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ERC-20 Paymaster Tutorial

Learn how to pay gas fees with ERC-20 tokens

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This tutorial extends the Get Started example to include paying for gas using an ERC-20 token.

You can also follow along to a video explanation, if that's your style.

① Start a New Project

Clone the example repository from Github.

git clone -b [email protected]:JohnRising/userOpExample.git

Enter the project directory, and install the dependencies.

cd userOpExample
npm install

② Set Configuration

Go to your Stackup dashboard and get an API key (or Paymaster URL) for Sepolia.

Remember: In production, these should be secrets.

🔑

Get a Paymaster Key

 
const rpcUrl ="https://public.stackup.sh/api/v1/node/ethereum-sepolia";
const paymasterUrl = "https://api.stackup.sh/v1/paymaster/API_KEY";

③ Set the ERC-20 Paymaster

Scroll down to the paymasterContext field.

To send an ERC-20 sponsored transaction, you need to change the paymasterContext to the erc20token type. We will sponsor the transaction using the 6TEST token. The code should look like this:

  const paymasterContext = {
    type: 'erc20token',
    token: '0x68f180fcCe6836688e9084f035309E29Bf0A2095', // Ethereum Mainnet wBTC
  };

④ Approve the paymaster to withdraw the token

Call data contain the instructions that will be executed on-chain. In order to pay for gas, the sender needs to tell the ERC-20 token that the paymaster is approved to withdraw the tokens.

You can just add this to the callData of your user operation. The paymaster withdraws the token at the end of the user operation, so you don't need a separate transaction to approve the token.

This example approves the withdrawal of 1000 6TEST tokens (equivalent to $1000 in ETH). In production, it is much better practice to only approve the necessary amount of tokens.

const paymasterContract = "0xE93ECa6595fe94091DC1af46aaC2A8b5D7990770";
const approvedWithdrawAmount = 1000000000;

const callTo = [token,
  // ... rest of the contracts you are calling
];

const callData = [
  erc20.interface.encodeFunctionData('approve', [paymasterContract, approvedWithdrawAmount]),
  // ... rest of your call data
];

⑤ Get 6TEST tokens

The 6TEST token can be minted directly to your smart contract address. The fastest way to do this is with an existing EOA and Etherscan.

  1. Navigate to https://sepolia.etherscan.io/address/0x3870419Ba2BBf0127060bCB37f69A1b1C090992B#writeContract

  2. Mint tokens to the sender.

    1. Connect to Etherscan
    2. Navigate to the mint function
    3. Paste the address of your new smart account into the spender field
    4. Enter a high amount of tokens in the amount field. The 6TEST token has six decimals and is equivalent to 1 USD, so make sure the amount is at least 10 million or so.
    5. Click the Write button.

  4. Check the balance when the mint transaction completes.

    1. Navigate to the Read Contract tab
    2. Find the balanceOf function.
    3. Enter the sender address in the account field
    4. Click Query

⑥ Run the script

In your terminal, run npm run dev. Your output should look something like this:

Account address: 0x90583F2C1A3b35552fCAc8DB672e064E4B58944A
Signed UserOperation: {
  sender: '0x90583F2C1A3b35552fCAc8DB672e064E4B58944A',
  nonce: '0x3a',
  initCode: '0x',
  callData: '0x18dfb3c7000000000000000000000000000000000000000000000000000000000000004000000000000000000000000000000000000000000000000000000000000000a000000000000000000000000000000000000000000000000000000000000000020000000000000000000000003870419ba2bbf0127060bcb37f69a1b1c090992b0000000000000000000000003870419ba2bbf0127060bcb37f69a1b1c090992b0000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000004000000000000000000000000000000000000000000000000000000000000000c00000000000000000000000000000000000000000000000000000000000000044095ea7b3000000000000000000000000e93eca6595fe94091dc1af46aac2a8b5d79907700000000000000000000000000000000000000000000000000000000000989680000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000044a9059cbb00000000000000000000000090583f2c1a3b35552fcac8db672e064e4b58944a000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000',
  callGasLimit: '0xde17',
  verificationGasLimit: '0x13ab5',
  preVerificationGas: '0xd898',
  maxFeePerGas: '0x42e2440f5',
  maxPriorityFeePerGas: '0x21',
  paymasterAndData: '0xe93eca6595fe94091dc1af46aac2a8b5d799077000000000000000000000000000000000000000000000000000000000654e9b8900000000000000000000000000000000000000000000000000000000000000000000000000000000000000003870419ba2bbf0127060bcb37f69a1b1c090992b000000000000000000000000000000000000000000000000000000007c75667a77115a8a2f94e7bd2be3ca66faa1f17594a80e48d8a89be9f84eb7766f080f740e55d945c921ff3764d7247de5c041d047662b8f0768ff035affecf82561812d1b',
  signature: '0x8083fb7b6c9d5db5d2738fb9e15087364c727a4fd3b270c423aa6b8e906222ec35c5fe11857af619e7540b0d4a41d1bee0872b9868303f159b06c3d0fd3e347b1c'
}
UserOpHash: 0x76afd00873cfac153699b084a14c8d893a287e7131ca13e22a31c0da48ecac99
Waiting for transaction...
Transaction hash: 0x31f30973c44fcea4dc2f5a7e1a7ecc33ee6e36a013f660619011b9c0baedc0fb
View here: https://jiffyscan.xyz/userOpHash/0x76afd00873cfac153699b084a14c8d893a287e7131ca13e22a31c0da48ecac99

🚧

Public RPCs tend to be slow

If you are using the default public bundler URL, it may take you longer to get your transaction on-chain.

Click here for a private bundler URL from Stackup.

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Updated 5 months ago