Sweep-UC: Swapping Coins Privately

Fair exchange (also referred to as atomic swap) is a fundamental operation in any cryptocurrency that allows users to atomically exchange coins. While a large body of work has been devoted to this problem, most solutions lack on-chain privacy. Thus, coins retain a public transaction history which is known to degrade the fungibility of a currency. This has led to a flourishing line of related research on fair exchange with privacy guarantees. Existing protocols either rely on heavy scripting (which also degrades fungibility and leads to high transaction fees), do not support atomic swaps across a wide range of currencies, or come with incomplete security proofs. To overcome these limitations, we introduce Sweep-UC (Read as Sweep Ur Coins.), the first fair exchange protocol that simultaneously is efficient, minimizes scripting, and is compatible with a wide range of currencies (more than the state of the art). We build Sweep-UC from modular sub-protocols and give a rigorous security analysis in the UC framework. Many of our tools and security definitions can be used in standalone fashion and may serve as useful components for future constructions of fair exchange.