TrustBlink: A zkSNARK-Powered On-Demand Relay for PoW Cross-Chain Verification With Low Costs

Cross-chain interoperability is critical to unlocking the full potential of blockchain ecosystems, as it enables distinct platforms to interact while preserving their respective advantages and characteristics. However, existing approaches often struggle to balance decentralization with minimal trust assumptions, frequently relying on federated validators or economic incentives. In contrast, trustless relays based on light client proofs incur substantial on-chain overhead due to continuous synchronization, verification, and storage requirements. We propose TrustBlink, an on-demand stateless relay based on zkSNARKs that compresses SPV-style proofs into succinct zero-knowledge arguments, enabling verification through a single on-chain operation. This approach significantly reduces both on-chain and off-chain verification costs and eliminates the need for continuous synchronization. TrustBlink further incorporates reusable randomness to mitigate upfront mining attacks and supports dynamic Proof-of-Work (PoW) difficulty adjustment without necessitating contract redeployment for each verification instance. We implemented a prototype of TrustBlink using the ZoKrates framework to facilitate interoperability between Bitcoin and Ethereum. Our evaluation demonstrates that TrustBlink maintains a constant gas cost of approximately 521,000 for block verification, irrespective of confirmation depth—representing a 55% reduction in cost compared to traditional relay schemes such as BTC Relay.