  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 trust 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 external
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 broad spectrum of use cases and capabilities, including message
passing, token transferring, and other computational logic operating on state
changes from different chains. To demonstrate the practicality of zkBridge, we
implemented a prototype bridge from Cosmos to Ethereum, a particularly
challenging direction that involves large proof circuits that existing systems
cannot efficiently handle. Our evaluation shows that zkBridge achieves
practical performance: proof generation takes less than 20 seconds, while
verifying proofs on-chain costs less than 230K gas. For completeness, we also
implemented and evaluated the direction from Ethereum to other EVM-compatible
chains (such as BSC) which involves smaller circuits and incurs much less
overhead.
