zk-SNARK-based ChainRelay from Bitcoin to Ethereum
The Relay cost is constant for any batch size
Difficulty target is encoded within the block header in Proof-of-Work blockchains. The offchain-program extracts the target value and utilizes it during the validation process.
To prevent attackers from submitting block headers that encode invalid target values which cannot be detected within the off-chain computation, the relay contract verifies its correctness before accepting a proof