Security Weaknesses in Chang and Wu´s Key Agreement Protocol for a Multi-Server Environment

Recently, Chang and Wu have proposed an efficient key agrement protocol suited for a multi-server environment. This work reviews Chang and Wu´s protocol and provides a security analysis on the protocol. Our analysis shows that Chang and Wu´s protocol does not achieve its fundamental goal not only of password security but also of mutual authentication. We demonstrate these security flaws by mounting an off-line password guessing attack and two impersonation attacks, the sever impersonation attack and the user impersonation attack on Chang and Wu´s protocol. In addition, we found that the protocol is vulnerable to an attack against perfect forward secrecy.