Cryptanalysis and Improvement of User Authentication Scheme using Smart Cards for Multi-Server Environments

For providing the login service in multi-server environments, Fan, Xu, and Li presented a remote user authentication scheme using smart cards. In this paper, we demonstrate that Fan-Xu-Li´s scheme is vulnerable to the parallel session attack. That is, when a legal user logs in a server, an adversary without knowing any secret information can easily impersonate the user to log in other authorized servers. It means that a serious security flaw exists in Fan-Xu-Li´s scheme. In addition to being practical, it is desirable to avoid relying on timestamps for security in their scheme. We therefore propose an improved scheme to overcome above disadvantages. As a unilateral authentication mechanism, our improved scheme is more suitable for real-life cryptographic applications than Fan-Xu-Li´s scheme