Cryptanalysis of security enhancement for the timestamp-based password authentication scheme using smart cards

Recently, Yang and Shieh proposed two password authentication schemes. In 2002, Chan and Cheng pointed out that Yang and Shieh´s timestamp-based authentication scheme was vulnerable to the forgery attack. In 2003, Sun and Yeh pointed out that Yang and Shieh´s scheme was vulnerable to the forgery attack and explained Chan and Cheng´s attack was unreasonable. Later, Shen et al. modified Yang and Shieh´s scheme to resist Chan and Cheng´s attack. However, in this paper, we shall point out that Shen et al.´s improvement is still vulnerable to the forgery attack.