Efficient verifier-based key agreement protocol for three parties without server's public key

Recently Lee et al. proposed an efficient verifier-based key agreement protocol for three parties without server’s public key. Lee et al. claimed the proposed protocol could resist various attacks and provide the perfect forward secrecy. However, in this paper, authors shall show that their protocol suffers from the impersonation attack. At the same time, authors shall propose an improved method to withstand the above attack. Keyword: authentication, cryptanalysis, impersonation attack, key agreement, password.