Horizon - a trustless harmony to ethereum bridge

• A bridge with cross-chain light clients, relayers, and prover full nodes, all trustless, no additional trust assumptions beyond the two blockchains that the bridge is connected to.
• A gas-efficient Harmony light client on Ethereum (could be generalized to other chains) that only requires checkpoint blocks (1 block every x blocks, where 1 ≤ x ≤ 16384, 16384 is the #blocks per epoch) to verify any number of Harmony transaction proofs by the clients.
• A constant-size Harmony light client proof that any user needs to send cross-chain (e.g., Ethereum) to claim their Harmony transaction.

• Bridge smart contract on Harmony
• Ethereum Light Client (ELC) smart contract on Harmony
• Ethereum Verifier (EVerifier) smart contract on Harmony
• Ethereum Prover (EProver) is an Ethereum full node or a client that has access to a full node
• Ethereum Relayer relays every Ethereum header information to ELC

• Bridge smart contract on Ethereum
• Harmony Light Client (HLC) smart contract on Ethereum
• Harmony Verifier (HVerifier) smart contract on Ethereum
• Harmony Prover (HProver) is a Harmony full node or a client that has access to a full node
• Harmony Relayer relays every checkpoint block header information to HLC

1. User locks ERC20 on Ethereum by transferring to bridge smart contract and obtains the hash of this transaction from blockchain
2. User sends the hash to EProver and receives proof-of-lock
3. User sends the proof-of-lock to bridge smart contract on Harmony
4. Bridge smart contract on Harmony invokes ELC and EVerifier to verify the proof-of-lock and mints HRC20 (equivalent amount)

1. User burns HRC20 on Harmony using Bridge smart contract and obtains the hash of this transaction from blockchain
2. User sends the hash to HProver and receives proof-of-burn
3. User sends the proof-of-burn to bridge smart contract on Ethereum
4. Bridge smart contract on Ethereum invokes HLC and HVerifier to verify the proof-of-burn and unlocks ERC20 (equivalent amount)