TLS Triple Handshakes (03 Mar 2014)

Today, the TLS WG mailing list meeting received a note about the work of Karthikeyan Bhargavan, Antoine Delignat-Lavaud, Cedric Fournet, Alfredo Pironti and Pierre-Yves Strub on triple handshake attacks against TLS. This is more complex than just a duplicated goto and I'm not going to try and reproduce their explanation here. Instead, I'll link to their site again, which also includes a copy of their paper.

In short, the TLS handshake hashes in too little information, and always has. Because of that it's possible to synchronise the state of two TLS sessions in a way that breaks assumptions made in the rest of the protocol.

I'd like to thank the researchers for doing a very good job of disclosing this. The note today even included a draft for fixing the TLS key derivation to include all the needed information to stop this attack, and it'll be presented at the WG meeting tomorrow.

In the mean time, people shouldn't panic. The impact of this attack is limited to sites that use TLS client-certificate authentication with renegotiation, and protocols that depend on channel binding. The vast majority of users have never used client certificates.

The client-certificate issues can be fixed with a unilateral, client change to be stricter about verifying certificates during a renegotiation, as suggested by the authors. I've included an image, below, that is loaded over an HTTPS connection that renegotiates with unrelated certificates before returning the image data. Hopefully the image below is broken. If not, then it likely will be soon because of a browser update. (I took the server down.)

Protocols that depend on channel binding (including ChannelID) need other changes. Ideally, the proposed update to the master-secret computation will be finalised and implemented. (For ChannelID, we have already updated the protocol to include a change similar to the proposed draft.)

It's likely that there are still concrete problems to be found because of the channel-binding break. Hopefully with today's greater publicity people can start to find them.