Improvement on a generalized scheme of proxy signature based on elliptic curves

In a generalized (t(1)/n(1)-t(2)/n(2)) proxy signature scheme with known signers, any t(1) or more original signers out of n(1) original signers (1 <= t(1) <= n(1)) can represent the original group to delegate the signing capability, and t(2) or more proxy signers Out Of n(2) proxy signers (1 <= t2 <= n(2)) 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 t'(1) (t(1) <= t'(1) <= n(1)) signers obtains a proxy signature (M-w, K, AOSID, M, R, S, APSID), the original signer group can collude to generate a generalized proxy signature without the agreement of the proxy group with the identities APSID. Hwang et al.'s generalized proxy signature scheme is unable to meet nonrepudiation. We propose an improved generalized proxy signature scheme which can resist our original signer group's collusion attack.