Cryptanalysis and attacks on designated verifier signature schemes

Designated verifier signature (DVS) scheme, which sacrifices the non-repudiation to realize the property that only original signer or the designated verifier can verify the signature and cannot convince others, is effective enough under the specific environment. For stronger security definition, designated verifier signature scheme is non-delegatable. That means any third-party if do not know the signer or the designated verifier´s private key cannot produce a valid signature. In this paper, we present a security analysis of the Islam et al.´s scheme. The result indicates that it does not meet the property of non-delegatable they claimed, and may be attacked by a malicious verifier who can generate proxy signatures for abuse. Moreover, we have found that Asaar et al.´s scheme is defective on the structure and delegatable by the malicious designated verifier.