Date Added: Nov 2011
In 2006, Liao et al. proposed a scheme over insecure networks. In 2006, Yoon-Yoo, and in 2008, Xiang et al. analyzed Liao et al.'s scheme and both of them pointed out, more or less, same vulnerabilities: like offline password guessing attack, impersonating the server by replay at-tack, denial of service attack on password changing and insider attack on it. But none of them suggested any solution to the pointed out attacks. This paper proposes an improved scheme with enhanced security, maintaining advantages of the original scheme and free from the attacks pointed out by Yoon-Yoo and Xiang et al..