An improved threshold proxy signature scheme based on the RSA cryptosystem

The proxy signature scheme, a variation of ordinary digital signature schemes, enables a proxy signer to sign messages on behalf of the original signer. In a (t, n) threshold proxy signature scheme, the original signer authorizes a proxy group with n proxy members. Only the cooperation of t or more proxy members is allowed to generate the proxy signature. Former threshold proxy signature schemes are all based on the difficulty of discrete logarithm. In 2003, for the first time, Hwang et al proposed a practical (t,n) threshold proxy signature scheme based on the difficulty of factoring the RSA modulus. By our observation, their scheme can't provide the properties of proxy protection, strong unforgeability, strong nonrepudiation, strong identifiability and known signers. To overcome their drawbacks, based on their scheme, we propose an improved version. The improved scheme provides the properties of not only proxy protection, strong unforgeability, strong identifiability and strong nonrepudiation, but also known signers.