Improvement on a Generalized Scheme of Proxy Signature Based on Elliptic Curves

In a generalized (t1/n1-t2/n2) proxy signature scheme with known signers, any t1 or more original signers out of n1 original signers (1 t1 n1) can represent the original group to delegate the signing capability, and t2 or more proxy signers out of n2 proxy signers (1 t2 n2) can represent the proxy group to sign message on behalf of the original group. In the paper, we show that Hwang et al.'s generalized proxy signature scheme is vulnerable to the original signers' forgery attack. After a malicious original group of t1 (t1 t1 n1) signers obtains a proxy sig- nature (Mw, K, AOSID, M, R, S, AP SID), the original signer group can collude to generate a generalized proxy signature without the agreement of the proxy group with the identities AP SID. Hwang et al.'s generalized proxy signa- ture scheme is unable to meet nonrepudiation. We propose an improved generalized proxy signature scheme which can resist our original signer group's collusion attack.