An Offline Dictionary Attack against zkPAKE Protocol

Password Authenticated Key Exchange (PAKE) allows a user to establish a strong cryptographic key with a server, using only knowledge of a pre-shared password. One of the basic security requirements of PAKE is to prevent offline dictionary attacks. In this paper, we revisit zkPAKE, an augmented PAKE that has been recently proposed by Mochetti, Resende, and Aranha (SBSeg 2015). Our work shows that the zkPAKE protocol is prone to offline password guessing attack, even in the presence of an adversary that has only eavesdropping capabilities. Therefore, zkPAKE is insecure and should not be used as a key exchange mechanism.