Mafia at the end 2

Tags

#Blockchain

You’re in the final step before catching them lacking. Prove yourself by winning at the casino and access the VIP room !

But remember, the house always win.

Note : To connect to the casino with Metamask, we recommend you to use another browser and a “trash” MetaMask wallet to avoid some weird behavior.

Author : Wefpen, Tzer

Flag format: PWNME{.........................}

Connect : nc mafia2.phreaks.fr 10020

by Wefpen and Tzer

Given Setup.sol and Casino.sol, we can see that the casino contract uses a PRNG to determine the outcome of the game. The PRNG is seeded with the prevrandao value from the previous block. We can use this information to predict the outcome of the game.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.26;

contract CasinoPWNME {

  bool public isWinner;

	uint256 public multiplier = 14130161972673258133;
	uint256 public increment = 11367173177704995300;
	uint256 public modulus = 4701930664760306055;
  uint private state;

  constructor (){
    state = block.prevrandao % modulus;
  }

  function checkWin() public view returns (bool) {
    return isWinner;
  }

  function playCasino(uint number) public payable  {

    require(msg.value >= 0.1 ether, "My brother in christ, it's pay to lose not free to play !");
    PRNG();
    if (number == state){
      isWinner = true;
    } else {
      isWinner = false;
    }
  }
  
  function PRNG() private{
    state = (multiplier * state + increment) % modulus;
  }

}

So we can see the state is at storage slot 0x4. We can get the value of the state by calling web3.eth.getStorageAt(casinoAddress, "0x4"). We can then calculate the next state by using the PRNG formula.

bool public isWinner;

uint256 public multiplier = 14130161972673258133;
uint256 public increment = 11367173177704995300;
uint256 public modulus = 4701930664760306055;
uint private state;

Here is the code to predict the outcome of the game:

const stateStorage = await web3.eth.getStorageAt(casinoAddress, "0x4");
console.log("State:", BigInt(stateStorage).toString());

const multiplier = BigInt("14130161972673258133");
const increment = BigInt("11367173177704995300");
const modulus = BigInt("4701930664760306055");

let state = BigInt(stateStorage) % modulus;
state = (multiplier * state + increment) % modulus;

Note: The code above is write to the casino website source code. and modify playCasino arguments from 0 to state value. (The challenge start instance error, so we can’t connect to the casino)

Mafia at the end 2 Flag: PWNME{th3_H0us3_41way5_w1n_bu7_sh0uld_be_4fr41d_0f_7h3_ul7im4te_g4m8l3r!}