About blockchain interoperability

•Describes key challenges to blockchain interoperability.•Introduce a formal context of blockchain interoperability.•Explores the meaning of this definition. A blockchain is designed to be a self-sufficient decentralised ledger: a peer verifying the validity of past transactions only needs to download the blockchain (the ledger) and nothing else. However, it might be of interest to make two different blockchains interoperable, i.e., to allow one to transmit information from one blockchain to another blockchain. In this paper, we give a formalisation of this problem, and we prove that blockchain interoperability is impossible according to the classical definition of a blockchain. Under a weaker definition of blockchain, we demonstrate that two blockchains are interoperable is equivalent to creating a ‘2-in-1’ blockchain containing both ledgers, thus limiting the theoretical interest of making interoperable blockchains in the first place. We also observe that all practical existing interoperable blockchain frameworks work indeed by exchanging already created tokens between two blockchains and not by offering the possibility to transfer tokens from one blockchain to another one, which implies a modification of the balance of total created tokens on both blockchains. It confirms that having interoperability is only possible by creating a ‘2-in-1’ blockchain containing both ledgers.