Weaknesses and improvement of Wang et al.'s remote user password authentication scheme for resource-limited environments

Recently, Wang et al. showed that two new verifier-free remote user password authentication schemes, Ku-Chen's scheme and Yoon et al.'s scheme, are vulnerable to an off-line password guessing attack, a forgery attack, and a denial-of-service attack, and then proposed an improved scheme for the real application in resource-limited environments. Unfortunately, we find that Wang et al.'s scheme is still vulnerable to an impersonation attack and an off-line password guessing attack. In addition, Wang et al.'s scheme is not easily reparable and is unable to provide perfect forward secrecy. Finally, we propose an improved scheme with better security strength.