Abstract: This paper analyzes a certificateless proxy signature scheme and points out it are vulnerable against two types of adversaries in certificateless cryptosystems. Type I adversary can replace the user’ public key to forge proxy delegations from any original signer to any proxy signer and compute the proxy signature; type II adversary（KGC） can generate the trapdoor system parameters according to the identity information of a pre-selected target user and then forge the proxy delegations from the target user to other users. To overcome these security problems, a new improved proxy signature scheme is proposed. Analysis shows that the new scheme has better security.

Key words: certificateless signature, proxy signature, public key replacement attacks, malicious KGC attacks, bilinear mapping

摘要： 分析了一种无证书代理签名方案，指出其针对于无证书密码系统中的两类敌手都不安全。类型I敌手可替换用户的公钥来伪造代理授权和代理签名；类型II敌手（KGC）可针对预先选择好的用户生成特殊的系统参数，然后伪造代理授权。为了克服这些安全问题，提出了一种改进的方案，分析表明，新方案具有更好的安全性。

关键词: 无证书签名, 代理签名, 公钥替换攻击, 恶意的KGC攻击, 双线性对