Ecosystems of multiple blockchains are now a reality. Multi-chain applications and protocols are perceived as necessary to enable scalability, privacy, and composability. Despite being a promising emerging research area, we recently have witnessed many attacks that have caused billions of dollars in losses. Attacks against bridges that connect chains are at the top of such attacks in terms of monetary cost, and no apparent solution seems to emerge from the ongoing chaos.

In this paper, we present our contribution to minimizing bridge attacks. In particular, we explore the concepts of cross-chain transaction, cross-chain logic, and the cross-chain state as the enablers of the cross-chain model. We propose Hephaestus, the first cross-chain model generator that captures the operational complexity of cross-chain applications. Hephaestus can generate cross-chain models from local transactions on different ledgers realizing arbitrary use cases and allowing operators to monitor their cross-chain applications. Monitoring helps identify outliers and malicious behavior, which can help programmatically to stop bridge hacks and other attacks. We conduct a detailed evaluation of our system, where we implement a cross-chain bridge use case. Our experimental results show that Hephaestus can process 600 cross-chain transactions in less than 5.5 seconds in an environment with two blockchains and requires sublinear storage.