On the security of an identity-based proxy multi-signature scheme

In 2000, Yi et al. proposed two proxy multi-signature schemes which enable two or more original signers to delegate their signing power to a proxy signer. Combining proxy multi-signature with identity-based cryptography, Wang et al. proposed an identity-based proxy multi-signature scheme in 2007. Their scheme is claimed to be secure in the random oracle model. However, in this study, the author shows that Wang et al.´s scheme is vulnerable to a forgery attack described in this study.