Weaknesses Analysis and Countermeasure of a Verifier-Based Key Agreement Protocol

In 2005, Lee et al. (S.W. Lee, H.S. Kim, K.Y. Yoo, Efficient verifier-based key agreement protocol for three parties without server´s public key, Applied Mathematics and Computation 167) proposed a efficient verifier-based key agreement protocol for three parties which does not require server´s public key. However, we find that the protocol is susceptible to an impersonation-of-initiator attack and an impersonation-of-responder attack. Further, we recommend a small change to the protocol that fixes the security problems.