Cross Chain Swaps with Circle’s CCTP and Axelar

Circle has created CCTP to natively transfer USDC across supported blockchains. In this tutorial, we’ll learn how to build a cross-chain USDC dApp using Circle’s Cross-Chain Transfer Protocol (CCTP) and Axelar’s General Message Passing (GMP).

With this approach, your users will be able to issue a single transaction with a GMP payload. On the backend, the application takes care of USDC bridging, plus any other action that the user wishes — as indicated in the payload. Axelar services, also working on the backend, can handle conversion and payment for destination-chain gas fees, so the user only has to transact once, using one gas token.

In this example, we will build a cross-chain swap dApp. It converts a native token from one chain to another chain, using native USDC as a routing asset. For example: send ETH to a contract on Ethereum and receive AVAX on Avalanche, or vice versa.

There are two parts we have to learn to achieve this:

1. Sending a native USDC token cross-chain.
2. Sending a swap payload cross-chain.

Part 1: Sending a native USDC token cross-chain

There are three components from Circle that we’ll use in this part:

1. MessageTransmitter contract – to mint USDC at the destination chain.
2. CircleBridge contract  – to burn USDC at the source chain.
3. Attestation API – to retrieve attestation to be used for minting USDC at the destination chain.

Let’s take a look at how to implement this step-by-step:

1. Burn the given amount of USDC by calling the function depositForBurnat the TokenMessenger contract. The example Solidity code is below. At this step, the contract does nothing except provide a function to burn the USDC in _depositForBurn function.

CrosschainNativeSwap.sol

// SPDX-License-Identifier: MIT
pragma solidity 0.8.9;

import {IERC20} from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import {SafeERC20} from "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
import {ITokenMessenger} from "./ITokenMessenger.sol";

contract CrosschainNativeSwap is AxelarExecutable, Ownable {
    IERC20 public usdc;
    ITokenMessenger public tokenMessenger;

    // mapping chain name => domain number;
    mapping(string => uint32) public circleDestinationDomains;

    constructor(
        address usdc_,
        address tokenMessenger_
    ) AxelarExecutable(gateway_) Ownable() {
        usdc = IERC20(usdc_);
        circleDestinationDomains["ethereum"] = 0;
        circleDestinationDomains["avalanche"] = 1;
    }

    function _sendViaCCTP(
        uint256 amount,
        string memory destinationChain,
        address recipient
    ) private isValidChain(destinationChain) {
        IERC20(address(usdc)).approve(address(tokenMessenger), amount);

        tokenMessenger.depositForBurn(
            amount,
            this.circleDestinationDomains(destinationChain),
            bytes32(uint256(uint160(recipient))),
            address(usdc)
        );
    }

}

ITokenMessenger.sol

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.9;

interface ITokenMessenger {
    // this event will be emitted when `depositForBurn` function is called.
    event MessageSent(bytes message);

    /**
    * @param _amount amount of tokens to burn
    * @param _destinationDomain destination domain
    * @param _mintRecipient address of mint recipient on destination domain
    * @param _burnToken address of contract to burn deposited tokens, on local
    domain
    * @return _nonce uint64, unique nonce for each burn
    */
    function depositForBurn(
        uint256 _amount,
        uint32 _destinationDomain,
        bytes32 _mintRecipient,
        address _burnToken
    ) external returns (uint64 _nonce);
}

That’s it for the initial contract using CCTP. We’ll continue to add our business logic. Let’s try to integrate this with Axelar network to complete our cross-chain swap dApp.

Part 2: Sending a swap payload cross-chain

In this part, we’ll add logic in our contract to send a payload cross-chain with Axelar network.

1. Upgrade our contract to include business logic and integrate with Axelar network.

CrosschainNativeSwap.sol

// SPDX-License-Identifier: MIT
pragma solidity 0.8.9;

import {IAxelarGasService} from "@axelar-network/axelar-cgp-solidity/contracts/interfaces/IAxelarGasService.sol";
import {IAxelarGateway} from "@axelar-network/axelar-cgp-solidity/contracts/interfaces/IAxelarGateway.sol";
import {AxelarExecutable} from "@axelar-network/axelar-gmp-sdk-solidity/contracts/executable/AxelarExecutable.sol";
import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
import {IERC20} from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import {SafeERC20} from "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
import {ITokenMessenger} from "./ITokenMessenger.sol";
import { StringToAddress, AddressToString } from "@axelar-network/axelar-gmp-sdk-solidity/contracts/libs/AddressString.sol";

contract CrosschainNativeSwap is AxelarExecutable, Ownable {
    IERC20 public usdc;
    ITokenMessenger public tokenMessenger;
    IAxelarGasService immutable gasReceiver;

    // mapping chain name => domain number;
    mapping(string => uint32) public circleDestinationDomains;
    // mapping destination chain name => destination contract address
    mapping(string => address) public siblings;

    error InvalidTrade();
    error InsufficientInput();
    error TradeFailed();

    event SwapSuccess(bytes32 indexed traceId, uint256 amount, bytes tradeData);

    event SwapFailed(
        bytes32 indexed traceId,
        uint256 amount,
        address refundAddress
    );

    event SwapPending(
        bytes32 indexed traceId,
        bytes32 indexed payloadHash,
        uint256 amount,
        string destinationChain,
        bytes payload
    );

    constructor(
        address usdc_,
        address gasReceiver_,
        address gateway_,
        address tokenMessenger_
    ) AxelarExecutable(gateway_) Ownable() {
        usdc = IERC20(usdc_);
        tokenMessenger = ITokenMessenger(tokenMessenger_);
        gasReceiver = IAxelarGasService(gasReceiver_);
        circleDestinationDomains["ethereum"] = 0;
        circleDestinationDomains["avalanche"] = 1;
    }

    modifier isValidChain(string memory destinationChain) {
        require(siblings[destinationChain] != address(0), "Invalid chain");
        _;
    }

    // Set address for this contract that deployed at another chain
    function addSibling(
        string memory chain_,
        address address_
    ) external onlyOwner {
        siblings[chain_] = address_;
    }

    /**
     * @dev Swap native token to USDC, burn it, and send swap payload to AxelarGateway contract
     * @param destinationChain Name of the destination chain
     * @param srcTradeData Trade data for the first swap
     * @param destTradeData Trade data for the second swap
     * @param traceId Trace ID of the swap
     * @param fallbackRecipient Recipient address to receive USDC token if the swap fails
     * @param inputPos Position of the input token in destTradeData
     */
    function nativeTradeSendTrade(
        string memory destinationChain,
        bytes memory srcTradeData,
        bytes memory destTradeData,
        bytes32 traceId,
        address fallbackRecipient,
        uint16 inputPos
    ) external payable isValidChain(destinationChain) {
        // Swap native token to USDC
        (uint256 nativeSwapAmount, uint256 usdcAmount) = _swapNativeToUsdc(
            srcTradeData
        );

        // Burns a specified amount of USDC tokens by calling `depositForBurn` function in Circle's TokenMessenger contract
        _sendViaCCTP(
            usdcAmount,
            destinationChain,
            this.siblings(destinationChain)
        );

        // encode the payload to send to the sibling contract
        bytes memory payload = abi.encode(
            destTradeData,
            usdcAmount,
            traceId,
            fallbackRecipient,
            inputPos
        );

        // Pay gas to AxelarGasReceiver contract with native token to execute the sibling contract at the destination chain
        _payGasAndCallContract(
            destinationChain,
            payload,
            msg.value - nativeSwapAmount
        );

        emit SwapPending(
            traceId,
            keccak256(payload),
            usdcAmount,
            destinationChain,
            payload
        );
    }

    function _payGasAndCallContract(
        string memory destinationChain,
        bytes memory payload,
        uint256 fee
    ) private {
        gasReceiver.payNativeGasForContractCall{value: fee}(
            address(this),
            destinationChain,
            AddressToString.toString(this.siblings(destinationChain)),
            payload,
            msg.sender
        );

        // Send all information to AxelarGateway contract.
        gateway.callContract(
            destinationChain,
            AddressToString.toString(this.siblings(destinationChain)),
            payload
        );
    }

    function _sendViaCCTP(
        uint256 amount,
        string memory destinationChain,
        address recipient
    ) private isValidChain(destinationChain) {
        IERC20(address(usdc)).approve(address(tokenMessenger), amount);

        tokenMessenger.depositForBurn(
            amount,
            this.circleDestinationDomains(destinationChain),
            bytes32(uint256(uint160(recipient))),
            address(usdc)
        );
    }

    function _swapNativeToUsdc(
        bytes memory tradeData
    ) private returns (uint256 amount, uint256 burnAmount) {
        // Calculate remaining usdc token in the contract
        uint256 preTradeBalance = IERC20(address(usdc)).balanceOf(
            address(this)
        );

        (uint256 nativeAmountIn, address router, bytes memory data) = abi
            .decode(tradeData, (uint256, address, bytes));

        // Swap native token to USDC
        (bool success, ) = router.call{value: nativeAmountIn}(data);

        // Revert if trade failed
        require(success, "TRADE_FAILED");

        // Calculate amount of USDC token swapped. This is the amount to be burned at the source chain.
        uint256 usdcAmount = IERC20(address(usdc)).balanceOf(address(this)) -
            preTradeBalance;

        // Return amount of native token swapped and amount of USDC token to be burned
        return (nativeAmountIn, usdcAmount);
    }

    function _refund(
        bytes32 traceId,
        uint256 amount,
        address recipient
    ) internal {
        SafeERC20.safeTransfer(IERC20(address(usdc)), recipient, amount);
        emit SwapFailed(traceId, amount, recipient);
    }

    // This function will be called by Axelar Executor service.
    function _execute(
        string memory /*sourceChain*/,
        string memory /*sourceAddress*/,
        bytes calldata payload
    ) internal override {
        // Decode payload
        (
            bytes memory tradeData,
            uint256 usdcAmount,
            bytes32 traceId,
            address fallbackRecipient,
            uint16 inputPos
        ) = abi.decode(payload, (bytes, uint256, bytes32, address, uint16));

        // This code inserts the usdcAmount into the tradeData bytes at a specific location, in order to properly position the data.
        assembly {
            mstore(add(tradeData, inputPos), usdcAmount)
        }

        (address srcToken, , address router, bytes memory data) = abi.decode(
            tradeData,
            (address, uint256, address, bytes)
        );

        // Approve USDC to the router contract
        IERC20(srcToken).approve(router, usdcAmount);

        // Swap USDC to native token
        (bool swapSuccess, ) = router.call(data);

        // If swap failed, refund USDC to the user at the destination chain.
        if (!swapSuccess)
            return _refund(traceId, usdcAmount, fallbackRecipient);

        // Emit success event so that our application can be notified.
        emit SwapSuccess(traceId, usdcAmount, tradeData);
    }
}

There’s a lot of new code added here. Let’s try to understand it, step by step:

Step 1 — Add the gasReceiver variable and initialize it in the constructor. This handles destination-chain gas token conversion and fee payment, so the user need not transact more than once.

Step 2 — Add the siblings variable and related functions so the admin can define connected contract addresses on the other chains.

Step 3 — Add the nativeTradeSendTrade function. The client will send a transaction to call this function. This is the most important function in our contract. Here are the implementation details:

• Swap native token to USDC with low-level contract call.
• Send the USDC to the destination chain via CCTP
• Construct the swap payload to send to the AxelarGateway contract. The payload will be relayed by Axelar Relayer service to the destination contract. The destination contract address is defined by addSibling function.
• Pay gas to the AxelarGasService contract with the native token. The required amount will be calculated off-chain by using AxelarJS-SDK on the client side. See more information about it here.
• Send destinationChain, destinationContractAddressand payload to the AxelarGateway contract.

Step 4 — Upgrade a contract to extend AxelarExecutable interface and override _execute function, so the contract can be called by Axelar Executor service at the destination chain.

• The function decodes payload to retrieve all information it needs for swap.
• We correct the amount in tradeData bytes before the swap.
• Approve USDC to the router contract and call the swap function, and refund if it fails.

And we’re done! Here is the demo that communicates with the completed contract.

Resources

• CrosschainNativeSwap contract: link
• Running Crosschain USDC Example with Hardhat: link
• Axelar Cross-chain USDC Demo: link

About Circle

Circle is a global financial technology firm that enables businesses of all sizes to harness the power of digital currencies and public blockchains for payments, commerce and financial applications worldwide. Circle is the issuer of USD Coin (USDC), one of the fastest growing dollar digital currencies powering always-on internet-native commerce and payments. Today, Circle’s transactional services, business accounts, and platform APIs are giving rise to a new generation of financial services and commerce applications that hold the promise of raising global economic prosperity for all through the frictionless exchange of financial value. Additionally, Circle operates SeedInvest, a leading startup fundraising platform in the U.S. Learn more at https://circle.com.