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Comment by Marsymars | Hacker News Reader

Sure, but you already have no way of knowing which one the other parties in your chats are doing.

I already sync my Signal backups to the cloud, because that's the most practical and time/cost-effective way to have a 3-2-1 backup system for my chats.

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There's a difference between someone in your chats acting adversarially and Signal supporting/encouraging adversarial behavior as part of the way the app works. If Signal published a change to the protocol that removed forward secrecy, we wouldn't consider it a non-event and say "well anyone could screenshot messages anyway," even though that may be true. They're calling this "secure backups," but in truth it appears to reduce security

I'd also wonder where this shared encryption key for message "backups" is stored. If it's available on all of my devices, I suspect it would be available on other devices as well?

I mean it says so right in the blog post

At the core of secure backups is a 64-character recovery key that is generated on your device. This key is yours and yours alone; it is never shared with Signal’s servers. Your recovery key is the only way to “unlock” your backup when you need to restore access to your messages. Losing it means losing access to your backup permanently, and Signal cannot help you recover it. You can generate a new key if you choose. We recommend storing this key securely (writing it down in a notebook or a secure password manager, for example).

i missed that paragraph, thanks for pointing it out. i wonder what algorithm they're using here, and if we could use third party tooling to decrypt these messages on a local computer? it might be a pathway to some cool experimental third-party Signal apps

The article says it is generated on your device and they don't have a copy. Sounds like a public-private keypair where you are responsible for managing the private key.

got it. doesn't Signal already have on-device keys with a session ratchet? why not back those keys up so one can decrypt the entire history on any device?

afaik the key material is regenerated for every message. new keys can be derived for every subsequent message you send, but only until you get a reply, then a new key exchange takes place. And the key material for message m1 cannot derive keys for the messages that came before m1. If the old key material gets properly deleted then there is only a very small window of compromise. backing up those keys would defeat the purpose of the ratchet.

yes, agreed, and isn't this feature re-encrypting all of the material without a ratchet or asymmetrical boxing?

Yes, it undoes all of the security features of Signal's encryption protocol.

I don't think it's appropriate to call someone you're talking to with disappearing messages turned off making a backup of the conversation so they have the (non-disappearing) message history if they drop their phone in a lake as "adversarial behavior".

If you don't want them to have a history only communicate via disappearing messages.

This post says disappearing messages are included in the backups. You have to enable disappearing messages with a timer of less than 24 hours to ensure that you can opt out.

Sure but the backup happens each day and then gets overwritten/deleted when the next days backup happens (which then deletes the disappearing messages that are expiring express the next backup). It just ensures you have access to any messages that you’re supposed to have access to according to the timers on said messages.

That's not how forward secrecy works. Ciphertext isn't "deleted" unless the key used to encrypt it is also deleted. That's the point of Signal's cutting edge protocol. This undoes all of that.

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