Dynamic Byzantine Reliable Broadcast [Technical Report]

Reliable broadcast is a powerful primitive guaranteeing that, intuitively, all processes in a distributed system deliver the same set of messages. There is a twofold reason why this primitive is appealing: (i) we can implement it deterministically in a completely asynchronous environment, unlike stronger primitives like consensus and total-order broadcast, and yet (ii) it is powerful enough to implement numerous useful applications like payment systems. The problem we tackle in this paper is that of dynamic reliable broadcast, i.e., enabling processes to join or leave the system. This is desirable property for long-lived applications supposed to be highly available, yet has been precluded in previous asynchronous reliable broadcast protocols. We introduce the first specification of a dynamic Byzantine reliable broadcast (DBRB) primitive that is amenable to an asynchronous implementation. Indeed, we present an algorithm that implements this specification in an asynchronous environment. Our algorithm ensures that if any correct process in the system broadcasts (resp. delivers) a message, then every correct process in the system delivers that message, or leaves the system. We assume that, at any point in time, 2/3 of the processes in the system are correct, which is tight. We also prove that even if only one process in the system can fail---and it can fail by merely crashing---then it is impossible to implement a stronger primitive, ensuring that if any correct process in the system broadcasts (resp. delivers) a message, then every correct process in the system delivers that message, including those that eventually leave.