Mehdi Mollazadeh - Imam Hussien University DSP Institute, Tehran, Iran.
Mohammad Baghery - Department of Mathematic and Cryptography, Imam Hussien University
Hamid Reza Maimani - Department of Mathematic, Tehran university and Institute for Studies in Theoretical Physics and Mathematics.

A group signature scheme allows a group member to sign messages anonymously on behalf of the group. However, in the case of a dispute, the identi4t of a signature can be revealed by a designated entity. Based on Song's forward-secure group signature schemes, Zhang, ll/u, and Wang proposed a new group signature scheme with forward security at ICICS 2003. Unfortunately, their scheme is insecure. In this paper, we improve Zhang's group signature scheme. Based on this improvement version, we propose a new practical forward secure group signature schemes. The prominent features, our scheme are: I) None of the I{ong attaclrs are applicable on our schene 2) The secarity of membership certificate is based on the Multiple Discrete Logarithm Problem .3) The total computation cost of signature and verification require only 9 modular exponentiations. Hence ,s very practical.

Cryptography, Group signature, Forward Security, Multiple Discrete Problem