Abstract
Recently, Hwang and Chen [5] proposed a variation of proxy signature scheme called the multi-proxy multi-signature scheme. In their scheme, an authorized group of proxy signers is allowed to sign message on behalf of a group of original singers. In this paper, we show that their multi-proxy multi-signature scheme is insecure against a kind of forgery attack in which an attacker can easily forge the signature for an arbitrary message. Furthermore, we also point out that the multi-proxy signature scheme in [2] also suffers from the same security flaw.