Cryptanalysis of simple three-party key exchange protocol

Recently, Lu and Cao published a novel protocol for password-based authenticated key exchanges (PAKE) in a three-party setting in Journal of Computers and Security, where two clients, each shares a human-memorable password with a trusted server, can construct a secure session key. They argued that their simple three-party PAKE (3-PAKE) protocol can resist against various known attacks. In this paper, we show that this protocol is vulnerable to a kind of man-in-the-middle attack that exploits an authentication flaw in their protocol and is subject to the undetectable on-line dictionary attack. We also conduct a detailed analysis on the flaws in the protocol and provide an improved protocol.