[elvisloops]

If the app takes your disappearing message, encrypts it with a static key that never changes and is never deleted, and uploads it to the cloud, then the message is never truly "disappearing." A "post compromise" event will allow the attacker to decrypt that ciphertext at any point in the future. All of this ratcheting is undone by backups.

[Ajedi32]

Disappearing messages were never a real thing in the first place. You can have a gentleman's agreement that the person you send your message to will delete it after reading it, there's no way to guarantee anything beyond that.

(Fair point though that probably "disappearing" messages shouldn't be included in backups since that obviously prevents them from being deleted. Idk if Signal implements that or not.)

[tptacek]

Disappearing messages are an opsec feature for trusted counterparties, not a cryptographic feature. They are very much a real thing.

[ragona]

> encrypts it with a static key

What type of static key? If it's just a big symmetric key that isn't derived from an asymmetric handshake of some type then no, that's not our current understanding of the PQ threat model.

[tptacek]

Part of the premise of FS/PCS is that "shit happens" to compromise keys even if the underlying cryptography is strong, so if you want a coherent end-to-end FS/PCS story, the claim would be that you need to be ratcheting everywhere.