Abstract: Security analyses of two certificateless signature schemes presented respectively by Su Wanli et al.and Zhang Yulei et al.are given.It is found that the two schemes are insecure against public-key replacement attacks and a forgery attack on the two schemes is given respectively.Two improved schemes for the two schemes are proposed.The improved schemes overcome the security flaws of original schemes，improve the security of the system，and retain some merits of the original schemes.

Key words: certificateless signature, public-key replacement attack, bilinear pairing

摘要： 对苏万力等提出的无证书签名方案和张玉磊等提出的无证书签名方案进行了安全性分析，指出这两个方案在公钥替换攻击下是不安全的，分别给出了这两个方案的一种伪造攻击。提出了这两个方案的改进方案，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: 无证书签名, 公钥替换攻击, 双线性对