Paper 2018/740

FairSwap: How to fairly exchange digital goods

Stefan Dziembowski, Lisa Eckey, and Sebastian Faust


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.

Available format(s)
Cryptographic protocols
Publication info
Published elsewhere. MINOR revision.ACM CCS 2018
Fair ExchangeSmart ContractsProvable SecurityDistributed File Sharing
Contact author(s)
lisa eckey @ crisp-da de
2018-09-05: last of 2 revisions
2018-08-15: received
See all versions
Short URL
Creative Commons Attribution


      author = {Stefan Dziembowski and Lisa Eckey and Sebastian Faust},
      title = {FairSwap: How to fairly exchange digital goods},
      howpublished = {Cryptology ePrint Archive, Paper 2018/740},
      year = {2018},
      doi = {10.1145/3243734.3243857},
      note = {\url{}},
      url = {}
Note: In order to protect the privacy of readers, does not use cookies or embedded third party content.