Im feeling pretty dumb even after reading the tldr. Can anyone who is well versed in this explain how this is better or safer? I read about the time, will it now be slower to send messages?
In the standard practical analysis of quantum threats to cryptography, your adversary is "harvesting and then decrypting". Everybody agrees that no adversary can perform quantum cryptography today, but we agree (to agree) that they'll plausibly be able to at some point in the future. If you assume Signal is carrying messages that have to be kept secret many years into the future, you have to assume your adversary is just stockpiling Signal ciphertexts in a warehouse somewhere waiting so that 15 or 20 years from now they can decrypt them.
That's why you want PQ key agreement today: to protect against a future capability targeting a record of the past. (It's also why you don't care as much about PQ signatures, because we agree no adversary can time travel back and MITM, say, a TLS signature verification).
To understand the importance of a PQ ratchet, add one more capability to the adversary. In addition to holding on to ciphertexts for 15-20 years, assume they will eventually compromise a device, or find an implementation-specific flaw in cryptography code that they can exploit to extract key material. This is a very realistic threat model; in fact, it's of much more practical importance than the collapse of an entire cryptographic primitive.
You defend against that threat model with "forward secrecy" and "post-compromise security". You continually update your key, so the compromise of any one key doesn't allow an attacker to retrospectively decrypt, or to encrypt future messages.
For those defenses to hold against a "harvest and decrypt" attacker, the "ratchet" mechanism you use to keep re-keying your session also needs to be PQ secure. If it isn't, attackers will target the ratchet instead of the messages, and your system will lose its forward and post-compromise secrecy.
Weakened, not broken. Quantum computers turns 128 bit AES into 64 bit equivalent. Which will still be extremely difficult for quantum computers due to the large computer size/number of steps required.
E.g. if you have 256 quantum computers, then each one of them needs to search only 60 bits of the key space to crack a 128-bit key (each one of them will only need to search 2^120 keys).
It's not really going to make much difference with near-future quantum computers. Especially since Grover's algorithm _has_ to complete all the 2^60 steps to produce a reliable result, you can't just run a quantum computer for a while, stop it, and then restart it.
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.
Show me a company anywhere that can provide security without user thought and deliberate action. It's a fantasy to believe anything you don't have to think about isn't theater. Hell, if you aren't thinking about it, you're one of the actors in that theater.
With disappearing messages off it was already reasonable to assume that a compromise of a counterparty's phone would result in exposure of all previous messages, so enabling backups wouldn't expose you to new risk.
That would cater to those who want to keep their chat history forever without exposing those with disappearing messages enabled to new risk.
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.)
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.
edit: Well, let me argue with myself for a moment. I don't think offering an encrypted backup feature undoes the PQ story. But FS/PCS is weakened, sure, since we're talking about all types of shit happening, not just currently known (or strongly theorized) attacks.
(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.)
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.
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.
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.
On a more serious note, if a quantum computer can break a key, a task requiring exponential complexity with key length on a classical computer, then breaking N keys is only a negligible additional cost in comparison.
So it kind of feels like it’s overrated in this case to be honest :)
I am excited to finally know what they mean by PCS after reading this article. It means that the session keys from their key agreement scheme (n ratchet) are generated new so an attacker doesn't get them again after a fairly specific sort of compromise. So from that I get that the off the record (OTR) protocol also has PCS. Which is a bit disappointing, I thought that they had come up with some new concept.
This key agreement doesn't happen that often. So a user isn't going to notice any slowness even if it was significantly slower.
> "What does this mean for you as a Signal user? First, when it comes to your experience using the app, nothing changes. Second, because of how we’re rolling this out and mixing it in with our existing encryption, eventually all of your conversations will move to this new protocol without you needing to take any action. Third, and most importantly, this protects your communications both now and in the event that cryptographically relevant quantum computers eventually become a reality, and it allows us to maintain our existing security guarantees of forward secrecy and post-compromise security as we proactively prepare for that new world."