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

In 1998, Dwork et al. first proposed an application of zeroknowledge, deniable authentication protocol. Thereafter, there were many researches about the deniable authentication schemes. In 2004, Shao demonstrated out that the previous schemes had a common weakness in which any third party can impersonate the intended receiver to verify the signature of the given message, and they proposed a new scheme based on ElGamal signature scheme. Although Shao claimed that his scheme 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 the deniable authentication scheme.