Making Speculative BFT Resilient with Trusted Monotonic Counters

TL;DR

SACZyzzyva enhances speculative Byzantine Fault Tolerance protocols by reducing replica requirements and only needing one active trusted counter, improving scalability and resilience to slow replicas.

Contribution

It introduces SACZyzzyva, a BFT protocol that tolerates slow replicas with only 3f+1 replicas and minimal trusted hardware, addressing scalability and robustness issues.

Findings

Low latency and high scalability demonstrated experimentally.

Proven optimal robustness of SACZyzzyva.

Trusted components do not increase fault tolerance unless in over two-thirds of replicas.

Abstract

Consensus mechanisms used by popular distributed ledgers are highly scalable but notoriously inefficient. Byzantine fault tolerance (BFT) protocols are efficient but far less scalable. Speculative BFT protocols such as Zyzzyva and Zyzzyva5 are efficient and scalable but require a trade-off: Zyzzyva requires only $3f + 1$ replicas to tolerate $f$ faults, but even a single slow replica will make Zyzzyva fall back to more expensive non-speculative operation. Zyzzyva5 does not require a non-speculative fallback, but requires $5f + 1$ replicas in order to tolerate $f$ faults. BFT variants using hardware-assisted trusted components can tolerate a greater proportion of faults, but require that every replica have this hardware. We present SACZyzzyva, addressing these concerns: resilience to slow replicas and requiring only $3f + 1$ replicas, with only one replica needing an active monotonic counter at any given time. We experimentally evaluate our protocols, demonstrating low latency and high scalability. We prove that SACZyzzyva is optimally robust and that trusted components cannot increase fault tolerance unless they are present in greater than two-thirds of replicas.