Guess New Random Number

Again, can you guess a random number? Be smart this time.

You might want to write a smart contract to defeat this level, and don't forget to withdraw the money back to your account! Note that all transactions in a block share the information (metadata) of the current block.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.11;

contract GuessNewRandomNumber {
    address player;

    constructor(address _player) payable {
        require(msg.value == 1 gwei);
        require(_player != address(0));
    }

    function solved() public view returns (bool) {
        return address(this).balance == 0 && address(player).balance == 0;
    }

    function guess(uint n) public payable {
        require(msg.value == 1 gwei);
        player = msg.sender;

        uint answer = uint(keccak256(
            abi.encodePacked(blockhash(block.number - 1), block.timestamp)));
        if (n == answer) {
            payable(msg.sender).transfer(2 gwei);
        }
    }
}

TIP

You have heard that blockhash() and block.timestamp is not a secure source of randomness (opens new window)!

You can solve 1-3 challenges from etherhack (opens new window) with a similar technique.