Visual Cryptography Explained#

Visual cryptography is a special encryption method where:

• A secret message is split across multiple images
• Each image alone looks like random noise or garbage
• When you overlay (stack) the images together, the secret message becomes visible to the human eye
• No complex math or computer decryption is needed!

Imagine you have a secret message. Instead of encrypting it with math, you break it down into pixel patterns across multiple transparent sheets. When someone places all sheets on top of each other and holds them up to the light, the message appears!

The images#

When we look at the two images, they appear to be completely random:

• scrambled1.png - looks like random noise/static
• scrambled2.png - looks like random noise/static

But when combined correctly, they reveal the hidden message!

Step 1: Understanding how to combine the images#

When combining two noisy images that contain a hidden message:

• The random noise in each image is different and unpredictable
• The hidden message is encoded in BOTH images at the same locations
• When we add the pixel values together, the message pixels become much brighter (because they’re bright in both images)
• The random noise stays relatively dark (because it’s different in each image)

Think of it this way: if both images have a white pixel in the same spot, adding them together gives an even brighter white. But if only one image has noise at a location, adding them gives a medium gray.

Step 2: Solution#

We load both images, convert them into arrays, and add their pixel values together.

Here’s the Python solution:

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from PIL import Image
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import numpy as np
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# Load both images
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img1 = Image.open('scrambled1.png')
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img2 = Image.open('scrambled2.png')
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# Convert to numpy arrays (keeps RGB format)
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array1 = np.asarray(img1)
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array2 = np.asarray(img2)
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# Add the two images together
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# Where both have bright pixels (the message), result is very bright
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# Where they differ (noise), result is medium brightness
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result = np.add(array1, array2)
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# Convert back to PIL Image
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result_img = Image.fromarray(result, mode="RGB")
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# Display and save the result
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result_img.show()
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result_img.save('flag.png')

Step 3: Read the flag#

After running the script, the hidden text appears in the generated flag.png file:

Flag: picoCTF{8cdf93c3}

⚡ Raikiri

🎉 Flag pwned!

💡 TL;DR / Lesson Learned

1. Each image alone is useless, the secret only appears when both are combined
2. Pixel addition is enough; no complex math or encryption is needed
3. Bright pixels in both images reinforce each other
4. Visual cryptography relies on human vision, not cryptographic algorithms
5. This challenge is about observation and image processing, not math