Repairing Efficient Threshold Group Signature Scheme

To enhance the efficiency of threshold group signature schemes, Yu and Chen, recently, proposed an efficient threshold group signature scheme. By using elliptic curves, the proposed scheme can use short secret key and reduces the load of signature verification. However, in this paper we find that there are many ambiguities in the proposed scheme. The verifiers cannot verify valid signatures, while adversaries can not only easily forge the signatures of individual members, but also forge group signatures without the knowledge of secret keys. Though we can modify it to withstand this forgery attack, the modified scheme cannot withstand a general coalition attack inherent in many threshold signature schemes.