Cryptanalysis of an Efficient Deniable Authentication Protocol Based on Generalized ElGamal Signature Scheme

In 1998, Dwork et al. first proposed an application of zero-knowledge, deniable authentication protocol.Thereafter, there were many researches about the deniable authentication schemes. In 2004, Shaodemonstrated out that the previous schemes had a common weakness in which any third party canimpersonate the intended receiver to verify the signature of the given message, and theyproposed a new scheme based on ElGamal signature scheme. Although Shao claimed that hisscheme could provide complete security and properties of a deniable authentication scheme,we will point out that Shao's scheme is unable to achieve the second requirement of thedeniable authentication scheme.