On Proxy (Multi-) Signature Schemes

Abstract

A proxy signature scheme allows a designed person, called a proxy signer, to sign messages on behalf of an original signer. Generalizing the concept of the proxy signature scheme, Yi et al. proposed the proxy multisignature scheme which allows a proxy signer to generate a proxy signature on behalf of two or more original signers. In this paper, we first analyze and improve the security of two proxy signature schemes, proposed by Sun and Hsieh at IS’99. Our analysis indicates that these two schemes suffer from the public key substitution attack and a kind of direct forgery. Then we show that the same attacks on Sun-Hsieh proxy signature schemes can be generalized to work on Yi et al.’s proxy multisignature schemes. Two proxy multi-signature schemes are consequently proposed to defeat these attacks. Finally, we point out that the Sun-Lee-Hwang threshold proxy signature scheme is also vulnerable to the same attacks above. An improved version is therefore proposed.