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Pixelated

Challenge information

Points: 100
Tags: picoCTF 2021, Cryptography
Author: SARA

Description:
I have these 2 images, can you make a flag out of them? 

scrambled1.png scrambled2.png

Hints:
1. https://en.wikipedia.org/wiki/Visual_cryptography
2. Think of different ways you can "stack" images

Challenge link: https://play.picoctf.org/practice/challenge/100

Solution

There are several ways to solve this challenge and here are two of them.

Stegsolve solution

You can use StegSolve to combine the pictures. However, I never got the current 1.4 version to work and used the former 1.3 version instead.

In StegSolve 1.3, open the scrambled1.png file. Then, in the Analyse-menu select Image Combiner and select the scrambled2.png file. A new window opens where you can step through various ways to combine the images: XOR, OR, AND, ADD, SUB, etc.

You will find the flag with the ADD-method.

Write a Python script

An alternative way to solve this challenge is to write a Python script with the help of the Python Imaging Library - Pillow and numpy

#!/usr/bin/python

from PIL import Image
from numpy import array

image1 = Image.open('scrambled1.png')
image2 = Image.open('scrambled2.png')

# Convert to arrays
array1 = array(image1)
array2 = array(image2)

# Combine/add the images
result = array1 + array2

# Save the result
Image.fromarray(result).save('flag.png')
print("Result saved as flag.png")

Then we run the script to combine the images

┌──(kali㉿kali)-[/mnt/…/picoCTF/picoCTF_2021/Cryptography/Pixelated]
└─$ ~/python_venvs/Pillow/bin/python pixelaated.py
Result saved as flag.png

Finally, use eog or feh to view the flag.png image to get the flag.

For additional information, please see the references below.

References