  Cross-Blockchain communication has gained traction due to the increasing
fragmentation of blockchain networks and scalability solutions such as
side-chaining and sharding. With SmartSync, we propose a novel concept for
cross-blockchain smart contract interactions that creates client contracts on
arbitrary blockchain networks supporting the same execution environment. Client
contracts mirror the logic and state of the original instance and enable
seamless on-chain function executions providing recent states. Synchronized
contracts supply instant read-only function calls to other applications hosted
on the target blockchain. Hereby, current limitations in cross-chain
communication are alleviated and new forms of contract interactions are
enabled. State updates are transmitted in a verifiable manner using Merkle
proofs and do not require trusted intermediaries. To permit lightweight
synchronizations, we introduce transition confirmations that facilitate the
application of verifiable state transitions without re-executing transactions
of the source blockchain. We prove the concept's soundness by providing a
prototypical implementation that enables smart contract forks, state
synchronizations, and on-chain validation on EVM-compatible blockchains. Our
evaluation demonstrates SmartSync's applicability for presented use cases
providing access to recent states to third-party contracts on the target
blockchain. Execution costs scale sub-linearly with the number of value updates
and depend on the depth and index of corresponding Merkle proofs.
