Optimistic Asynchronous Byzantine Agreement

Agreement problems are a fundamental building block for constructing reliable distributed systems. While robust and eÆcient protocols exist in the crash-failure setting, protocols resilient against a Byzantine adversary tend to have problems with either eÆciency or security. This paper proposes an optimistic approach to the Byzantine agreement problem, combining the eÆciency of fully synchronous protocols with the robustness of asynchronous ones. If the system satis es certain timing assumptions, and all parties are honest (the optimistic case), the protocol reaches optimal performance. Otherwise, if the timing assumptions are violated or parties become corrupted the protocol invokes an asynchronous \fallback" protocol. Neither liveness nor security are violated in the transition, and the performance overhead is minimal. Furthermore, no expensive cryptographic operations are needed in the optimistic case, unlike in most practical Byzantine agreement protocols. Thus, the optimistic approach gives a maximum of security while being more eÆcient than most (less secure) protocols.