The Basics

Posted on Wed 07 August 2024 in ctf

This is a writeup of the blockchain challenge The Basics in the n00bz CTF 2024.

The attached file evm.txt contained the following hash:

5f346113370265fdc29ff358a314601257ff00

Using https://evmd.xyz/ we are able to decompile the bytecode which results in the following results:

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0;

/// @title            Decompiled Contract
/// @author           Jonathan Becker <jonathan@jbecker.dev>
/// @custom:version   heimdall-rs v0.8.3
///
/// @notice           This contract was decompiled using the heimdall-rs decompiler.
///                     It was generated directly by tracing the EVM opcodes from this contract.
///                     As a result, it may not compile or even be valid solidity code.
///                     Despite this, it should be obvious what each function does. Overall
///                     logic should have been preserved throughout decompiling.
///
/// @custom:github    You can find the open-source decompiler here:
///                       https://heimdall.rs

contract DecompiledContract {
    fallback() external payable {
        if (0xfdc29ff358a3 == (0x1337 * msg.value)) {
            selfdestruct(0);
        }

    }
}

To retriev the flag we need to resolve the if clause for msg.value e.g. with JavaScript:

// 0xfdc29ff358a3 == (0x1337 * msg.value)
flag = '0x' + (0xfdc29ff358a3 / 0x1337).toString(16);
console.log('n00bz{' + flag + '}');

This gives us the flag: n00bz{0xd34db33f5}