### Rapidash: Foundations of Side-Contract-Resilient Fair Exchange

##### Abstract

Fair exchange is a fundamental primitive enabled by blockchains, and is widely adopted in applications such as atomic swaps, payment channels, and DeFi. Most existing designs of blockchain-based fair exchange protocols consider only the participating users as strategic players, and assume the miners are honest and passive. However, recent works revealed that the fairness of commonly deployed fair exchange protocols can be broken entirely in the presence of user-miner collusion. In particular, a user can bribe the miners to help it cheat — a phenomenon also referred to as Miner Extractable Value (MEV). In this work, we provide the first formal treatment of side-contract-resilient fair exchange where users and miners may enter into arbitrary contracts on the side. We propose a new fair exchange protocol called Rapidash, and prove that the protocol is incentive compatible in the presence of user-miner collusion. In particular, we show that Rapidash satisfies a coalition-resistant Nash equilibrium absent external incentives. Further, even when there exist arbitrary but bounded external incentives, Rapidash still protects honest players and ensures that they cannot be harmed. Last but not least, our game-theoretic formulations also lay the theoretical groundwork for studying side-contract-resilient fair exchange protocols. Finally, to showcase the instantiability of Rapidash with a wide range of blockchain systems, we present instantiations of Rapidash that are compatible with Bitcoin and Ethereum while incurring only a minimal overhead in terms of costs for the users.

Note: This paper subsumes Ponyta (https://eprint.iacr.org/2022/582) and previous versions of this paper.

Available format(s)
Category
Cryptographic protocols
Publication info
Preprint.
Keywords
blockchain fair exchange side-contract-resilient
Contact author(s)
haochung @ andrew cmu edu
elisawem @ andrew cmu edu
runting @ gmail com
t srikrishnan @ gmail com
History
2022-10-22: last of 2 revisions
See all versions
Short URL
https://ia.cr/2022/1063

CC BY

BibTeX

@misc{cryptoeprint:2022/1063,
author = {Hao Chung and Elisaweta Masserova and Elaine Shi and Sri AravindaKrishnan Thyagarajan},
title = {Rapidash: Foundations of Side-Contract-Resilient Fair Exchange},
howpublished = {Cryptology ePrint Archive, Paper 2022/1063},
year = {2022},
note = {\url{https://eprint.iacr.org/2022/1063}},
url = {https://eprint.iacr.org/2022/1063}
}

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