Improvement of a Three-party Key Exchange Protocol

Recently, Lu and Cao proposed a novel three-party key exchange protocol. Guo et al pointed out that their protocol cannot resist man-in-the-middle attack and undetectable on-line dictionary attack, and then an improved protocol was proposed in 2008.However, we show that Guo et alpsilas protocol is still vulnerable to undetectable on-line dictionary attack. To overcome the security flaw, an improved scheme is proposed.