Cryptology ePrint Archive: Report 2021/897

A Rational Protocol Treatment of 51% Attacks

Christian Badertscher and Yun Lu and Vassilis Zikas

Abstract: Game-theoretic analyses of cryptocurrencies and---more generally---blockchain-based decentralized ledgers offer insight on their economic robustness and behavior when even their underpinning cryptographic assumptions fail. In this work we utilize the recently proposed blockchain adaptation of the rational protocol design (RPD) framework [EUROCRYPT '18] to analyze 51% double-spending attacks against Nakamoto-style proof-of-work based cryptocurrencies. We first observe a property of the originally proposed utility class that yields an unnatural conclusion against such attacks, and show how to devise a utility that avoids this pitfall and makes predictions that match the observable behavior---i.e., that renders attacking a dominant strategy in settings where an attack was indeed observed in reality. We then propose a generic remedy to the underlying protocol parameters that provably deter adversaries controlling a majority of the system's resources from attacks on blockchain consistency, including the 51% double-spending attack. This can be used as guidance to patch systems that have suffered such attacks, e.g., Ethereum Classic and Bitcoin Cash, and serves as a demonstration of the power of game-theoretic analyses.

Category / Keywords: applications / blockchains and distributed ledgers, rational cryptography

Original Publication (with major differences): IACR-CRYPTO-2021

Date: received 30 Jun 2021, last revised 1 Jul 2021

Contact author: yunlu mail at gmail com, vassilis zikas at gmail com, christian badertscher at iohk io

Available format(s): PDF | BibTeX Citation

Version: 20210701:082753 (All versions of this report)

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