Arbitrary Message Bridge

The native Arbitrary Message Bridge (AMB) allows user to send arbitrary data between Gnosis Chain and Ethereum. This allows Gnosis contracts to send data and trigger contract functions on Ethereum and other chains, and vice versa.

The AMB is a key bridge primitive that is used inside higher-order bridges like the Omnibridge native token bridge, and is part of the Tokenbridge Architecture.

Due to the light client finality requirements (at least 23mins on Ethereum), the transactions will take approx. 30mins to be signed by the bridge. However, users can still use 3rd party bridges (Jumper.exchange, Connext, Hop) without any impact.

Overview

	Detail
Frontend URL	N/A
Trust Model	4-of-7 Validator Multisig
Governance	8-of-16 Multisig
Governance Parameters	Validators
Bug Bounty	up to $2m
Bug Reporting	Immunefi

Contracts

Ethereum

Ethereum

Contract	Address
Omnibridge Multi-Token Mediator	0x88ad09518695c6c3712AC10a214bE5109a655671
AMB (Foreign)	0x4C36d2919e407f0Cc2Ee3c993ccF8ac26d9CE64e
AMB/OmniBridge wETH Router Helper	0xa6439Ca0FCbA1d0F80df0bE6A17220feD9c9038a

Gnosis

Gnosis

Contract	Address
AMB/Omnibridge Multi-Token Mediator	0xf6A78083ca3e2a662D6dd1703c939c8aCE2e268d
AMB (Home)	0x75Df5AF045d91108662D8080fD1FEFAd6aA0bb59
AMB Helper Contract	0x7d94ece17e81355326e3359115D4B02411825EdD

Sepolia-Chiado

Sepolia - Chiado

Contract	Address
AMB (Sepolia)	0xf2546D6648BD2af6a008A7e7C1542BB240329E11
Validator Contract (Sepolia)	0xa0bd95dd2570632c8640ab5bc213f3a0ea33e26a
AMB (Chiado)	0x8448E15d0e706C0298dECA99F0b4744030e59d7d
Validator Contract (Chiado)	0x9e8a89ebcb83065eaaf4b7ff720caa5e6b25c976
AMB Helper Contract (Chiado)	0x3cc500B3c01D04C265c9293cB35BA2Fd8eA6dc1b

Bridge Validators

• See Bridge Validator

Bridge Governance

• See Bridge Governance

How it works

Terminology

• Home (Native) Network: Of the two networks being bridged between, the home or native network is the one with fast and inexpensive operations. All bridge operations to collect validator confirmations are performed on this side of the bridge. It is the Gnosis Chain in this case.
• Foreign Network: Can be any EVM chain, generally it refers to Ethereum.
• Originating Contract: An arbitrary contract where the message originates, typically this is where the user interacts and requests for a function to be invoked on another network.

Call a cross-chain method via AMB:

function requireToPassMessage (address _contract,
                                bytes _data,
                                uint256 _gas) external;

param	details
_contract	address of contract on other network
_data	encoded bytes of the method selector and the params that will be called in the contract on the other side
_gas	gas to be provided in execution of the method call on the other side

Foreign Network to Home Network

1. User calls foo() on the originating contract
2. Originating contract calls requireToPassMessage() on Foreign Bridge contract, and encodes foo(), target address, and gas limit used on the other chain for executing a message.
3. UserRequestForAffirmation event is emitted, and listening validators relay the message to the Home side where signatures are collected
4. executeAffirmation() is called on the Home Bridge contract by a validator once enough signatures are collected.
5. Home bridge contract decodes the message and calls foo() on the target contract.

Home Network to Foreign Network

1. User calls foo() on an originating contract
2. Originating contract calls requireToPassMessage() on Home Bridge contract, and encodes foo(), target address, and gas limit used on the other chain for executing a message.
3. Signatures are collected from validators by calling submitSignatures(), and once enough are collected CollectedSignatures() event is emitted.
4. Message is relayed to the Foreign Bridge contract, and executeSignatures() is called
5. Foreign bridge contract decodes the message and calls foo() on target contract

How to check if AMB is down (not relaying message)

In certain circumstances, i.e. hardfork, AMB will be planned for downtime (not relaying message) to ensure security of the bridge. Planned downtime will be announced in public channel like Discord and Twitter, prior to the event.
There is also another way to check whether the AMB is down or not by reading maxGasPerTx value on AMB contract.

In the current configuration, maxGasPerTx is set to 4000000 on Ethereum and 2000000 on Gnosis Chain.

The AMB is down when maxGasPerTx is set to 0, only by owner of the contract.

By setting maxGasPerTx to 0, the condition in _sendMessage() will not pass, meaning, the bridge is down/stopped.

Security Considerations for Receiving a Call

Concern	Remediation
Authorization	Check the address of invoking contract using messageSender()
Authorization	Check that msg.sender is the address of the bridge contract
Replay Attack	transactionHash() allows for checking of a hash of the transaction that invoked the requireToPassMessage() call. The invoking contract (in some cases, the mediator contract) is responsible for providing a unique sequence (can be a nonce) as part of the _data param in the requireToPassMessage() function call

AMB Components

Component	Description
System Contracts	AMB Implementation Contracts (Home Bridge and Foreign Bridge), Governance Multisigs, gas limit helpers, failed call management helpers (for when gas estimate was insufficient), and fee management helpers to collect fees
Bridge validator	Containerized microservices that listen for on-chain events and send confirmations to relay messages. More on them here.
DevOps	Bridge validator,Bridge UI, docker compose, ansible playbooks
dApp Contracts	extensions (pair mediator contracts on both sides of the AMB), such as the Omnibridge

Use Cases of AMB

• ERC-to-ERC Bridges: AMB-ERC-TO-ERC mode enables the transfer of ERC tokens to the Foreign Mediator, which will interact with Foreign AMB Bridge to mint wrapped ERC-667 tokens on the Home Network. Complimentarily, the mode enables the transfer ERC20 or ERC-667 tokens to the Home Mediator, which will interact with Home AMB Bridge to unlock ERC20 tokens on the Foreign network. This is used by the Omnibridge.
• ERC-to-Native Bridges: ERC-TO-NATIVE mode enables the user to send ERC20 tokens to the Foreign Bridge and receive native coins from the Home Bridge Complimentarily, then can send native coins to the Home Bridge to unlock ERC20 tokens from the Foreign Bridge. The home network nodes must support a consensus engine that allows using a smart contract for block reward calculation. This mode is used by the xDai Bridge
• Message Passing: ARBITRARY-MESSAGE mode enables the capability to invoke a Home/Foreign Bridge contract to send a message that will be executed on the other Network. This can be an arbitrary contract method invocation.

Resources

• Token bridge Documentation