[TLS] TLS 1.3, Raw Public Keys, and Misbinding Attacks

[John Mattsson <[e-mail]>]

Hi,

I looked into what RFC 8446(bis) says about Raw Public Keys. As correctly stated in RFC 8446, TLS 1.3 with signatures and certificates is an implementation of SIGMA-I:

SIGMA does however require that the identities of the endpoints (called A and B in [SIGMA]) are included in the messages. This is not true for TLS 1.3 with RPKs and TLS 1.3 with RPKs is therefore not SIGMA. TLS 1.3 with RPKs is vulnerable to what Krawczyk’s SIGMA paper calls misbinding attacks:

“This attack, to which we refer as an “identity misbinding attack”, applies to many seemingly natural and intuitive protocols. Avoiding this form of attack and guaranteeing a consistent binding between a session key and the peers to the session is a central element in the design of SIGMA.”

“Even more significantly we show here that the misbinding attack applies to this protocol in any scenario where parties can register public keys without proving knowledge of the corresponding signature key.”

As stated in Appendix E.1, at the completion of the handshake, each side outputs its view of the identities of the communicating parties. On of the TLS 1.3 security properties are “Peer Authentication”, which says that the client’s and server’s view of the identities match. TLS 1.3 with PRKs does not fulfill this unless the out-of-band mechanism to register public keys proved knowledge of the private key. RFC 7250 does not say anything about this either.

I think this needs to be clarified in RFC8446bis. The only reason to ever use an RPK is in constrained IoT environments. Otherwise a self-signed certificate is a much better choice. TLS 1.3 with self-signed certificates is SIGMA-I.

It is worrying to find comments like this:

“I'd like to be able to use wireguard/ssh-style authentication for my app. This is possible currently with self-signed certificates, but the proper solution is RFC 7250, which is also part of TLS 1.3.”
https://github.com/openssl/openssl/issues/6929

RPKs are not the proper solution.

(Talking about misbinding, does RFC 8446 say anything about how to avoid selfie attacks where an entity using PSK authentication ends up talking to itself?)

Cheers,
John Preuß Mattsson

[SIGMA] https://link.springer.com/chapter/10.1007/978-3-540-45146-4_24