Abstract: Present a security analysis of the Xu-zhang-feng identity based threshold proxy signature scheme，which can not resist original singer changing attack，at the same time，this scheme can’t withstand conspiracy attack，any [t]（or more） proxy signers can conspire to gain the private keys of other [n-t] proxy signers，and can forge their valid signature.An improved threshold proxy signature scheme is proposed，which can resolve the security problem existing in the Xu-zhang-feng identity based proxy threshold signature scheme.

Key words: threshold proxy signature, identity based, original signer changing attack

摘要： 通过对Xu-zhang-feng的基于身份的门限代理签名方案分析，发现该方案不能抵抗原始签名人的改变攻击，同时该方案也不能抵御合谋攻击，任意不少于t个代理人都可以合谋获得其他n-t个代理人的私钥从而伪造他们的签名。在此基础上提出了改进的基于身份的门限代理方案，克服了原方案的缺陷。

关键词: 门限代理签名, 基于身份, 原始签名人改变攻击