Tag: Flag Checker

Flag Checker

Flag checkers are a common type of challenge in the Reverse Engineering category.

An executable or script called a flag checker takes the player's input and outputs whether that input matches the flag.

These challenges may also be called "crackmes" in a broader sense. (automatically translated from Japanese)

·Updated May 3, 2026

Flag Checker Challenges

Optimized expression

Daily AlpacaHackTopic: OptimizationReleased: Mar 26, 2026

127 solves

tan90909090

Tags

Even if an expression looks complex, it may be an optimized form of a simpler expression.

Beginner Hint

This challenge is in the Rev (Reverse Engineering) category.
Flag checkers are a common theme in Rev problems.
A flag checker executable or script accepts player input and tells you whether it matches the flag.
The provided files are the ELF binary chal and the C source code chal.c.
The goal is to find an input (the flag) that makes the binary print Correct! by analyzing the source code.