Session-state Reveal Is Stronger Than Ephemeral Key Reveal: Attacking the NAXOS Authenticated Key Exchange Protocol

In the paper “Stronger Security of Authenticated Key Exchange” [1,2], a new security model for authenticated key exchange protocols (eCK) is proposed. The new model is suggested to be at least as strong as previous models for key exchange protocols. The model includes a new notion of an Ephemeral Key Reveal adversary query, which is claimed in e. g. [2–4] to be at least as strong as the Session-state Reveal query. We show that Session-state Reveal is stronger than Ephemeral Key Reveal, implying that the eCK security model is incomparable to the CK model [5, 6]. In particular we show that the proposed NAXOS protocol from [1, 2] does not meet its security requirements if the Session-state Reveal query is allowed in the eCK model. We discuss the implications of our result for some related protocols proven correct in the eCK model, and discuss the interaction between Session-state Reveal and protocol transformations.