Paper 2023/1211

Optimal Flexible Consensus and its Application to Ethereum

Joachim Neu, Stanford University
Srivatsan Sridhar, Stanford University
Lei Yang, Massachusetts Institute of Technology
David Tse, Stanford University

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.

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Cryptographic protocols
Publication info
Contact author(s)
jneu @ stanford edu
svatsan @ stanford edu
leiy @ csail mit edu
dntse @ stanford edu
2023-08-10: approved
2023-08-10: received
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      author = {Joachim Neu and Srivatsan Sridhar and Lei Yang and David Tse},
      title = {Optimal Flexible Consensus and its Application to Ethereum},
      howpublished = {Cryptology ePrint Archive, Paper 2023/1211},
      year = {2023},
      note = {\url{}},
      url = {}
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