Abstract: This paper analyzes two public channel proxy signature schemes，we point out that both schemes are insecure；the scheme of [4] can’t resist the change original signer attack.In the scheme of [5]，original signer can successfully forge a valid proxy signature scheme.A new public channel signature scheme is proposed.Analysis shows that the proposed scheme satisfies all security properties of the strong proxy signature scheme.

Key words: digital signature, proxy signature, forgery attack

摘要： 对两个不需要安全信道的代理签名方案进行了分析，发现它们并不满足不可伪造性。在[4]中不能抵抗原始签名人改变攻击，在[5]中原始签名人可以伪造一个有效的代理签名。提出一个改进的强代理签名方案，新方案在不增加计算复杂性的前提下，保证了签名的安全性，同时代理授权过程中仍然保持不需要安全信道。

关键词: 数字签名, 代理签名, 伪造攻击