Attacks on the (enhanced) Yang-Shieh authentication

The Yang-Shieh authentication is a time-stamp based password authentication scheme that uses smart cards 1]]. In 2 and 3]], various attacks on this scheme are described. However, an enhancement of the scheme is proposed in 3]] and enables the scheme to resist these existing attacks. In this paper, we show two new attack that can break the enhanced scheme. We further point out that the fundamental computational assumption of the Yang-Shieh authentication scheme is incorrect.