The Password Change Phase is Still Insecure

In 2004, W. C. Ku and S. M. Chen proposed an efficient remote user authentication scheme using smart cards to solve the security problems of Chien et al.’s scheme. Recently, Hsu and Yoon et al. pointed out the security weaknesses of the Ku and Chen’s scheme Furthermore, Yoon et al. also proposed a new efficient remote user authentication scheme using smart cards. Yoon et al. also modified the password change phase of Ku and Chen’s scheme. This paper analyzes that password change phase of Yoon et al’s modified scheme is still insecure.