关键词: 无证书公钥密码体制, 代理签名, 广义指定验证者签名, 替换公钥攻击

Abstract: It is pointed out that Fan et al’s certificateless proxy signature scheme and Ming et al’s certificateless universal designated verifier signature scheme are both insecure against public key replacement attack.At the same time，Fan et al’s scheme can not resist the original signer changing attack.An adversary can forge a valid signature on the same message which is generated by these proxy signers on behalf of this attacker himself.Besides，it is pointed out that there exists a defect in Ming et al’s security proof of certificateless signature by using the replay technique directly.By hashing the public key with the proxy authorized certificate to avoid these attacks，the improved schemes can improve the security efficiently and also retain the other merits of the original schemes.

Key words: certificateless public-key cryptography, proxy signature, universal designated verifier signature, public key replacement attack