Horizon

Horizon - a trustless harmony to ethereum bridge

Key points

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

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

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