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Gas Fee Payment in wBTC using Meta Transactions (OpenGSN & ERC-2771)

In this example, we will show how wBTC can be used for gas fee payments using the OpenGSN and ERC-2771 standard on BOB Sepolia (Testnet). This enables users to transact without the necessity to own ETH.

Live Demo

Check out the live demo at demo-meta-transactions.gobob.xyz.

Example Code

Check out the code of the demo in this repository.

OpenGSN​

OpenGSN, or the Ethereum Gas Station Network, is a decentralized solution abstracting gas fee payments away from users. OpenGSN enables gasless transactions, allowing users to interact with smart contracts without needing ETH. It provides use cases such as privacy-focused transactions, payments in ERC-20 tokens, off-chain payments, and onboarding subsidies. Learn more in OpenGSN docs.

Smart contract changes​

To enable OpenGSN functionality in smart contracts, developers need to make specific modifications. The smart contracts must inherit from the ERC2771Recipient contract. It's crucial to note that, when working with GSN recipient contracts, developers must refrain from using msg.sender directly. Instead, they should utilize _msgSender(), a function provided by ERC2771Recipient. This ensures accurate retrieval of user addresses during transactions and facilitates seamless integration with the Gas Station Network. Read the full smart contract modification instructions here.

preview

Using the dApp​

This application contains a simple form that allows you to transfer wBTC between accounts with the gas fee paid in wBTC. Simply enter the wBTC amount and the recipient's EVM address and the transaction will be sent to the Open Gas Network relay and relayed using the wBTC paymaster.

note

Before the first relayed transaction is done, the paymaster smart contract has to be approved to spend your wBTC. That is why there will be a transaction request before the first relayed transfer transaction.

Try it out​

Connecting MetaMask and funding your account​

  1. Go to demo-meta-transactions.gobob.xyz
  2. Fund your account with Sepolia ETH from the Sepolia testnet faucet.
  3. Transfer Sepolia ETH to BOB using our Bridge.
  4. Connect with your MetaMask account and get wBTC by clicking on the 'Get Tokens' button in the application header. (You will receive 30,000 wBTC that you can spend.)
  5. Add wBTC to your MetaMask, wBTC address is 0x2868d708e442A6a940670d26100036d426F1e16b

Making a transfer​

  1. Input the amount you wish to send and the recipient's address into the form fields.
  2. Click on 'Approve & Transfer' to approve the paymaster to spend your wBTC and to sign the relayed transaction.
  3. Wait for the transfer transaction to be relayed.

Local development​

  • WBTC contract address: 0x2868d708e442A6a940670d26100036d426F1e16b
  • Paymaster contract address: 0x25Aa86d188E37A47dd2011535534E53Cf994559d
  • OpenGSN relayer: https://gsn-relay-sepolia.gobob.xyz/

Installing the project​

  1. Install pnpm
  2. Run pnpm install

Starting the project​

  1. Run pnpm run dev
  2. Open localhost:5173 in browser.

Interacting with OpenGSN relay​

To allow simple interaction with the relay ethers.Contract instance is created using OpenGSN provider:

const getRelayedContract = async (contractType: ContractType) => {
  if (!window.ethereum) {
    throw new Error('Injected provider not found!')
  }

  const { address, abi } = contracts[contractType]

  const config = {
    preferredRelays: ['https://gsn-relay-sepolia.gobob.xyz/'],
    performDryRunViewRelayCall: false,
    gasPriceSlackPercent: 1000,
    maxPaymasterDataLength: 100,
    paymasterAddress: erc20PaymasterAddress,
  }

  const gsnProvider = await RelayProvider.newProvider({
    provider: window.ethereum,
    config,
    overrideDependencies: { asyncPaymasterData: getErc20PaymasterData },
  }).init()

  const ethersProvider = new providers.Web3Provider(gsnProvider)

  const relayedContract = new Contract(address, abi, ethersProvider.getSigner())

  return relayedContract
}

Then the Contract instance can be used in a standard way:

const transferTx = await relayedContract.transfer(
  form.address,
  atomicAmount.toString()
)

await transferTx.wait()