Comment by elvisloops 18 hours ago
Yes, but you don't need a complicated ratcheting protocol if you've eliminated forward secrecy in other ways. This post is about "post compromise security," but there is already no post-compromise security after the cloud backups feature
It's not optional because you don't know whether the people you are communicating with have it enabled. One person in a group chat with the feature enabled undoes the forward secrecy for everyone in the group chat.
A cloud backup eliminates any forward secrecy. It used to be that in Signal, when you have a message on your device and it is deleted (or a disappearing message disappears), then it is truly gone and can never be recovered. Now with backups, since the key that was used to encrypt it to the cloud remains on your device, it can be recovered even after the message is deleted or disappears.
The only way to "truly" opt-out is to, as you say, set a disappearing message timer for <24 hours.
Yeah, and all of that's already true right now because messages are stored on those users' devices already. You'll be heartbroken to hear that those users can also take a screenshot of your disappearing messages and send it to anyone. There are fundamental limitations to what a messaging app can protect you from.
While the analog hole will always exist, and you can't make it actually impossible, Snapchat's quite good at that screenshot thing. Both platforms have APIs to prevent, or at least notify on the use of screenshot. It's weird that signal doesn't use any of them.
i know of ~3 currently working methods to take screenshots on snapchat
it isn't "weird that signal doesn't use any of them" because it does [1] use both, just not for giving a false sense of security to your correspondents
[1] https://support.signal.org/hc/en-us/articles/360043469312-Sc...
this is the same argument as saying "you shouldn't have remote delete requests". Yes, people can screenshot or export. That doesn't mean you shouldn't have a nicety that generally works pre-compromise or pre-evil. Locks just keep honest people honest.
Do you also think it's "strange" that they're introducing that (optional!) feature while also storing all the messages on your device? The cloud backup is strictly more secure than that on-device database. Their blog post on the subject also explicitly says it won't include disappearing messages that disappear within 24 hours.