[toast0]

Prekeys are to start a session with someone, it's basically a public key. You generate a new public private keypair, do a DHE to establish the session secrets, send your new public key along with the encrypted message. If you send more messages to the same person, they use the same session.

TLS is fine enough for messages in flight, but a lot of messengers store message archives on their servers, and there may be tens of thousands of employees who have potential access to that; not sure if that's really what anybody's mom needs.

[lmm]

> Prekeys are to start a session with someone, it's basically a public key. You generate a new public private keypair, do a DHE to establish the session secrets, send your new public key along with the encrypted message.

At which point you have essentially decayed to conventional PKI and don't get any the security properties that you were supposed to get from the fancy Signal protocol (i.e. PFS).

[bjoli]

Well, you always have to verify keys. No security is guaranteed without it. That is the case everywhere. Those pre-keys are just there to start a session, which in my communication with my brother case has lasted for as long as I had my phone (about 3 years). That session creates new key material with every message, providing forward secrecy.

I think it is a pretty elegant solution to key distribution, even though I wouldn't plan any bomb attempts without first validating the fingerprints.

[toast0]

The fancy protocol gives you PFS by generating new keys for each messaging round trip between parties. Anything in flight is subject to decryption if you can grab the keys from an endpoint before they're cycled.

The upside is that users can communicate with each other without needing both endpoints online simultaneously (which is fairly hard to guarantee, given all the battery saving stuff in mobile OSes and lack of 100% network coverage).

The downside is the key cycle time is much longer than they would be if all communications were done with both parties online, but it's still much shorter than a conventional PKI (ex PGP).

[lmm]

So how fast does cycling happen if the server is under an attacker's control and trying to ensure that cycling is delayed as long as possible? It's messy and complicated, and certainly doesn't leave you with the simple "messages can never be read in the future" security property that Signal's advocates claim.

Regular key rotation is good practice when using PGP or similar - have a master key that you use only for signing subkeys, generate a new subkey every month (say) and destroy the ones older than 2 months each time - though admittedly UI/tool support for doing this is limited.

[toast0]

> So how fast does cycling happen if the server is under an attacker's control and trying to ensure that cycling is delayed as long as possible?

If you're getting bi-directional communication with your partner, you're getting key cycling. The server can only delay key cycling in a session by delaying one direction of communication.

The server can't hand out the same pre-key to multiple users, because the client will delete pre-key pairs on first use, with the exception of a "last resort" pre-key. The server could exclusively hand out the last resort pre-key to all users attempting to contact you, and refuse to accept new pre-keys. Then the first flight of messages from users establishing a new session would not be PFS, but any messages sent once you respond would have PFS.

Adding, of course, the server could also hand out incorrect keys and man in the middle all the steps; so long as users don't verify the keys. And a malicious client and server could conspire to include the correct keys for verification and the MITM keys for transport. This would be visible in the shipped code, but if the backdoored client is only distributed to a limited set of users, it wouldn't be subject to random reverse engineering like the normal client is. But I assume everybody downloads multiple copies of apks from different networks and compares to ensure they're byte identical ;)

[lmm]

> If you're getting bi-directional communication with your partner, you're getting key cycling. The server can only delay key cycling in a session by delaying one direction of communication.

That implies the client has to have a code path for sending multiple messages without cycling. And it forces a tough choice between losing messages that are received out of order and not destroying keys quite as quickly as nominally expected.

Maybe it's fine, but for me it pushed the complexity threshold over the point where I could feel any confidence in it. I'm comfortable with traditional PKI. I'm comfortable with the online-only OTR/axolotl ratchet. But I'm very dubious of having this many edge cases.

> The server could exclusively hand out the last resort pre-key to all users attempting to contact you, and refuse to accept new pre-keys. Then the first flight of messages from users establishing a new session would not be PFS, but any messages sent once you respond would have PFS.

Assuming there's no way for the client to end up on the initial-message codepath.

Again, yeah, maybe it's fine. But it all just feels so hacky and fiddly. These edge cases aren't where anyone studying the protocol is going to spend any time, but it's security-critical that they be implemented right.

[bjoli]

I just re-read your message, and thought I should clarify: The client has two ratchets going. One is an opportunistic DH ratchet, and the other is a hash-based one that provides forward secrecy if the contents of the last DH ratchet was not intercepted and decrypted. Which, if you have verified the keys and no device key change has happened, it hasn't.

If you have a successful compromise of one message, a missed message is all it takes for the ratchet to self-heal and you have lost the ability to decrypt future messages. It is PFS+ in a sense.