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Coin Flip

The goal of this level is to correctly guess the outcome of a coin flip 10 times in a row. The number of consecutiveWins is initialized as 0 within the constructor(). If a wrong guess is passed at any point, the consecutiveWins is reset to 0.

The issue with this contract is that Ethereum is a deterministic Turing machine, meaning there is no inherent randomness on the EVM. Developers mistakenly use data related to blocks, such as block.number, or block.timestamp, to "achieve" randomness, however these values are not random and can be exploited. Here, flip() introduces what appears to be the start of randomness by calculating the blockhash:

uint256 blockValue = uint256(blockhash(block.number - 1));

The blockValue is then divided by FACTOR which is used to determine coinFlip - the coin flip result. FACTOR is the third state variable, which is equal to 57896044618658097711785492504343953926634992332820282019728792003956564819968 - we know what factor the blockhash is being divided by every single guess as it is available to us, removing any credible randomness. A check is then made to see whether coinFlip == 1, returning true or falseusing Solidity's ternary operator:

bool side = coinFlip == 1 ? true : false;

Then, _guess is provided to flip() as a parameter. Within the function, the user _guess is checked against side to see whether the correct guess was provided. This is problematic as we are able to calculate the correct guess using the blockhash, FACTOR, and returning whether the division of the two is equal to 1. This is demonstrated in the AttackCoinFlip.sol contract provided in this folder.

In AttackCoinFlip.sol we calculate the correct guess using the logic directly from CoinFlip.sol:

function calculateGuess() private view returns (bool) {
        uint256 blockValue = uint256(blockhash(block.number - 1));
        uint256 coinFlip = blockValue / FACTOR;

        return coinFlip == 1;
}

And with calculateGuess(), we can return the correct guess every time in a separate external function:

function flip() external {
        bool correctGuess = calculateGuess();
        require(target.flip(correctGuess), "Guess failed");
}

In order to complete this level, simply call flip() 10 times.