This paper presents GREP, a highly scalable and efficient group rekeying protocol with the following merits. First, it rekeys the group with only two messages, introducing an overhead which is small, constant, and independent of the group size. Second, GREP considers collusion as a first-class attack. Third, GREP efficiently recovers the group from a collusion attack without recourse to a total member reinitialization. The recovery cost smoothly grows with the group size, and gradually increases with the attack severity. GREP achieves these results by organizing nodes into logical subgroups and exploiting the history of node joining events. This allows GREP to establish a total ordering among subgroups and among nodes in each subgroup, so making collusion recovery highly scalable and efficient. We evaluate performance from several standpoints, and show that GREP is deployable in large-scale networks of customary, even resource constrained, platforms.
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