Cryptology ePrint Archive: Report 2018/740

FairSwap: How to fairly exchange digital goods

Stefan Dziembowski and Lisa Eckey and Sebastian Faust

Abstract: We introduce FairSwap -- an efficient protocol for fair exchange of digital goods using smart contracts. A fair exchange protocol allows a sender S to sell a digital commodity x for a fixed price p to a receiver R. The protocol is said to be secure if R only pays if he receives the correct x. Our solution guarantees fairness by relying on smart contracts executed over decentralized cryptocurrencies, where the contract takes the role of an external judge that completes the exchange in case of disagreement. While in the past there have been several proposals for building fair exchange protocols over cryptocurrencies, our solution has two distinctive features that makes it particular attractive when users deal with large commodities. These advantages are: (1) minimizing the cost for running the smart contract on the blockchain, and (2) avoiding expensive cryptographic tools such as zero-knowledge proofs. In addition to our new protocols, we provide formal security definitions for smart contract based fair exchange, and prove security of our construction. Finally, we illustrate several applications of our basic protocol and evaluate practicality of our approach via a prototype implementation for fairly selling large files over the cryptocurrency Ethereum.

Category / Keywords: cryptographic protocols / Fair Exchange, Smart Contracts, Provable Security, Distributed File Sharing

Original Publication (with minor differences): ACM CCS 2018

Date: received 13 Aug 2018, last revised 5 Sep 2018

Contact author: lisa eckey at crisp-da de

Available format(s): PDF | BibTeX Citation

Version: 20180905:110612 (All versions of this report)

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