Abstract: Security analyses of an identity-based designated verifier signature scheme presented by Li Mingxiang et al. are given.It is found that the scheme exists two vital security flaws：the scheme is universal forgeable.And the scheme is not a strong designated verifier signature scheme since a non-designated-verifier can also check the validity of signatures.An improved scheme is proposed.This scheme overcomes the security flaws of the original scheme，improves the security of the system，and retains some merits of the original scheme.

Key words: identity-based cryptography, designated verifier signature, bilinear pairing

摘要： 对李明祥等提出的一个基于身份的强指定验证者签名方案进行了安全性分析，指出其存在两个重大的安全缺陷：（1）该方案是可以普遍伪造的;（2）该方案不是一个指定验证者签名方案，非指定验证者也可以验证签名的有效性。提出了改进方案，克服了原方案的缺陷，提高了系统的安全性，并保留了原方案的优点。

关键词: 基于身份的密码体制, 指定验证者签名, 双线性对