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TenableCTF 2023 - Cyberpunk Cafe writeup

Cyberpunk Cafe was a challenge at TenableCTF 2023 from the ‘Stego’ category.
We received a single text file for this challenge.

The challenge

The challenge.txt file had the following contents

Thanks for visiting the Cyberpunk Cafe! Please use the code below to see what we have to offer:

0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001111111000100111110110000011111110000000010000010101111101111010000100000100000000101110101010011110111100101011101000000001011101010000010110010100010111010000000010111010101011111000100110101110100000000100000101010110000110010001000001000000001111111010101010101010101011111110000000000000000010111110010000100000000000000000001001111100010110001100110111110000000001111110000010101110101111011001010000000010011011110111101111100111101110100000000011011000011111100100100101001001000000000000101111011101101011011110000000000000011110100010001100010100111100100100000000001010100000010010010011011010101000000001101000110100111111010010101010000000000000011111111001100110101110100101100000000000001001100101000000111101001001000000001001111111111000110100010010010010000000010111001111000010111101101000101000000000001010111001111010100000101101000000000000011110110010111010111100111010010000000011100111110010100110100100100000100000000001100010111100101111010111111000000000001111011000001110110100001111101110000000000000000100101011100000110001010100000000111111101111010010010110101010101000000001000001011001110010000101000110100000000010111010001010000011111111111001100000000101110100110011011110111110011101000000001011101011011000100101111000110110000000010000010000011110010011011000100000000000111111100011011111010000111000001000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

The first thing that I could think of when seeing that string was that maybe it forms some patterns if printed with a specific number of characters per row.

So I looked at the length of that string: 1681 characters.
If you factorize 1681, you will see that it is 41 squared. This gave me quite an obvious option: a 41x41 square.

Also, if it is a square it is probably going to be a QR code.
This part is quite similar to an older challenge that I solved, so I used a similar approach.

from PIL import Image
import numpy as np

# Target string goes here
obfuscated = ''

# The obfuscated text is 1681 characters long
# 1681 = 41x41, we can probably arrange it in a 41x41 image
WIDTH = 41
HEIGHT = 41

image_pixels = np.zeros([41,41])

for line_nr in range(41):
	start = line_nr * WIDTH
	stop = (line_nr+1) * WIDTH
	line = obfuscated[start:stop]
	for pixel_nr, pixel in enumerate(line):
		if pixel == '1':
			image_pixels[line_nr][pixel_nr] = 255
		elif pixel == '0':
			image_pixels[line_nr][pixel_nr] = 0

img = Image.fromarray(image_pixels.astype('uint8'))
img.save('result.png')

The code is quite simple, it starts with a 41x41 numpy array and then it goes through the array of zeros and ones:

  • if the current char is a 0 it turns the corresponding pixel black
  • if the current char is a 1, the corresponding pixel turns white

After running it, I got a 41x41 black and white image.
This is how it looks after magnifiying it a bit (it was 41x41 … so it was small)

Resulting QR Code

Scan the code and you get the flag: flag{br1ng_b4ck_phys1c4l_menu5}