How to Create an Off-Chain NFT Allowlist

$
node -v

$
mkdir offchain-allowlist && cd offchain-allowlist
>
npm init -y
>
npm install --save-dev hardhat
>
npx hardhat

$
npx hardhat test

$
npm install @openzeppelin/contracts

1
//SPDX-License-Identifier: MIT
2
pragma solidity ^0.8.4;
3
4
import "hardhat/console.sol";
5
import "@openzeppelin/contracts/utils/Counters.sol";
6
import "@openzeppelin/contracts/access/Ownable.sol";
7
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol";
8
import "@openzeppelin/contracts/utils/cryptography/ECDSA.sol";
9
10
contract NFTAllowlist is ERC721Enumerable, Ownable {
11
    using Counters for Counters.Counter;
12
    
13
    Counters.Counter private _tokenIds;
14
15
    // Signature tracker
16
    mapping(bytes => bool) public signatureUsed;
17
    
18
    constructor() ERC721("NFT Allowlist Demo", "NAD") {
19
        console.log("Contract has been deployed!");
20
    }
21
22
    // Allowlist addresses
23
    function recoverSigner(bytes32 hash, 
24
													 bytes memory signature) 
25
						 public pure returns (address) {
26
		    bytes32 messageDigest = keccak256(
27
	        abi.encodePacked(
28
	          "\x19Ethereum Signed Message:\n32", 
29
            hash
30
	        )
31
		    );
32
		    return ECDSA.recover(messageDigest, signature);
33
		}
34
35
    // Presale mint
36
    function preSale(uint _count, bytes32 hash, bytes memory signature) public {
37
        require(recoverSigner(hash, signature) == owner(), "Address is not allowlisted");
38
        require(!signatureUsed[signature], "Signature has already been used.");
39
40
        for (uint i = 0; i < _count; i++) {
41
            _mintSingleNFT();
42
        }
43
44
        signatureUsed[signature] = true;
45
    }
46
    
47
    function _mintSingleNFT() private {
48
        uint newTokenID = _tokenIds.current();
49
        _safeMint(msg.sender, newTokenID);
50
        _tokenIds.increment();
51
    }
52
}

$
npx hardhat compile

1
const { ethers } = require("hardhat");
2
const hre = require("hardhat");
3
4
async function main() {
5
6
  // Define a list of allowlisted wallets
7
  const allowlistedAddresses = [
8
    '0x70997970c51812dc3a010c7d01b50e0d17dc79c8',
9
    '0x3c44cdddb6a900fa2b585dd299e03d12fa4293bc',
10
    '0x90f79bf6eb2c4f870365e785982e1f101e93b906',
11
    '0x15d34aaf54267db7d7c367839aaf71a00a2c6a65',
12
    '0x9965507d1a55bcc2695c58ba16fb37d819b0a4dc',
13
  ];
14
15
  // Select an allowlisted address to mint NFT
16
  const selectedAddress = '0x90f79bf6eb2c4f870365e785982e1f101e93b906'
17
18
  // Define wallet that will be used to sign messages
19
  const walletAddress = '0xf39fd6e51aad88f6f4ce6ab8827279cfffb92266';
20
  const privateKey = '0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80';
21
  const signer = new ethers.Wallet(privateKey);
22
  console.log("Wallet used to sign messages: ", signer.address, "\n");
23
24
  let messageHash, signature;
25
26
  // Check if selected address is in allowlist
27
  // If yes, sign the wallet's address
28
  if (allowlistedAddresses.includes(selectedAddress)) {
29
    console.log("Address is allowlisted! Minting should be possible.");
30
31
    // Compute message hash
32
    messageHash = ethers.utils.id(selectedAddress);
33
    console.log("Message Hash: ", messageHash);
34
35
    // Sign the message hash
36
    let messageBytes = ethers.utils.arrayify(messageHash);
37
    signature = await signer.signMessage(messageBytes);
38
    console.log("Signature: ", signature, "\n");
39
  }
40
41
  const factory = await hre.ethers.getContractFactory("NFTAllowlist");
42
  const [owner, address1, address2] = await hre.ethers.getSigners();
43
  const contract = await factory.deploy();
44
45
  await contract.deployed();
46
  console.log("Contract deployed to: ", contract.address);
47
  console.log("Contract deployed by (Owner/Signing Wallet): ", owner.address, "\n");
48
49
  recover = await contract.recoverSigner(messageHash, signature);
50
  console.log("Message was signed by: ", recover.toString());
51
52
  let txn;
53
  txn = await contract.preSale(2, messageHash, signature);
54
  await txn.wait();
55
  console.log("NFTs minted successfully!");
56
57
}
58
59
main()
60
  .then(() => process.exit(0))
61
  .catch((error) => {
62
    console.error(error);
63
    process.exit(1);
64
  });

$
npx hardhat run scripts/run.js

$
Wallet used to sign messages:  0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266 
>
>
Address is allowlisted! Minting should be possible.
>
Message Hash:  0x52d01c65d2e6acff550def14b5ce5bf353ac7ad53b132fc531c5d085d77c4ee3
>
Signature:  0x55d2baf93dff9184dea51cc81c7837c0d65e01962d7292f03afa80cedf3dcdb948ada6cae3bc49ae56d3546019ebd62cc56efe1ef9c946a7c4fe66f21596e6c91c 
>
>
Contract has been deployed!
>
Contract deployed to:  0x5FbDB2315678afecb367f032d93F642f64180aa3
>
Contract deployed by (Owner/Signing Wallet):  0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266 
>
>
Message was signed by:  0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266
>
NFTs minted successfully!