A secure improvement on the user-friendly remote authentication scheme with no time concurrency mechanism

Sun proposed an efficient remote authentication scheme using smart cards which ensures low communication and computation cost, but the user cannot choose his/her password at will. Thus, Wu and Chieu presented an improvement. However, Yang and Wang pointed out that Wu and Chieu's scheme is vulnerable to the password guessing and forgery attacks. To preserve the advantage of Sun's scheme and Wu and Chieu's scheme and to amend the security weakness, we propose an improvement in this paper. Moreover, our scheme needs no time concurrency mechanism and ensures mutual authentication.