# Sending CCIP messages In the previous two lessons, we deployed three smart contracts. On the source chain (Sepolia): 1. `Sender`: To send the cross-chain message. On the destination chain (base Sepolia): 2. `Receiver`: to receive the cross-chain message. 3. `Vault`: into which we call `deposit` to deposit the USDC we sent from the `Sender` .contract to the `Receiver` contract using CCIP. Let's use these contracts to perform a token and data transfer cross-chain using CCIP. ## Fund the Sender with LINK We first need to fund our `Sender` contract with some LINK on Sepolia to pay the fees. Switch your network back to Sepolia in MetaMask. For this, we will follow the same steps as in Section 5 Lesson 5, so revisit that lesson if you need a recap of how to do this. From MetaMask, send `3` LINK to the `Sender` contract address: ::image{src='/chainlink-fundamentals/6-chainlink-ccip-messages/assets/send-link.png' style='width: 100%; height: auto;' alt='send-link'} ## Approve the Sender and Receiver contracts We need to approve both `Sender` and `Receiver` contracts to spend USDC: - `Sender`: so this contract can transfer the USDC from your wallet to itself, ready for the cross-chain transfer. - `Vault`: so it can send USDC from your EOA to itself when calling `deposit`. Let's do a quick recap on how to do this. ### Approving the Sender The `Sender` has been deployed to Sepolia, so we will use [Etherscan](https://sepolia.etherscan.io/) to perform this approval. 1. Head to the [USDC contract on Sepolia Etherscan](https://sepolia.etherscan.io/address/0x1c7D4B196Cb0C7B01d743Fbc6116a902379C7238) 2. Click on the **Contract** and then the **Write as Proxy** tabs. 3. Click **Connect to Web3** and connect your wallet: ::image{src='/chainlink-fundamentals/6-chainlink-ccip-messages/assets/etherscan-sepolia.png' style='width: 100%; height: auto;' alt='etherscan-sepolia'} 4. Click on the `approve` function: - Paste the `Sender` address as the `spender` - Set the `value` as `1000000` - 1 USDC - Click **Write** - Confirm the transaction in MetaMask ::image{src='/chainlink-fundamentals/6-chainlink-ccip-messages/assets/sepolia-approve.png' style='width: 100%; height: auto;' alt='sepolia-approve'} 5. Confirm the `Sender` has been added as a spender by: - Clicking the **Read as Proxy** tab. - Clicking **Connect to Web3** and connecting your wallet. - Clicking the `allowance` function . - Pasting your MetaMask address as the `owner`. - Pasting the `Sender` address as the `spender`. - Clicking **Query**. - `1000000` will be returned if the `Sender` was successfully added as a spender. ::image{src='/chainlink-fundamentals/6-chainlink-ccip-messages/assets/sepolia-allowance.png' style='width: 100%; height: auto;' alt='sepolia-allowance'} ### Approving the Vault The `Vault` has been deployed to Base Sepolia. So we will use [Basescan](https://sepolia.basescan.org/) to approve this contract. 1. Head to the [USDC contract on Base Sepolia](https://sepolia.basescan.org/address/0x036CbD53842c5426634e7929541eC2318f3dCF7e) 2. Click on the **Contract** and then the **Write as Proxy** tabs. 3. Click **Connect to Web3** and connect your wallet. 4. Click on the `approve` function: - Paste the `Vault` address as the `spender`. - Set the `value` as `1000000` - `1` USDC. - Click **Write**. - Confirm the transaction in MetaMask. ::image{src='/chainlink-fundamentals/6-chainlink-ccip-messages/assets/basescan.png' style='width: 100%; height: auto;' alt='basescan'} 5. Confirm the `Vault` has been added as a spender by: - Clicking the **Read as Proxy** tab. - Clicking **Connect to Web3** and connecting your wallet. - Clicking the `allowance` function . - Paste your MetaMask address as the `owner` . - Pasting the `Vault` address as the `spender`. - Clicking **Query**. - `1000000` will be returned if the `Vault` was successfully added as a spender. ::image{src='/chainlink-fundamentals/6-chainlink-ccip-messages/assets/allowance-base.png' style='width: 100%; height: auto;' alt='allowance-base'} ## Sending the cross-chain message FINALLY! We are ready to send the cross-chain message! - Head back to Remix. - Switch back to Sepolia in MetaMask. - Expand the `Sender` contract dropdown, click on the `transferTokens` function, and enter the following function parameters: - `_receiver`: the `Receiver` contract address. - `_amount`: `1000000`. - `_target`: the `Vault` contract address. - Click **transact** and sign the transaction in MetaMask to send the message cross-chain: ::image{src='/chainlink-fundamentals/6-chainlink-ccip-messages/assets/transfer-usdc.png' style='width: 100%; height: auto;' alt='transfer-usdc'} - Once your transaction has confirmed on Sepolia, copy the transaction hash: ::image{src='/chainlink-fundamentals/6-chainlink-ccip-messages/assets/transaction-hash.png' style='width: 100%; height: auto;' alt='transaction-hash'} - Head to the [CCIP Explorer](https://ccip.chain.link/) and paste the transaction hash to see the status of your CCIP message: ::image{src='/chainlink-fundamentals/6-chainlink-ccip-messages/assets/transaction-details.png' style='width: 100%; height: auto;' alt='transaction-details'} - Once finality has been reached, you will see the status in the CCIP explorer: ::image{src='/chainlink-fundamentals/6-chainlink-ccip-messages/assets/success.png' style='width: 100%; height: auto;' alt='success'} ## Checking the data was executed Let's check that the USDC was successfully automatically deposited into the vault. - In MetaMask, connect to Base Sepolia. - Expand the `Vault` contract dropdown in the **Deployed Contracts** section and find the `balances` function. - Paste your MetaMask address as the `address` and click **balances**. - If the data has been successfully executed, `1000000` will be returned: ::image{src='/chainlink-fundamentals/6-chainlink-ccip-messages/assets/balances.png' style='width: 100%; height: auto;' alt='balances'} This was a very challenging lesson so great job at getting to the end! I recommend you take a break here to digest all of that information before moving onto the next section. ## Challenge! So now you know how to write a smart contract to bridge USDC tokens from Sepolia to Base Sepolia, the next challenge is to use this same smart contract to transfer [CCIP-BnM](https://sepolia.etherscan.io/token/0xfd57b4ddbf88a4e07ff4e34c487b99af2fe82a05#writeContract) tokens to ZKsync Sepolia and automatically send the tokens to a vault! To do this you will need to: 1. Add the ZKsync Sepolia chain to MetaMask. 2. Add the CCIP-BnM token on Sepolia and ZKsync Sepolia to Metamask. 3. Modify and re-deploy the `Sender` smart contract: - `DESTINATION_CHAIN_SELECTOR` to be the [selector for ZKsync Sepolia](https://docs.chain.link/ccip/directory/testnet/chain/ethereum-testnet-sepolia-zksync-1). - Change the token to be [CCIP-BnM](https://sepolia.etherscan.io/token/0xfd57b4ddbf88a4e07ff4e34c487b99af2fe82a05#writeContract) rather than USDC. - (dev-only challenge): Can you modify the smart contracts to allow the function caller to specify the token and destination chain? 4. Modify the `Vault` smart contract - Change the token to be [CCIP-BnM](https://sepolia.etherscan.io/token/0xfd57b4ddbf88a4e07ff4e34c487b99af2fe82a05#writeContract) rather than USDC. - (dev-only challenge): Can you modify the `Vault` to allow an owner to be able to add allowed tokens? 6. Re-deploy the `Receiver` and `Vault` smart contracts: - To do this, you will need to use the Remix ZKsync plugin to compile and deploy your smart contract! - Simply click on the plugins tab on the bottom of the left sodebar, type "zksync" into the search bar and then click **Activate** on the plugin. It will now be available in the sidebar. - If you get stuck, [this lesson on Updraft](https://updraft.cyfrin.io/courses/solidity/simple-storage/zksync-plugin) will take you through using the ZKsync plugin step-by step. - You will also need to get some Zksync Sepolia ETH to pay for the gas. Watch [this Updraft lesson](https://updraft.devcyfrin.com/courses/blockchain-basics/basics/making-your-first-transaction-on-zksync) if you need a refresher of how to do this. ::image{src='/chainlink-fundamentals/6-chainlink-ccip-messages/assets/zksync-plugin.png' style='width: 100%; height: auto;' alt='zksync-plugin'} 7. Get some test CCIP tokens (CCIP-BnM for Burn and Mint) by [calling `drip()` on the token contract using Etherscan](https://sepolia.etherscan.io/token/0xfd57b4ddbf88a4e07ff4e34c487b99af2fe82a05#writeContract). 9. Call `transferTokens` as before and check the CCIP explorer to check the status of your cross-chain transfer. 10. Check your balance of ZKsync Sepolia CCIP-BnM tokens has now increased in the Vault! See how awesome it is to deploy contracts and transfer your tokens cross-chain to ZKsync Speolia and automatically perform actions! Extra cookies for you if you managed the extra challenge:)