FlagChecker - Google CTF 2025 Writeup

A simple C++ binary that validates user input character-by-character using nested math equations. Extract the equations to calculate the flag.

🚩 Challenge Overview

  • Platform/Event: Google CTF 2025
  • Difficulty: Easy
  • Points: 110
  • Solves: 541
  • Category: Reversing
  • Tags: ELF x64, Static Analysis, Equation Solving, Z3 Solver

📝 Description

A simple C++ binary that validates user input character-by-character using nested math equations. Extract the equations to calculate the flag.

💡 Solution / Approach

  1. Open binary in IDA Pro. Locate the flag verification loop.

  2. The code performs linear transformations: (input[i] ^ key[i]) + 5 == target[i].

  3. Extract arrays key and target from data section.

  4. Write a simple Python Z3 script to solve the equations and rebuild the flag.