Cryptanalysis and Improvement of Digital Multisignature Scheme Based on RSA

Zhang et al. proposed a sequential multisignature scheme based on RSA. The scheme has advantages of low computation and communication costs, and so on. However, we find a problem in their scheme that the verifier can not distinguish whether the multisignature is signed by all the signers of the group or only by the last signer. Thus, any single signature created by the last signer can be used as a multisignaturr created by the whole group members. This paper proposes an improved scheme that can overcome the defect. In the new scheme, the identity messages of all the signers are added in the multisignature and used in verification phase, so that the verifier can know the signature is generated by which signers. Performance analysis shows that the proposed scheme costs less computation than the original scheme in both signature and verification phases. Furthermore, each partial signature is based on the signer＇s identity certificate, which makes the scheme more secure.