SoK: Communication Across Distributed Ledgers

Communication across distributed systems, each running its own consensus, is a problem previously studied under the assumption of trust across systems. With the appearance of distributed ledgers or blockchains, numerous protocols have emerged, which attempt to achieve trustless communication between distrusting ledgers and participants. Cross-chain communication thereby plays a fundamental role in cryptocurrency exchanges, sharding, bootstrapping and extension 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. We provide the first systematic exposition of protocols for cross-chain communication. First, we formalize the underlying research problem and show cross-chain communication is impossible without a trusted third party, contrary to common beliefs in the blockchain community. We then develop a framework for evaluating existing and designing new crosschain protocols, based on use case, trust model and security assumptions of interlinked blockchains. Finally, we identify security and privacy challenges faced by protocols in the cross-chain setting. This Systematization of Knowledge (SoK) offers a comprehensive guide for designing protocols bridging the numerous distributed ledgers available today and aims to facilitate clearer communication between academia and industry in