On Password-Authenticated Key Exchange Security Modeling

Deciding which security model is the right one for Authenticated Key Exchange (AKE) is well-known to be a difficult problem. In this paper, we examine definitions of security for Password-AKE (PAKE) in the style proposed by Bellare et al. at Eurocrypt 2000. Indeed, there does not seem to be any consensus, even when narrowing the study down to this particular authentication method and model style, on how to precisely define fundamental notions such as accepting, terminating, and partnering. The aim of this paper is to begin addressing this problem. We first show how definitions vary from paper to paper. We then propose and thoroughly motivate a definition of our own, and use the opportunity to correct a minor flaw in a more recent and more PAKE-appropriate model proposed by Abdalla et al. at Public Key Cryptography 2005. Finally, we argue that the uniqueness of partners holding with overwhelming probability ought to be an explicitly required and proven property for AKE in general, but even more so in the password case, where the optimal security bound one aims to achieve is no longer a negligible value. To drive this last point, we exhibit a protocol that is provably secure following the Abdalla et al. definition, and at the same time fails to satisfy this property.