Paper 2025/176

HyperLoop: Rationally secure efficient cross-chain bridge

Abstract

Cross-chain bridges, realizing the transfer of information and assets between blockchains, form the core of blockchain interoperability solutions. Most existing bridge networks are modeled in an honest-malicious setting, where the bridge nodes are either honest or malicious. Rationality allows the nodes to deviate from the protocol arbitrarily for an economic incentive. In this work, we present HyperLoop, an efficient cross-chain multi-signature bridge and prove that it is safe and live game-theoretically, under the more realistic rational-malicious model. As rational bridge nodes are allowed to deviate from the protocol and even collude, a monitor mechanism is necessitated, which we realize by introducing whistle-blower nodes. These whistle-blowers constantly check the operations of the bridge and raise complaints to a complaint resolution network in case of discrepancies. To enforce punishments, it is necessary for the nodes to stake an amount before participating as bridge nodes. Consequently, a cap on the volume of funds transferred over the bridge is established. We describe a sliding window mechanism and establish a relation between the stake and the sliding window limit necessary for the safety of the bridge. Our design yields an economic, computation, and communication-efficient bridge. We realize and deploy our bridge prototype bridging Ethereum and Polygon chains over testnets. For a 19-node bridge network, each bridge node takes an average of only 3 msec to detect and sign a source chain request, showing the highly efficiency and low-latency of the bridge.