feature request: Implement fallback keys to improve E2EE reliability #1228

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opened 2025-12-18 20:11:20 +00:00 by nex · 4 comments

nex commented

2025-12-18 20:11:20 +00:00

Owner

Fallback keys were added in spec v1.2, and they allow devices to continue receiving new encrypted messages even when their one time keys are exhausted, with the added risk of replay attacks. Synapse currently supports this, and I believe Tuwunel added support for them recently.

We should implement support for uploading and replacing fallback keys, as well as serving them to consumers where needed. This will likely greatly improve the reliability of encryption when people don't spend every waking moment on matrix.

Right now continuwuity only supports one-time keys, however, if a device is offline for a long time, its one time key pool may become exhausted, and further attempts to send messages to that device will result in unable-to-decrypt errors, which may end up being unrecoverable under some circumstances. [Fallback keys](https://spec.matrix.org/v1.17/client-server-api/#one-time-and-fallback-keys) were added in spec v1.2, and they allow devices to continue receiving new encrypted messages even when their one time keys are exhausted, with the added risk of replay attacks. Synapse currently supports this, and I believe Tuwunel added support for them recently. We should implement support for uploading and replacing fallback keys, as well as serving them to consumers where needed. This will likely greatly improve the reliability of encryption when people don't spend every waking moment on matrix.

nex added this to the 0.5.5 milestone

2025-12-18 20:11:20 +00:00

nex added the

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2025-12-18 20:11:20 +00:00

ginger modified the milestone from 0.5.5 to (deleted)

2026-02-15 15:10:42 +00:00

eve commented

2026-04-24 08:38:48 +00:00

Contributor

I'm going to start working on this if no one is doing it.

nex commented

2026-04-24 11:33:38 +00:00

Author

Owner

@eve We've already drawn up a plan for this and I did start on this, but haven't gotten very far. If you want to work on it, feel free - happy to share the design ideas we came up with in https://matrix.to/#/#dev:continuwuity.org if it helps at all!

@eve We've already drawn up a plan for this and I did *start* on this, but haven't gotten very far. If you want to work on it, feel free - happy to share the design ideas we came up with in https://matrix.to/#/#dev:continuwuity.org if it helps at all!

👍 1

eve commented

2026-04-24 17:27:04 +00:00

Contributor

@nex I'm happy to work on this I think, it seems interesting. For now I'm reading through the spec and code to make sure I understand everything. I'll probably come chat with you about the way to implement it before I get started on writing the code.

eve referenced this issue

2026-04-27 11:14:18 +00:00

Add support for fallback keys #1710

ginger commented

2026-05-12 14:27:55 +00:00

Owner

Fixed by !1710.

ginger closed this issue

2026-05-12 14:27:55 +00:00