Paper 2011/219

On the Security of TLS-DHE in the Standard Model

Tibor Jager, Florian Kohlar, Sven Schäge, and Jörg Schwenk

Abstract

TLS is the most important cryptographic protocol in use today. However, up to now there is no complete cryptographic security proof in the standard model, nor in any other model. We give the first such proof for the core cryptographic protocol of TLS ciphersuites based on ephemeral Diffie-Hellman key exchange (TLS-DHE), which include the cipher suite TLS DHE DSS WITH 3DES EDE CBC SHA mandatory in TLS 1.0 and TLS 1.1. It is impossible to prove security of the TLS Handshake in any classical key-indistinguishability-based security model (like e.g. the Bellare-Rogaway or the Canetti-Krawczyk model), due to subtle issues with the encryption of the final Finished messages of the TLS Handshake. Therefore we start with proving the security of a truncated version of the TLS Handshake protocol, which has also been considered in previous work on TLS. Then we define the notion of authenticated and confidential channel establishment (ACCE) as a new security model which captures precisely the security properties expected from TLS in practice, and show that the combination of the TLS Handshake protocol with the TLS Record Layer can be proven secure in this model.

Note: Fixed a notational issue concerning the encryption of the Finished messages.