On the Security of Two Group Signature Schemes with Forward Security

A group signature scheme allows a group member of a group to sign messages on behalf of the group anonymously. In case of dispute, a special entity of the group, group manager, can reveal the signer of a valid group signature. In 2005, Zhang et al. proposed a new group signature with forward security based on their earlier scheme in ICICS 2003. Recently, Zhou et al. proposed a dynamic group signature scheme with forward security at GCC 2007, In the year of 2008, Zhang and Geung pointed out the scheme is insecure and suggested an improvement. In this paper, we analyze a security analysis of Zhang et al.’s group signature scheme and Zhou et al.’s group signature scheme. We also discuss why the improved Zhou et al.’s scheme by Zhang et al. is still insecure.