Cross Chain Transfer Protocol

Introduction

Circle's Cross-Chain Transfer Protocol (CCTP) facilitates the transfer of USDC between distinct blockchains, providing a more streamlined experience. By integrating CCTP with programmable wallets in your applications, you enable seamless USDC transactions across multiple chains. This reduces the need for users to manage separate wallets for different blockchains, potentially broadening your audience and providing increased liquidity and adaptability. It's designed to provide users with a cohesive and efficient digital currency experience.

Outcomes of this Interactive Quickstart Guide

By the conclusion of this interactive guide, you will learn how to facilitate a transfer of USDC across blockchains using a Programmable Wallet. Specifically, the guide will demonstrate a scenario wherein a user conducts a USDC transfer leveraging the Cross-Chain Transfer Protocol (CCTP). You will follow step-by-step instructions to create and execute a USDC transfer from Sepolia, an Ethereum testnet, to Mumbai, the Polygon Testnet, via a programmable wallet. This practical walkthrough is designed to equip you with the knowledge to implement CCTP in your own applications.

Completing a Cross-Blockchain Transfer: Step-By-Step Process

1. 1

   Approve USDC Contract to Initiate Transfers

   To utilize CCTP effectively, it's necessary to authorize the USDC contract to conduct transfers on a user's behalf. In this guide, we approve only a small amount of USDC for demonstration. However, for operational efficiency, you might want to approve a larger amount to minimize the frequency of this action in a production environment.

2. 2

   Initiate the Burning of USDC

   Following the user's request to move USDC from Ethereum to Polygon, leverage the provided tools to initiate burning the specified amount of USDC from the Ethereum wallet created for this transaction.

   
3. 3

   Obtain Circle’s Attestation

   Circle monitors the burn event on the Ethereum blockchain and issues an attestation. Retrieve this attestation using the relevant procedures, which serves as the authorization required to mint the corresponding USDC amount on the Polygon blockchain.

   
4. 4

   Trigger USDC Minting

   Utilize the retrieved attestation to start the minting process. The attestation confirms the amount of USDC to mint on the Polygon blockchain, and the amount to allocate to the specified Polygon wallet.

   

Understanding Smart Contracts

Smart contracts are computer protocols that automate the execution of agreements, operating under a set of predefined conditions. Once deployed onto a blockchain, they follow an "if-then" logic to perform actions automatically when specific conditions are met. The immutable nature of blockchain ensures that the smart contract code cannot be changed once established, which reinforces their reliability as they will always execute exactly as programmed. A smart contract typically consists of code that defines its operations, a data storage area, memory, and a unique public address that can be interacted with through its functions. For a comprehensive exploration of smart contracts, you are encouraged to read further on the dedicated resource.

Exploring the Cross-Chain Transfer Protocol (CCTP)

The Cross-Chain Transfer Protocol (CCTP) serves as an integral tool within the blockchain infrastructure to guarantee the secure and seamless transfer of USDC across various blockchain networks. By adopting CCTP, users can benefit from a protected and efficient means of transferring USDC between supported chains, which currently include Ethereum, Arbitrum, Optimism, and Avalanche. Ongoing developments suggest plans to expand support to additional networks. To learn more about CCTP, see Cross-Chain Transfer Protocol in the Circle Developer Documentation.

To get started, Create a Circle Web3 Services account in the console.

Prerequisites

Before you get started, ensure that you have:

• Have a Web3 Services account
• Created your API key
• Created a developer-controlled wallet

Burn USDC

In the process of transferring USDC from Ethereum to Polygon, you need to give the TokenMessenger contract the necessary permissions to execute the USDC transfer on your behalf. In essence, you're empowering the TokenMessenger contract to access and transfer the specified amount of USDC from your source wallet.

For this, we use the Programmable Wallets contract execution API:

The second step of the process facilitates a burn of the specified amount of USDC on Ethereum Sepolia (the source chain) that you are looking to move to Polygon Mumbai (the destination chain).

To do this, you again use Programmable Wallets contract execution API:

const encodedDestinationAddress = web3.eth.abi
                    .encodeParameter('address', destinationAddress);

Just like your first contract execution request, the other parameters - walletId, feeLevel, idempotencyKey, and EntitySecretCiphertext - were automatically populated in the request.

Validating the Burn of 1 USDC in your Developer Console

After initiating the burn of USDC through our API, you can check whether it was successful in our Developer Console.

1. 1

   Login to the Web3 Services Console

   To begin the process of validating your burn of USDC, log in to the Web3 Services Console.

2. 2

   Navigate to the List of Wallets

   Within the console, locate the Wallets page under Dev Controlled Wallet. This page displays all of the wallets you created thus far. Open the Wallets page.

3. 3

   Open the Wallet Details Screen

   Find and click on the Ethereum Sepolia wallet from which you initiated the USDC burn.

4. 4

   View Your Wallet Balance

   On the wallet details screen, scroll down to the bottom to find the section displaying your wallet balances. As you successfully created your Sepolia Wallet, an initial deposit of 10 USDC was made. If you have successfully burned 1 USDC, the total wallet balance should now reflect 9 USDC, indicating the accurate deduction of the burned amount.

   

By following these step-by-step instructions and reviewing your wallet balance in the Web3 Services Console, you can confidently validate the burn of your USDC tokens and verify the correct adjustment in your wallet's overall balance.

Approve Burn

As a result of interacting with the TokenMessenger contract, Circle generates a unique ID for the transaction. Starting with the returned transaction ID, you call Circle's Attestation service to fetch an attestation that the burn took place. This attestation proves that the USDC was removed from circulation on the source chain.

The attestation is used on the destination chain to mint new USDC. You can make API calls to get the transaction's transactionHash, retrieve the logs of the depositForBurn transaction, and use Circle's Attestation API to fetch the attestation signature. The steps have been broken down for you here, but you can use the following "Try It Out" component to run all of these steps together.

1. 1

   Fetching the Deposit Transaction Object from Programmable Wallets.

   Call the Programmable Wallets API to fetch the transaction object for the depositForBurn transaction. Make a GET request using the provided code and the transaction ID. Retrieve the transaction object from the response data.

   Code

   SDK

   NodeJS

2. 2

   Decoding and Creating messageBytes and messageHash with a Web3 Library

   Using a Web3 library like web3 or ethers, decode and create messageBytes and messageHash from the transaction object's txHash. Follow the provided code snippet to extract the relevant information.

   JavaScript

   // get messageBytes from EVM logs using txHash of the transaction.
   const transactionReceipt = await web3.eth.getTransactionReceipt(
   transaction.txHash,
   )
   const eventTopic = web3.utils.keccak256('MessageSent(bytes)')
   const log = transactionReceipt.logs.find((l) => l.topics[0] === eventTopic)
   const messageBytes = web3.eth.abi.decodeParameters(['bytes'], log.data)[0]
   const messageHash = web3.utils.keccak256(messageBytes)
   

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3. 3

   Fetch Attestation Signature from Circle's Iris API

   Retrieve the attestation signature from Circle's Iris API using the messageHash obtained in the previous step. Use the provided code inside a loop to query the API until the attestation status is "complete". Access the attestation signature from the response.

   JavaScript

   // Get attestation signature from iris-api.circle.com
   let attestationResponse = { status: 'pending' }
   while (attestationResponse.status != 'complete') {
   const response = await fetch(
   `https://iris-api-sandbox.circle.com/attestations/${messageHash}`,
   )
   attestationResponse = await response.json()
   await new Promise((r) => setTimeout(r, 2000))
   }
   

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   The attestation service is rate-limited. Please limit your requests to less than 10 per second.

Mint USDC

To mint USDC on the destination blockchain, we're using the messageBytes from the depositForBurn event and the attestation signature (attestationResponse.attestation). We're fetching both from Circle's attestation services and calling the receiveMessage function on the Message Transmitter contract in the destination blockchain network.

Using the Programmable Wallets smart contract execution API:

Once the message has been received, the specified amount of USDC is minted at the recipient's address on the destination blockchain.

This returns a transactionId after you submit the receiveMessage request. You can use this transactionId to look up the status and details of the final transaction.

Congratulations on your Progress!

Throughout this process, you've successfully:

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