Breaking an improved password authenticated key exchange protocol for imbalanced wireless networks

Yeh et al. (2003) proposed a password-authenticated key exchange protocol which is claimed to be an improvement of Zhu et al.´s (2002) password-authenticated key exchange protocol to defend against the so-called undetectable on-line password guessing attack. In this paper, we show that the improved protocol of Yeh et al. is insecure. In fact, an adversary can recover the password, off-line, by executing a single run of the improved protocol. We also point out the misconception of the so-called undetectable on-line password guessing attack.