Paper 2019/1128

SoK: Communication Across Distributed Ledgers

Alexei Zamyatin and Mustafa Al-Bassam and Dionysis Zindros and Eleftherios Kokoris-Kogias and Pedro Moreno-Sanchez and Aggelos Kiayias and William J. Knottenbelt

Abstract

Communication across distributed systems, where each system runs its own consensus, is a problem previously studied only within a single trust domain (e.g., a datacenter). With the appearance of distributed ledgers or blockchains, numerous protocols requiring robustness against adversarial behavior have emerged. Cross-chain communication thereby plays a fundamental role in cryptocurrency exchanges, sharding, as well as the bootstrapping and migration of distributed ledgers. Unfortunately, existing proposals are designed ad-hoc for specific use-cases, making it hard to gain confidence on their correctness and to use them as building blocks for new systems. In this paper, we provide the first systematic exposition of protocols for cross-chain communication. Through formalization of the underlying research question, which reduces to the fair exchange problem, we identify threat and network model assumptions, necessary for designing correct cross-chain communication protocols. We overview main applications, derive a classification and provide a comparative analysis of existing approaches. Further, we survey and classify techniques for verifying state cross-chain and mechanisms for constructing locks.