关键词: 数字签名, 代理签名, 门限代理签名, 门限代理门限签名

Abstract: Hwang et al，have proposed a new threshold-proxy threshold-signature scheme in 2005.In this scheme，only any t or more original signers of n original signers can cooperatively authorize a proxy group of m proxy signers and then only c or more proxy signers can cooperatively generate threshold-proxy threshold-signature.Compared with other threshold multi-proxy multi-signature，this scheme has the advantage of providing the fair protection for the original and proxy groups.Unfortunately，this paper points out that Hwang’s scheme is vulnerable to insider attack.That is，malicious inside members can forge a threshold-proxy threshold-signature for a message while participating in a normal message signing process.Furthermore，this paper proposes two improved schemes on the base of analyzing the insider attack.

Key words: digital signature, proxy signature, threshold proxy signature, threshold-proxy threshold-signature