which disables encryption: with great suspicion.

[turt2live]

How are new clients to the room meant to do encryption if they don't know the algorithm?

Can we please relax the restrictions on what goes into the algorithm? It affects client authors too, and it's ridiculous to exclude something because it doesn't affect the core protocol operation.

[richvdh]

@KitsuneRal: I think there might be a misunderstanding here.

there's no good way for a client to handle any readable content of the room once that state is broken
...
this room is irreparably broken along with all of its encrypted messages already in DAG.

m.room.encryption is not supposed to define how you interpret received events, so I don't agree that the existing messages are invalidated once m.room.encryption is redacted.

again - repeated encryption events are not proper

there is nothing wrong with repeated m.room.encryption events?

[KitsuneRal]

m.room.encryption is not supposed to define how you interpret received events, so I don't agree that the existing messages are invalidated once m.room.encryption is redacted.

That's a pretty strong statement. Then what's the purpose of algorithm there?

there is nothing wrong with repeated m.room.encryption events?

Except they should be treated with great suspicion because there's no defined semantics for any m.room.encryption event except the first one?

[richvdh]

Then what's the purpose of algorithm there?

It indicates to clients how they should encrypt messages in this room.

there's no defined semantics for any m.room.encryption event except the first one?

I don't think that's true. I can't find anything in the spec about this, but my mental model is that clients should keep a record of rooms which are encrypted (along with the encryption algorithm), and give the user a warning if they send a message in a room which was previously encrypted and is no longer (or if the encryption is 'weaker', for some handwavey definition of weaker).

But certainly it's valid for there to be multiple encryption events in a room, changing minor settings.

[turt2live]

There's also the problem of clients joining a room which has a redacted encryption event: They won't have the prior knowledge that it was encrypted and will assume it's not.

ftr riot-web does exactly as you describe: it ignores changes to the encryption event it wasn't expecting.

[uhoreg]

I've created https://github.com/matrix-org/matrix-doc/issues/2272 for clarifying the semantics of m.room.encryption events. I'd suggest ignoring m.room.encryption for the purposes of this MSC, and discussing in that issue for a future MSC, as I think that it's a larger issue that shouldn't hold this one up.

[richvdh]

I've updated the MSC to (1) change the create event so that all keys are preserved, and (2) clarify that I don't think the notifications property should be preserved, and (2b) to record why I think this is important.

Further input welcome...

[KitsuneRal]

Typo fixing - however, might be entirely unnecessary if we keep algorithm around :)

[richvdh]

Nothing else seems to be happening here, so I'd like to propose FCP:

@mscbot fcp merge

[mscbot]

Team member @richvdh has proposed to merge this. The next step is review by the rest of the tagged people:

• @KitsuneRal
• @anoadragon453
• @ara4n
• @dbkr
• @erikjohnston
• @richvdh
• @turt2live
• @uhoreg

No concerns currently listed.

Once at least 75% of reviewers approve (and none object), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[turt2live]

This MSC is currently assigned to v10, though apparently we need to unblock MSC2174 too. #3604

[turt2live]

This went back to not having an assigned room version (pulled out of v10).

[turt2live]

This is now formally assigned to room version 11: #3820

[turt2live]

Spec PR: matrix-org/matrix-spec#1604

[turt2live]

Merged 🎉