Comments on a novel user authentication and key agreement scheme

In 2013, Sun et al. showed that the related works' authentication schemes proposed by [2-7] are vulnerable to an insider attack and fail to provide mutual authentication. These two attacks can be successfully plotted by an adversary, since the private key of the server can compute all the legal users’ private keys. They then proposed a new remote user authentication and key agreement scheme for the mobile client-server environment. However, we find that their scheme is still vulnerable to insider attack (Sun et al.) and how to avoid such an insider attack on the client-server environment is still an open problem. Keyword: user authentication, key agreement, client-server environment, insider attack, mutual authentication