You don't have to enable the Signal backups feature, but you have no way of knowing whether the recipient of your messages has. One person in a group chat with that enabled will undo all of the forward secrecy you're describing.
The expectation is that what happens inside Signal is secure, and the features Signal provides are secure. If the idea is that nobody is going to enable this feature, then why build it? If the idea is that many people are going to enable this feature, then this entire cryptographic protocol is meaningless.
I've yet to see a protocol that lets you convincingly insert fake messages into both sides of your own chat history, especially in a way that isn't detectable by say, sqlite rowid order, but that would be an interesting idea for where to take this sort of thing.
If you are just looking for "secure(TM)[X]", you are making a mistake somewhere anyway.
If your life or livelihood depends on it, you learn what the impact of every choice is and you painstakingly keep to your opsec.
Somewhere between the two user action becomes a necessity. You need to judge where that point is for you and take responsibility for it because nobody else can guarantee it.
But practically, it probably has more risk as people bypassing employer or legal controls think it’s “secure”. So they have conversations that they wouldn’t have.
(Note: I didn't actually dig into the backup implementation, but my guess is that it's more of a KDF -> symmetric design, rather than the sorts of asymmetric negotiation you'd find in multi-party messaging.)
(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.)
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.
The backups feature doesn't open up any new vulnerability that didn't inherently exist in sending messages to someone else you might not fully trust. One person in a group chat can also take pictures of their phone's screen & upload your messages to the public.
It solves the problem: How can a group of people (two or more people) securely communicate with each other.
The group has to mutually decide their risk profile, and then decide which features of the application to use. And each person in the group has to decide whether they can trust others in the group to follow the agreed upon opsec. Signal cannot solve these social problems.
On the other hand, if an adversary captures one of the group participants' phone and breaks device security, and the chat was recorded on that device, then they can access all recorded chats. By the same token, no cryptography can protect against a malicious group participant who records messages.
In the same scenario, cloud backups seem to merely imply that the same adversary can obtain the cloud backup key and therefore decipher the cloud backups if they get their hands on it. They won't need that, however, since the group chat history is already stored on the device. If no chats were recorded on the device at all the situation would be different.
I jest, and Signal's support for backups do really increase exposure to this risk, but just trying to say its a matter of degree not a fundamentally new change. People that have been using sigtop[0] to backup their Signal messages to plaintext also create the same exposure risk.
