Security Analysis of Hsieh-Sun's Deniable Authentication Protocol

In 2004, Hsieh and Sun showed that Fan et al.'s deniable protocol does not posse the denial property; that is, a cheating receiver can prove the source of the received message to a third party. Furthermore, they also presented a modification of Fan et al.'s protocol to overcome the security flaw. However, the current paper points out an error in Hsieh-Sun's cryptanalysis on Fan et al.'s deniable authentication protocol and demonstrate that Hsieh-Sun's improved deniable authentication protocol still does not posse the deniable property and is susceptible to a malicious receiver's impersonation attack.