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.