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CTF#04 Solution

Text File Saved as Zip

Number of files: 1

Flag pattern: flag#**********

Flag: flag#gandhalf13

Solvable on which OS: Windows

This solution’s OS: Windows 11

Software: WinZip / WinRar, Text editor

Estimated solution time: 3 minutes

Internet access required: For software installation only

Details

Goal

The user will try to open the file and realize it’s in the wrong format, and then try to see which format it is. Opening the file in a text editor (e.g., Notepad), with or without changing the file’s extension to .txt, will immediately reveal the flag.

Implementation

We received the file wrong format.zip:

When we try to open the file, we get an error message:

Something’s not working. This must be a mistake. But maybe it is deliberate? We also realize the file’s size is tiny: 15 bytes in total.

If the file is so small, it won’t hurt to try to see what’s inside it.
So, let’s open it in a text editor:

(Changing the file’s extension to .txt will launch a text editor by default to open the file)

And that’s our flag.

 

And there’s our flag.

Even if we open the file in a text editor without changing the file’s extension, the result is still the same:

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