Cryptanalysis of a Non-interactive Deniable Authentication Protocol Based on Factoring

A deniable authentication protocol allows a sender to transfer an authenticated message to a receiver in such a way that the receiver cannot prove to a third party about the source of the message. In recent years, many deniable authentication protocols have been proposed. In 2005, Lu et al. proposed a secure and non-interactive deniable authentication protocol based on factoring. Although Lu et al. claimed that their protocol could provide complete security and properties of a deniable authentication protocol, we will point out that Lu et al. protocol is unable to achieve the second requirement of being the deniable authentication protocol.