On HTLC-Based Protocols for Multi-Party Cross-Chain Swaps

TL;DR

This paper investigates HTLC-based protocols for multi-party cross-chain asset swaps, providing a complete characterization of the swap digraphs where such protocols are feasible, addressing a key open problem in blockchain interoperability.

Contribution

It offers a comprehensive analysis and characterization of swap digraphs that support HTLC-based protocols for multi-party cross-chain asset swaps.

Findings

Characterized swap digraphs supporting HTLC protocols

Identified limitations of existing protocols for arbitrary digraphs

Provided insights into efficient cross-chain swap designs

Abstract

In his 2018 paper, Herlihy introduced an atomic protocol for multi-party asset swaps across different blockchains. His model represents an asset swap by a directed graph whose nodes are the participating parties and edges represent asset transfers, and rational behavior of the participants is captured by a preference relation between a protocol's outcomes. Asset transfers between parties are achieved using smart contracts. These smart contracts are quite involved and they require storage and processing of a large number of paths in the swap digraph, limiting practical significance of his protocol. His paper also describes a different protocol that uses only standard hash time-lock contracts (HTLC's), but this simpler protocol applies only to some special types of digraphs. He left open the question whether there is a simple and efficient protocol for cross-chain asset swaps in arbitrary digraphs. Motivated by this open problem, we conducted a comprehensive study of \emph{HTLC-based protocols}, in which all asset transfers are implemented with HTLCs. Our main contribution is a full characterization of swap digraphs that have such protocols.