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Call a Contract From Bitcoin

Call a Contract From Bitcoin

In this tutorial you will create an omnichain contract for minting ERC-20 tokens on ZetaChain. Omnichain contracts can be called from any connected blockchain, but in this tutorial you will call the contract from Bitcoin.

Overview:

  • A user sends tBTC to a TSS address on Bitcoin with a memo containing an omnichain contract address and the recipient's address.

  • ZetaChain detects the transaction and calls the onCrossChainCall function of the omnichain contract.

  • The onCrossChainCall

    • checks if the cross-chain call was made from Bitcoin. If not, it reverts.
    • mints ERC-20 tokens (the same amount as tBTC sent in the first step) on ZetaChain and sends them to the recipient.

Set Up Your Environment​

Clone the Hardhat contract template:

git clone https://github.com/zeta-chain/template

Install dependencies:

cd template
yarn add --dev @openzeppelin/contracts

Create the Contract​

Create a new omnichain contract Minter that expects to see a recipient address in the message:

npx hardhat omnichain Minter recipient:address
contracts/Minter.sol
// SPDX-License-Identifier: MIT
pragma solidity 0.8.7;

import "@zetachain/protocol-contracts/contracts/zevm/SystemContract.sol";
import "@zetachain/protocol-contracts/contracts/zevm/interfaces/zContract.sol";
import "@openzeppelin/contracts/token/ERC20/ERC20.sol";

contract Minter is zContract, ERC20 {
    error SenderNotSystemContract();
    error WrongChain();

    SystemContract public immutable systemContract;
    uint256 public immutable chain;

    constructor(
        string memory name,
        string memory symbol,
        uint256 chainID,
        address systemContractAddress
    ) ERC20(name, symbol) {
        systemContract = SystemContract(systemContractAddress);
        chain = chainID;
    }

    function onCrossChainCall(
        zContext calldata context,
        address zrc20,
        uint256 amount,
        bytes calldata message
    ) external virtual override {
        if (msg.sender != address(systemContract)) {
            revert SenderNotSystemContract();
        }
        address recipient = abi.decode(message, (address));
        address acceptedZRC20 = systemContract.gasCoinZRC20ByChainId(chain);
        if (zrc20 != acceptedZRC20) revert WrongChain();

        _mint(recipient, amount);
    }
}

Contract's constructor accepts a name of the token, a symbol, a chain ID of the chain from which the contract is allowed to be called (in our example, we will provide Bitcoin Testnet's chain ID) and a system contract address.

When onCrossChainCall is called, the contract checks if the call was made from the allowed chain. If not, it reverts. If the call was made from the allowed chain, the contract converts the message from bytes into the recipient address and mints new ERC-20 tokens to that address.

Modify the Deploy Task​

tasks/deploy.ts
const main = async (args: any, hre: HardhatRuntimeEnvironment) => {
  // ...
  const contract = await factory.deploy(
    "Wrapped tBTC",
    "WTBTC",
    18332,
    systemContract
  );
  // ...
};

task("deploy", "Deploy the contract", main);

Deploy the Contract​

Clear the cache and artifacts, then compile the contract:

npx hardhat compile --force

Deploy the contract to ZetaChain:

npx hardhat deploy --network zeta_testnet
🔑 Using account: 0x1bE17D79b60182D7F3573576B7807F6C20Ae7C99

🚀 Successfully deployed contract on ZetaChain.
📜 Contract address: 0xE26F2e102E2f3267777F288389435d3037D14bb3
🌍 Explorer: https://athens3.explorer.zetachain.com/address/0xE26F2e102E2f3267777F288389435d3037D14bb3

Calling the contract from Bitcoin Testnet​

Ensure that you have an account and tBTC on the Bitcoin Testnet. You can get some from a faucet.

Use the send-btc command to send tBTC to the TSS address on Bitcoin Testnet set as the --recipient:

npx hardhat send-btc --amount 0.001 --memo 629eEe97B95Bd6e04B0885De58eF016177a709Ae2cD3D070aE1BD365909dD859d29F387AA96911e1 --recipient tb1qy9pqmk2pd9sv63g27jt8r657wy0d9ueeh0nqur

The memo contains the following:

  • the address of the omnichain contract on ZetaChain that will be called once the cross-chain transaction is processed without the 0x prefix: 629eEe97B95Bd6e04B0885De58eF016177a709Ae.
  • a list of arguments that will be passed to the contracts as the message bytes. In our case, it's a single value, the recipient's address without the 0x prefix: 2cD3D070aE1BD365909dD859d29F387AA96911e1.

You can learn more about how to construct the memo in the Bitcoin section.

The send-btc command will return the transaction hash. You can use the cctx command to track the status of the cross-chain transaction:

npx hardhat cctx --tx TX_HASH

Add the token contract address 0x629eEe97B95Bd6e04B0885De58eF016177a709Ae to Metamask (or any other wallet) to be able to check the balance of the ERC-20 token minted by the contract.

Once the transaction is processed, you will be able to see the udpated balance of the WTBTC token in Metamask.