Cross-blockchain smart contract interoperability

Abstract

Blockchain technologies' rapid evolution has resulted in the fragmentation of decentralized applications across multiple networks. Since blockchain protocols do not permit the autonomous retrieval of external data, deployed smart contracts are restricted to their host blockchain. The resulting lock-in effect contradicts the initial proposition of smart contracts to enable independent decentralized applications. Therefore, cross-blockchain interoperability methods are needed to enable smart contracts to interact across blockchain network boundaries. Current approaches to blockchain interoperability suffer from centralization, high transaction costs, or require economic rationality assumptions. The execution of present cross-blockchain smart contract messaging protocols incurs high delays and costs, as multiple transactions must be executed sequentially. In addition, these protocols do not remedy the tight binding between smart contracts and their host blockchain. This thesis presents novel cross-blockchain smart contract interoperability concepts that are trustless, efficient, and do not require financial preconditions. The first contribution depicts a chain relay utilizing verifiable off-chain computations to off-load the validation of Proof-of-Work (PoW) block headers. As a result, on-chain validation costs are reduced by orders of magnitude without compromising security. The concept is evaluated based on a prototype relaying Bitcoin and Ethereum block headers to blockchains that support the Ethereum Virtual Machine (EVM). In recent years, Proof-of-Stake (PoS) blockchains have become increasingly popular. The second contribution of this thesis introduces an efficient chain relay scheme addressing PoS blockchains that guarantee finality. The consensus protocol's finality guarantee enables the chain relay to operate on a subset of finalized block headers. As a result, transaction costs are minimized without compromising the relay's security. The introduced cross-chain state access methods depict the foundation for smart contract interoperability across blockchain networks. As a third contribution, we present the concept of smart contract forks to mitigate the close dependency of smart contracts on their host blockchain. The business logic and state of smart contracts are migrated to other blockchains that support the same execution environment. As a migration result, smart contract users benefit from the properties the target blockchain provides, and lock-in effects are alleviated. The fourth contribution of this thesis is a concept for smart contract synchronization, which enables instant read-only cross-blockchain function calls. Replica contracts are created on target blockchains and updated according to the initially deployed instance. We evaluate smart contract forks and synchronizations based on a prototype that supports EVM-compliant blockchains.