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Comment by elvisloops | Hacker News Reader

Strange that they are posting about the "signal ratchet" when they just removed it by launching cloud backups that use a static key? Since those cloud backups include disappearing messages, that feature completely undoes all of the forward secrecy in this protocol.

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Signal can't protect you against the other party you are communicating with. They can backup the conversation, or screenshot it, or take a photo of the screen with another camera. They could also retell in their words what you sent.

You know (with pretty high certainty) that your conversational partner is using Signal. The security level of Signal affects your estimation of the security level of your partner.

That backup system presumably uses symmetric encryption, which is not nearly as vulnerable to quantum-accelerated attacks.

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

>but there is already no post-compromise security after the cloud backups feature

The feature is opt in, so I really don't see the issue here.

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.

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...

What are they, other than the analog hole?

Screen security so that I can't see the app on my phone in preview during app switching isn't the same thing as stopping screenshots.

android emulator (i think bluestacks still works), web snapchat client + BetterSnap extension (you can even save the original media file!), on graphene it seems to detect screenshots but not video recording (likely not intentional, there was an open issue to block screenshot detection but no devs were interested iirc)

it's a lost cause except maaaybe provisioning drm keys but even then, as you say, analog hole

re: screen security isn't the same thing - that's what i mean, signal does use those very APIs but not for a half-assed snitching feature

Oh interesting. Yeah, I mean, iOS has the API so it seems silly that they don't do anything about it there, but I guess if you support a diverse userbase like they have to, then user education is impossible and a false sense of security would be a bad thing to give to uaers

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.

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