SoK: Communication across distributed ledgers LNCS Zamyatin, Alexei Al-Bassam, Mustafa Zindros, Dionysis Kokoris Kogias, Eleftherios Moreno-Sanchez, Pedro Kiayias, Aggelos Knottenbelt, William J. Since the inception of Bitcoin, a plethora of distributed ledgers differing in design and purpose has been created. While by design, blockchains provide no means to securely communicate with external systems, numerous attempts towards trustless cross-chain communication have been proposed over the years. Today, cross-chain communication (CCC) plays a fundamental role in cryptocurrency exchanges, scalability efforts via sharding, extension of existing systems through sidechains, and bootstrapping of new blockchains. Unfortunately, existing proposals are designed ad-hoc for specific use-cases, making it hard to gain confidence in their correctness and composability. We provide the first systematic exposition of cross-chain communication protocols. We formalize the underlying research problem and show that CCC is impossible without a trusted third party, contrary to common beliefs in the blockchain community. With this result in mind, we develop a framework to design new and evaluate existing CCC protocols, focusing on the inherent trust assumptions thereof, and derive a classification covering the field of cross-chain communication to date. We conclude by discussing open challenges for CCC research and the implications of interoperability on the security and privacy of blockchains. Springer Nature 2021 info:eu-repo/semantics/conferenceObject doc-type:conferenceObject text http://purl.org/coar/resource_type/c_5794 https://research-explorer.ista.ac.at/record/10325 Zamyatin A, Al-Bassam M, Zindros D, et al. SoK: Communication across distributed ledgers. In: <i>25th International Conference on Financial Cryptography and Data Security</i>. Vol 12675. Springer Nature; 2021:3-36. doi:<a href="https://doi.org/10.1007/978-3-662-64331-0_1">10.1007/978-3-662-64331-0_1</a> eng info:eu-repo/semantics/altIdentifier/doi/10.1007/978-3-662-64331-0_1 info:eu-repo/semantics/altIdentifier/issn/0302-9743 info:eu-repo/semantics/altIdentifier/e-issn/1611-3349 info:eu-repo/semantics/altIdentifier/isbn/9-783-6626-4330-3 info:eu-repo/semantics/altIdentifier/e-isbn/978-3-662-64331-0 info:eu-repo/semantics/altIdentifier/wos/000712016200001 info:eu-repo/semantics/openAccess