OptiSwap: Fast Optimistic Fair Exchange

Selling digital commodities securely over the Internet is a challenging task when Seller and Buyer do not trust each other. With the advent of cryptocurrencies, one prominent solution for digital exchange is to rely on a smart contract as a trusted arbiter that fairly resolves disputes when Seller and Buyer disagree. Such protocols have an optimistic mode, where the digital exchange between the parties can be completed with only minimal interaction with the smart contract. In this work we present OptiSwap, a new smart contract based fair exchange protocol that significantly improves the optimistic case of smart contract based fair exchange protocols. In particular, OptiSwap has almost no overhead in communication complexity, and improves on the computational overheads of the parties compared to prior solutions. An additional feature of OptiSwap is a protection mechanism against so-called grieving attacks, where an adversary attempts to violate the financial fairness of the protocol by forcing the honest party to pay fees. We analyze OptiSwap's security in the UC model and provide benchmark results over Ethereum.