Writeup: Reversing - BlueHens CTF 2024

Problems I Can Do

• Intro to Reverse
• Cut The Flag

Intro to Reverse

Just a classic flagchecker.
(Try using [dogbolt.org](http://dogbolt.org/))

I uploaded the file to dogbolt.org as instructed in the challenge.

The Reverse Code (Python)

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var_48 = "ucaqbvl,n*d\\'R#!!l"
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flag = ""
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for i in range(len(var_48)):
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    # Increase the value of i for each character in var_48.
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    char = chr(ord(var_48[i]) + i)
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    flag += char
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print("Flag:", flag)

After running the script, I got the flag:

udctf{r3v3ng3_101}

Cut The Flag

I received the file CutTheFlag v0.0.zip and extracted its contents.
Inside the CutTheFlag v0.0.zip file, I found the game:

It’s a game where we have to cut the flags. As we cut them, the cutter speed increases, making it harder to press the button in time as the number of cuts grows.

To tackle this, I used dnSpy to reverse engineer the game. I located the file, which is found in , and edited the code.Assembly-CSharp.dll CutTheString_Data\Managed\Assembly-CSharp.dll

In the GameManager class, I modified the NextLevel() method. Instead of incrementing curLevel by 1 with this.curLevel++, I changed it to increment by 20 using this.curLevel += 20. This was to test if jumping ahead by 20 levels would work as intended.

After making the change, I got the expected result — the level increased by 20.

What??????

I tried playing the game again, and I finally got the flag!

The flag is:

UDCTF{a_d1fF3R3nT_p3rsp3ct1v3}

I brainstormed on the Cut The Flag challenge with Autsada Wiriya..