Crosschain

This directory contains contracts for sending and receiving cross chain messages that follows the ERC-7786 standard.

• CrosschainLinked: helper to facilitate communication between a contract on one chain and counterparts on remote chains through ERC-7786 gateways.
• ERC7786Recipient: generic ERC-7786 crosschain contract that receives messages from a trusted gateway.

Additionally there are multiple bridge constructions:

• BridgeFungible: Core bridging logic for crosschain ERC-20 transfer. Used by BridgeERC20, BridgeERC7802 and ERC20Crosschain,
• BridgeERC20: Standalone bridge contract to connect an ERC-20 token contract with counterparts on remote chains,
• BridgeERC7802: Standalone bridge contract to connect an ERC-7802 token contract with counterparts on remote chains.

Helpers

CrosschainLinked

ERC7786Recipient

Bridges

BridgeFungible

BridgeERC20

BridgeERC7802

import "@openzeppelin/contracts/crosschain/CrosschainLinked.sol";

Core bridging mechanism.

This contract contains the logic to register and send messages to counterparts on remote chains using ERC-7786 gateways. It ensure received messages originate from a counterpart. This is the base of token bridges such as BridgeFungible.

Contracts that inherit from this contract can use the internal CrosschainLinked._sendMessageToCounterpart to send messages to their counterpart on a foreign chain. They must override the ERC7786Recipient._processMessage function to handle messages that have been verified.

import "@openzeppelin/contracts/crosschain/ERC7786Recipient.sol";

Base implementation of an ERC-7786 compliant cross-chain message receiver.

This abstract contract exposes the receiveMessage function that is used for communication with (one or multiple) destination gateways. This contract leaves two functions unimplemented:

• CrosschainLinked._isAuthorizedGateway, an internal getter used to verify whether an address is recognised by the contract as a valid ERC-7786 destination gateway. One or multiple gateway can be supported. Note that any malicious address for which this function returns true would be able to impersonate any account on any other chain sending any message.

• ERC7786Recipient._processMessage, the internal function that will be called with any message that has been validated.

ERC-7786 requires the gateway to ensure messages are not delivered more than once. Therefore, we don't need to keep track of the processed receiveId.

@custom:stateless

import "@openzeppelin/contracts/crosschain/bridges/BridgeERC20.sol";

This is a variant of BridgeFungible that implements the bridge logic for ERC-20 tokens that do not expose a crosschain mint and burn mechanism. Instead, it takes custody of bridged assets.

import "@openzeppelin/contracts/crosschain/bridges/BridgeERC7802.sol";

This is a variant of BridgeFungible that implements the bridge logic for ERC-7802 compliant tokens.

import "@openzeppelin/contracts/crosschain/bridges/abstract/BridgeFungible.sol";

Base contract for bridging ERC-20 between chains using an ERC-7786 gateway.

In order to use this contract, two functions must be implemented to link it to the token:

• BridgeERC20._onSend: called when a crosschain transfer is going out. Must take the sender tokens or revert.
• BridgeERC20._onReceive: called when a crosschain transfer is coming in. Must give tokens to the receiver.

This base contract is used by the BridgeERC20, which interfaces with legacy ERC-20 tokens, and BridgeERC7802, which interface with ERC-7802 to provide an approve-free user experience. It is also used by the ERC20Crosschain extension, which embeds the bridge logic directly in the token contract.