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The Power of Random Symmetry-Breaking in Nakamoto Consensus

Authors Lili Su, Quanquan C. Liu, Neha Narula

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Author Details

Lili Su
  • Northeastern University, Boston, MA, USA
Quanquan C. Liu
  • Massachusetts Institute of Technology, Cambridge, MA, USA
Neha Narula
  • Massachusetts Institute of Technology, Cambridge, MA, USA

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Lili Su, Quanquan C. Liu, and Neha Narula. The Power of Random Symmetry-Breaking in Nakamoto Consensus. In 35th International Symposium on Distributed Computing (DISC 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 209, pp. 39:1-39:19, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2021) https://doi.org/10.4230/LIPIcs.DISC.2021.39

Abstract

Nakamoto consensus underlies the security of many of the world’s largest cryptocurrencies, such as Bitcoin and Ethereum. Common lore is that Nakamoto consensus only achieves consistency and liveness under a regime where the difficulty of its underlying mining puzzle is very high, negatively impacting overall throughput and latency. In this work, we study Nakamoto consensus under a wide range of puzzle difficulties, including very easy puzzles. We first analyze an adversary-free setting and show that, surprisingly, the common prefix of the blockchain grows quickly even with easy puzzles. In a setting with adversaries, we provide a small backwards-compatible change to Nakamoto consensus to achieve consistency and liveness with easy puzzles. Our insight relies on a careful choice of symmetry-breaking strategy, which was significantly underestimated in prior work. We introduce a new method - coalescing random walks - to analyzing the correctness of Nakamoto consensus under the uniformly-at-random symmetry-breaking strategy. This method is more powerful than existing analysis methods that focus on bounding the number of convergence opportunities.

Subject Classification

ACM Subject Classification
  • Theory of computation → Distributed computing models
Keywords
  • Nakamoto consensus
  • Byzantine consensus
  • blockchain
  • symmetry-breaking
  • coalescing random walks

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