Forgery Attacks on Lee-Chang´s Strong Designated Verifier Signature Scheme

In 2008, Lee-Chang pointed out that Saeednia et al.´s scheme would reveal the identity of the signer if the secret key of the signer is compromised. Then, they proposed a new strong designated verifier signature scheme that provides signer ambiguity, even if the secret key of the signer is compromised. Lee-Chang claimed that their proposed scheme is more secure and suitable for the purpose of a strong designated verifier signature scheme. Unlike their claims, this paper shows that Lee-Chang scheme still cannot withstand our proposed forgery attacks which an adversary can forge a signature without knowing the secret key of the signer or the designated verifier.