Efficient Synchronous Byzantine Consensus

TL;DR

This paper introduces new synchronous Byzantine consensus protocols that improve fault tolerance and reduce decision rounds by leveraging relaxed synchrony assumptions, benefiting Byzantine fault-tolerant systems and cryptographic applications.

Contribution

It presents protocols achieving Byzantine fault tolerance with n=2f+1 in a synchronous setting, reducing consensus rounds significantly compared to previous solutions.

Findings

Consensus commits every 3 rounds in the common case.

Synchronous Byzantine agreement achieved in expected 8 rounds.

Protocol tolerates up to f Byzantine faults with 2f+1 replicas.

Abstract

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 threshold to $n=2f+1$ by utilizing a relaxed synchrony assumption. We present a synchronous state machine replication protocol that commits a decision every 3 rounds in the common case. The key challenge is to ensure 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 expected 8 rounds. The best previous solution (Katz and Koo, 2006) requires expected 24 rounds. Our protocols may be applied to build Byzantine fault tolerant systems or improve cryptographic protocols such as cryptocurrencies when synchrony can be assumed.