Revisiting the “An Improved Remote user Authentication Scheme with Key Agreement”

Recently, Kumari et al., pointed out that Chang et al.’s scheme “Untraceable dynamic-identity-based remote user authentication scheme with verifiable password update” has several drawbacks and does not provide any session key agreement. Hence, they proposed an improved remote user authentication scheme with key agreement based on Chang et al.’s protocol. They claimed that the improved method is secure. However, we found that their improvement still has both anonymity breach and smart card loss password guessing attack which cannot be violated in the ten basic requirements advocated for a secure identity authentication using smart card by Liao et al. Thus, we modify their protocol to encompass these security functionalities which are needed in a user authentication system using smart card.