---
title: "Solving a CTF flag checker"
url: "https://reamcp.dev/zh-cn/cases/solving-a-ctf-flag-checker-34ece1"
source: "https://rea.tools/showcase/ctf"
source_type: "website"
author: "REA team"
author_handle: "REA team"
stars: 0
category: "CTF 与学习"
categories: ["ctf-learning"]
tags: []
added: "2026-10-13T03:57:24.604Z"
updated: "2026-10-13T03:57:24.605Z"
locale: "zh-cn"
content_locale: "en"
---

# Solving a CTF flag checker

Inspects a DownUnderCTF binary, finds its checking functions, turns the masks and sums into equations and confirms the solved flag against the original program.

## Description

Why it is listed: A CTF challenge: the intended, legitimate use.

## Original README

Use REA to solve DownUnderCTF's masked squares flag checker. Find the checking functions, extract the masks and compare a recovered flag with the original program.