Analysis of a secure conference scheme for mobile communication

Dynamic participation is a feature of the secure conference schemes that allows new conferees to join and the old conferees to leave. The conferees who have left should not be able to decrypt the secure conference communication anymore. A secure conference scheme with dynamic participation was proposed in M.S. Hwang and W.P. Yang (1995) and later it was modified with the self-encryption mechanism in K.F. Hwang and C.C. Chang (2003) for a better performance. In this paper we analyze both the original scheme and the modified version. We show that both of them are subject to the active and passive attacks presented in this paper. Our active attack works in the way that a colluding group of attackers can still obtain the conference key even after they all leave the conference. The passive attack does not need any attacker to ever participate the conference. The conference key can be compromised with a large probability as long as the number of conferees is large.