中文摘要：

2003年,李等指出Chan-Wei的门限代理签密方案不满足强不可伪造性、强不可否认性和强识别性,同时提出了一个改进方案.笔者给出了一种替换私钥攻击方案,指出李等的方案也不满足强不可伪造性,指定接收人可以利用替换私钥生成有效的代理签名.为克服李等方案的安全性缺陷,给出了一个门限代理签密改进方案.

英文摘要：

In 2003, Li et al. showed that Chan-Wei＇s threshold proxy signcryption scheme did not satisfy strong unforgeability, strong nonrepudiation and strong identifiability. Additionally, they presented an improvement to counter the weaknesses. However, secret key substitution attack on Li et al. ＇s scheme to show that their scheme is also vulnerable to the forge attack. The specified receiver can forge a valid threshold proxy signature for any message by secret key substitution. To eliminate the weaknesses, we propose a threshold proxy signcryption scheme to improve Li et al. ,s scheme.