Security weakness of Tseng’s fault-tolerant conference-key agreement protocol

A fault-tolerant conference-key agreement protocol establishes a shared key among participants of a conference even when some malicious participants disrupt key agreement processes. Recently, Tseng proposed a new fault-tolerant conference-key agreement protocol that only requires a constant message size and a small number of rounds. In this paper, we show that the Tseng’s protocol cannot provide forward and backward confidentiality during a conference session for the proposed attack method. We also show that a simple countermeasure—re-randomizing short-term keys of some participants—to avoid the proposed attack can be broken by extending the proposed attack method.