zapper.network

Cross-chain bridge that enables trust-minimised communication between the Bitcoin network and EVM-compatible blockchains (like Ethereum, Polygon, etc.). The architecture leverages Bitcoin script primitives (P2WSH + HTLCs or threshold signatures) on the Bitcoin side, while using smart contracts on the EVM side to handle verification and locking logic. It ensures secure and verifiable operations without relying on centralised custodians.

On both Bitcoin and EVM sides, HTLCs ensure atomicity—either both sides of the swap succeed, or neither does. This prevents fund loss and removes the need for trust.

Custom scripts lock BTC using a hashlock + timelock. The EVM smart contracts mirror this behavior to coordinate swap execution or refunds.

On the EVM side, the bridge integrates with the 1inch aggregation API to fetch real-time DEX swap rates. This enables dynamic, optimized pricing and lets users instantly swap their BTC-equivalent (e.g., wBTC) to any EVM token (like ETH, USDC, etc.).

Relayer Service(watch-tower): The relayer is a lightweight off-chain service that monitors both the Bitcoin and EVM chains. It performs two key functions:

Secret Relay: After a user claims funds on one chain by revealing a secret (preimage), the relayer detects it and propagates the same secret to the other chain to unlock the corresponding HTLC, ensuring atomicity.

Resolver Transactions: It submits resolver transactions on the EVM side to trigger smart contract functions (e.g., swap execution, release, or refund), acting as a trusted-but-verifiable facilitator.