  In 2000, Hwang and Li proposed a new remote user authentication scheme using
smart cards. In the same year, Chan and Cheng pointed out that Hwang and
Li&#8217;s scheme is not secure against the masquerade attack. Further, in
2003, Shen, Lin and Hwang pointed out a different type of attack on Hwang and
Li&#8217;s scheme and presented a modified scheme to remove its security
pitfalls. This paper presents an improved scheme which is secure against
Chan-Cheng and all the extended attacks.
