joaoaguiam

address public grantSigner;

function stake(address _userUportAddress, uint _expiryDate, bytes _signature) public payable whenNotPaused {
    bytes32 hashMessage = keccak256(abi.encodePacked(_userUportAddress, msg.value, _expiryDate));
    address signer = hashMessage.toEthSignedMessageHash().recover(_signature);

    require(signer == grantSigner, "Signature is not valid");
    require(block.timestamp < _expiryDate, "Grant is expired");
    require(userStakedAddress[_userUportAddress] == 0, "User has already staked!");