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Tag: Flag Checker

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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)

minaminao
·Updated May 3, 2026
Flag Checker Challenges

AlpacaForm

Daily AlpacaHackTopic: .NETReleased: May 28, 2026

81 solves
Rev

by

shiragi

shiragi

"This challenge can be solved on Linux alone."

Beginner Hint 1: Challenge Overview (AI-translated)
  • The distributed files are a .NET application written in C# and a flag checker that determines whether the entered string is the correct flag.
  • AlpacaForm.exe is the executable file used to launch the app on Windows. The C# code containing the actual validation logic is in AlpacaForm.dll.
  • .dll stands for Dynamic Link Library. In general, it is a file that contains code or data loaded and used by executable files or other programs.
Beginner Hint 2: Reverse Engineering (AI-translated)
  • AlpacaForm.dll can be decompiled and read in a form close to C# using a .NET decompiler such as ILSpy.
  • For example, try running ilspycmd AlpacaForm.dll to decompile it (Ref).
  • After decompiling it, look for the function that validates the entered string.
alpaca-form.tar.gz

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