zkBridge: Trustless Cross-chain Bridges Made Practical

Blockchains have seen growing traction with cryptocurrencies reaching a market cap of over 1 trillion dollars, major institution investors taking interests, and global impacts on governments, businesses, and individuals. Also growing significantly is the heterogeneity of the ecosystem where a variety of blockchains co-exist. Cross-chain bridge is a necessary building block in this multi-chain ecosystem. Existing solutions, however, either suffer from performance issues or rely on honesty assumptions of committees that significantly lower the security. Recurring attacks against bridges have cost users more than 1.5 billion USD. In this paper, we introduce zkBridge, an efficient cross-chain bridge that guarantees strong security without extra trust assumptions. With succinct proofs, zkBridge not only guarantees correctness, but also significantly reduces on-chain verification cost. We propose novel succinct proof protocols that are orders-of-magnitude faster than existing solutions for workload in zkBridge. With a modular design, zkBridge enables a few useful capabilities, including message passing, token transferring, and other computational logic operating on state changes from different chains. We fully implemented zkBridge between Cosmos and Ethereum and evaluated the end-to-end performance. The experiment shows that zkBridge achieves practical performance: it can generate a block header proof within 2 minutes, while verifying proofs on-chain costs less than 220K gas (the same as Groth16). Relaying a transaction from Cosmos to Ethereum costs 210K gas.