Cryptanalysis of a robust smart card authentication scheme for multi-server architecture

User authentication is an important security issue for network based services. Multi-server authentication scheme resolves the repeated registration problem of single-server authentication scenario where the user has to register at different servers to access different types of network services. Recently, Pippal et al. proposed a smart card authentication scheme for multi-server architecture. They claimed that their scheme has some advantages and can resist kinds of attacks. In this paper, we analyze the weaknesses of Pippal et al.´s scheme, and point out that their scheme cannot provide correct authentication, cannot resist impersonation attack, stolen smart card attack, and insider attack. Besides, their scheme is non-extensible when a new server added into the system.