A Secure and Efficient Fault-Tolerant Group Key Agreement Protocol

A fault-tolerant group key agreement (GKA) protocol establishes a shared key among participants of a group even when some malicious participants disrupt key agreement processes. In 2010, Zhao et al. proposed a new efficient fault-tolerant GKA protocol, named EGKA, with lower computational cost and average communication cost than all previously fault-tolerant GKA protocols, and claimed the EGKA is resistant to passive and active attacks. Unfortunately, we show that the EGKA protocol seems not fault-tolerant in several non-trivial cases as it is vulnerable to the different key attack and the message tampering attack, in which an honest participant can be easily excluded from the set of participants. To remedy these security flaws, we propose an improved EGKA protocol, which covers all the identified weaknesses of Zhao et al.'s scheme and is still more efficient than the existing fault-tolerant GKA protocols.