Help with Challenge [SafeMath]

Question

I am currently taking a course in Solidity and we get presented with weekly challenges that we should try to solve. This time the general topic is SafeMath and the challenge looks the following:

contract HopAndMaltOracle{
    // Contract oracle
    address public oracle;

   modifier onlyOracle(){
        require(msg.sender == oracle);
       _;
    }
    uint public maltMarketCap;
    uint public hopMarketCap;

    // Callback function
    event MaltMarketCapChanged();
    event HopMarketCapChanged();

    constructor(uint _maltCap, uint _hopCap) public payable {
        oracle = msg.sender;
        maltMarketCap = _maltCap;
        hopMarketCap = _hopCap;
        emit MaltMarketCapChanged();
        emit HopMarketCapChanged();
    }

    function updateMaltMarketCap(uint _cap) public onlyOracle {
        maltMarketCap = _cap;
        emit MaltMarketCapChanged();
    }

    function updateHopMarketCap(uint _cap) public onlyOracle {
        hopMarketCap = _cap;
        emit HopMarketCapChanged();
    }

    function IDidIt() public{
        require(address(this).balance % 2 == 1);
        msg.sender.transfer(address(this).balance);
    }
}

The goal is to reduce the balance of the contract to 0 (by somehow calling IDidIt()). Now looking at the contract it seems to me like none of the methods can be invoked since all of them are either using the onlyOracle modifier or are the constructor (except for IDidIt() ).

My idea would be to send Ether to the contract in order to increase the balance to something that would satisfy

require(address(this).balance % 2 == 1);

but I can't find a way to do so since the fallback function is not acceppting my payments (tried with:

eth.sendTransaction({from:myAddress,to:contractAddress,value:1});

The starting balance of the contract is 2 Ether. I am not asking for a solution, but a nudge in the right direction would definitely help :)

Answer 1

The constructor is payable. The oracle stores the address of the deployer

Answer 2

It turned out there are some ways to force Ether on a contract without having to directly send it through fallback or other functions, these are:

• transfer to a contract named as the receiver of a mining reward by a coinbase transaction
• transfer to a contract if it is the destination of a selfdestruct command
• if Ether was sent to the address of the contract before it was deployed

The second option turned out to be the correct one for this exercise. I created a contract with a payable method, deposited 1 Wei on it and then called suicide(challengeContractAddress), which transfered the balance of 1 Wei to the challenge contract and allowed me to call IDidIt().

Thanks to everyone for helping!

Answer 3

Firstly, why do you have your constructor as payable ? You don't even store the msg.value.

Secondly, you should have your fallback function as payable if you want to send it an ether value.