Comments on Two Multi-Server Authentication Protocols

Recently, Tsai and Liao et al. each proposed a multi-server authentication protocol. They claimed their protocols are secure and can withstand various attacks. But the authors found some security loopholes in each protocol. This paper will show the attacks on their schemes. The authors have analyzed the security of Tsai's and Liao-Wang's protocols. Although, they claim their protocols can resist against various attacks, the authors have showed that their protocols are indeed insecure against some attack that the authors have described in this paper.