Comments on two password based protocols

Recently, M. Hölbl et al. and I. E. Liao et al. each proposed an user authentication protocol. Both claimed that their schemes can withstand password guessing attack. However, T. Xiang et al. pointed out I. E. Liao et al.’s protocol suffers three kinds of attacks, including password guessing attacks. We present an improvement protocol to get rid of password guessing attacks. In this paper, we first point out the security loopholes of M. Hölbl et al.’s protocol and review T. Xiang et al.’s cryptanalysis on I. E. Liao et al.’s protocol. Then, we present the improvements on M. Hölbl et al.’s protocol and I. E. Liao et al.’s protocol, respectively.