Gear Bridges

Gear Bridge is an implementation of a trustless ZK-based cross-chain bridge facilitating the transfer of assets between Ethereum and Gear-based blockchains, such as the Vara network.

Security

Ethernal team have performed partial audit of the code, which covered the following scope for the commit d42251c:

• prover
• plonky2 circuits for crypto primitives
• ethereum smart-contracts

Warning

One of the conclusions of this audit was that malicious node which relayer is connected to can cause the proof generation process to be halt. So all deployed relayers MUST use their own dedicated gear node which is known to be non-malicious.

They've also performed another audit which covered the following scope for the commit 8b1018bd:

• gear programs
• ethereum smart-contracts

High-Level Design

The Gear <-> Eth transfer protocol allows relaying messages from Gear-based blockchains to Ethereum and back. These messages contain generic data defined by protocols built on top of the bridge. The protocol doesn't guarantee the order in which messages are relayed.

This repository contains the implementation of a token bridging protocol built on top of a more generic messaging protocol.

Components of Gear <-> Eth bridge

Detailed diagram

Protocol part

• Pallet-Gear-Eth-Bridge Built-in Actor: a Built-in Actor - the entry point into the generic bridging protocol. Receives messages from any actor on the Gear network and relays them to pallet-gear-eth-bridge.
• Pallet-Gear-Eth-Bridge: Receives messages from the pallet-gear-eth-bridge built-in actor and stores them in the binary Merkle trie. This Merkle trie gets slashed at the end of each ERA. Also stores and updates hashed GRANDPA authority set.
• Relayer(proxy)¹: Accepts proofs of Merkle trie root inclusion and, if they're valid, stores Merkle trie roots in memory. Deployed behind ERC-1967 Proxy.
• Verifier: A contract capable of verifying plonk proofs created by gnark. The submitted proofs are plonky2 proofs wrapped by gnark.
• MessageQueue(proxy)¹: Used to recover messages from Merkle tries. A user can request a message to be relayed further onto Ethereum by providing proof of inclusion of a message actually included in the Merkle trie, given that this Merkle root was already relayed by gear->eth protocol relayer (or another party). This is also the exit point of the generic Gear -> Eth bridging protocol.
• Checkpoint-Light-Client: Lazy ethereum light client that maintains sync committee validator list and is capable of verifying block headers using it.
• Ethereum-Event-Client: Program on Gear that's capable of verifying that some event was included into some block. To check validity of the block it requests data from checkpoint-light-client.
• Historical-Proxy: Program on Gear that maintains historical eth-events-* program addresses and redirects requests to a eth-events-* responsible of processing requested transaction. Serves as an exit point from core Ethereum -> Gear protocol.
• Gear -> Eth Protocol Relayer: Reads Gear state, generates ZK-proofs, and submits them to Ethereum. Capable of creating two types of ZK-proofs: proof of authority set changes and proof of inclusion of Merkle trie root into the storage of pallet-gear-eth-bridge. Proofs of authority set changes are intermediate and stored in on-chain proof storage while proofs of Merkle trie root inclusion are submitted to Ethereum.
• Eth -> Gear Protocol Relayer: Relayer that monitors sync committee changes and blocks signed by it on Ethereum and updates state of checkpoint-light-client using it.

Token transfer part

• VFT: A program capable of transferring, burning, and minting vft tokens. It repeats the implementation of the ERC20 standard on the Gear network, the standard implementation vft can be found here.
• VFT-Manager: Receives vft tokens from users, burns/locks them, and emits a message to the pallet-gear-eth-bridge built-in actor. This message contains information about which token is being bridged, how much of it, and the recipient of funds on the Ethereum network. Also manages Eth -> Gear token transfers by verifying ethereum events using historical-proxy program and minting/unlocking corresponding tokens. It can work with both Ethereum and Gear supplied tokens. When token supply is based on Ethereum it will perform burns and mints of corresponding VFT tokens. When token supply is based on Gear it will perform locks and unlocks of corresponding VFT tokens.
• ERC20Manager(proxy)¹: Mirroring behaviour of vft-manager but on Ethereum side and without sending request to a pallet-gear-eth-bridge.
• Bridging Payment: Program on Gear and smart-contract on Ethereum that have the same functions. When bridging request is sent to them, they collect fees and relay this request to the corresponding smart-contract/program responsible of executing this request. Then some token relayer can observe events signaling that request have been processed and fee is paid and process this request on the other chain, using its own funds as tx fees. bridging payment services are fully in control of relayer that've deployed them, so to perform bridging using them one should trust the owner.
• Gear -> Eth Token Relayer and Eth -> Gear Token Relayer: These relayers hook to the events produced by bridging payment services and perform cross-chain actions to guarantee message delivery. For example, gear -> eth token relayer collects bridging fees from user on Gear. When merkle root that contain this message will be relayed to Ethereum, this relayer will send transaction to the MessageQueue that will trigger transfer from ERC20Manager to the user.

Workflow of Gear -> Ethereum Token² Transfer

• The user submits his bridging request to frontend
• frontend submits approve call to the vft program. Approve should allow vft-manager to spend amount of tokens that user have requested to bridge.
• frontend submits user request to the bridging-payment.
• bridging-payment takes fee and submits a message to the vft-manager to initiate bridging.
• The vft-manager burns/locks vft tokens and emits a message to the pallet-gear-eth-bridge built-in actor.
• The pallet-gear-eth-bridge built-in actor relays the message to pallet-gear-eth-bridge.
• The pallet-gear-eth-bridge stores the message in a Merkle trie.
• Eventually, the gear->eth protocol relayer (or another party) relays the message to the Relayer contract, and it gets stored there.
• gear->eth token relayer sees that user message was relayed and submits a Merkle proof of inclusion to the MessageQueue contract.
• The MessageQueue contract reads the Merkle root from the Relayer contract, checks the Merkle proof, and relays the message to the ERC20Manager.
• The ERC20Manager releases funds to the user's account on Ethereum.

Workflow of Ethereum -> Gear Token² Transfer

• The user submits his bridging request to frontend
• frontend submits approve call to the corresponding ERC20 contract. Approve should allow BridgingPayment to spend amount of tokens that user have requested to bridge.
• frontend submits user request to the BridgingPayment.
• The ERC20Manager locks/burns ERC20 tokens and emits an event.
• Eventually, the eth->gear protocol relayer (or another party) sumbits to checkpoint-light-client ethereum block which has block number bigger than one where event have been emitted.
• Eventually, the eth->gear token relayer (or another party) submits this event to historical-proxy
• historical-proxy verifies this event by sending a message to the corresponding eth-events-*.
• eth-events-* verifies that block where event is present is a valid finalized block on ethereum by calling checkpoint-light-client.
• historical-proxy routes user bridging request to vft-manager
• vft-manager sends message to a vft program that corresponds to a ERC20 token that've been locked/burned in step 4. This message is either transfer(in the case when token supply is on Gear) or mint(in the case when token supply is on Ethereum).
• vft program mints/transfers tokens to a user address.

Build and run

Prerequisites

Install rust:

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

Install golang: golang installation

Install forge:

curl -L https://foundry.paradigm.xyz | bash
foundryup

Note

To build ring crate follow BUILDING.md.

Run relayer

Build workspace:

cargo build --release -p relayer

And then run

./target/release/relayer --help

to see required parameters to start different kinds of relayers.

Footnotes

1. (proxy) means that this contract is deployed behind ERC-1967 Proxy ↩ ↩² ↩³

2. Gear itself is not a blockchain network and has no native token. This refers to the token of any network built on Gear technology, such as Vara. ↩ ↩²