On the Security of Two Signature Schemes

Special properties of group signature and short signature make them become hot topic of current e-commerce research. To further improve efficiency, recently, Song Han et al gave an efficient group signature scheme without using the proof of knowledge in ECMN2004, and Rui Zhang et al proposed a new short signature scheme without random oracle in ACNS2005. A common advantage of the two schemes is very efficient. In this paper, we first analyze the security of the HWL group signature scheme and show that the scheme is universal forgeable, namely, arbitrary one can forge a group signature in the name of any group member, while his identity cannot be revealed by the group manager. Then through analyzing the ZFI short signature scheme, we show that the short signature scheme exists key substitution attack. At the same time, we also give the corresponding improved ways to overcome the flaws of the schemes above by introducing the proof of knowledge, respectively