Optimal Flexible Consensus and its Application to Ethereum

TL;DR

This paper introduces a modular flexible consensus mechanism that allows individual clients to choose their safety-liveness tradeoff, optimizing for safety in high-value applications like Ethereum without system-wide changes.

Contribution

It presents the first construction enabling optimal safety-liveness tradeoff for each client, adaptable to existing protocols like Ethereum, through an add-on with minimal modifications.

Findings

Achieves optimal safety-liveness tradeoff for all clients simultaneously.

Modular construction that can be added on top of existing consensus protocols.

Demonstrated implementation within Ethereum's consensus API.

Abstract

Classic BFT consensus protocols guarantee safety and liveness for all clients if fewer than one-third of replicas are faulty. However, in applications such as high-value payments, some clients may want to prioritize safety over liveness. Flexible consensus allows each client to opt for a higher safety resilience, albeit at the expense of reduced liveness resilience. We present the first construction that allows optimal safety--liveness tradeoff for every client simultaneously. This construction is modular and is realized as an add-on applied on top of an existing consensus protocol. The add-on consists of an additional round of voting and permanent locking done by the replicas, to sidestep a sub-optimal quorum-intersection-based constraint present in previous solutions. We adapt our construction to the existing Ethereum protocol to derive optimal flexible confirmation rules that clients can adopt unilaterally without requiring system-wide changes. This is possible because existing Ethereum protocol features can double as the extra voting and locking. We demonstrate an implementation using Ethereum's consensus API.