Practical Synchronous Byzantine Consensus

We present new protocols for Byzantine state machine replication and Byzantine agreement in the synchronous and authenticated setting. The celebrated PBFT state machine replication protocol tolerates f Byzantine faults in an asynchronous setting using 3f + 1 replicas, and has since been studied or deployed by numerous works. In this work, we improve the Byzantine fault tolerance to n = 2f + 1 by utilizing the synchrony assumption. The key challenge is to ensure a quorum intersection at one honest replica. Our solution is to rely on the synchrony assumption to form a post-commit quorum of size 2f +1, which intersects at f + 1 replicas with any pre-commit quorums of size f + 1. Our protocol also solves synchronous authenticated Byzantine agreement in fewer rounds than the best existing solution (Katz and Koo, 2006). A challenge in this direction is to handle non-simultaneous termination, which we solve by introducing a notion of virtual participation after termination. Our protocols may be applied to build practical synchronous Byzantine fault tolerant systems and improve cryptographic protocols such as secure multiparty computation and cryptocurrencies when synchrony can be assumed.