Volga 2017 Quals – VC

The VC task is under the category Crypto.

VC

There are files A.png and B.png. But where’s the flag?
A.png
B.png

Solution:
First, create two layers in Adobe photoshop or whatever image editor you have.
On the first layer (layer 0), insert the image A.png and B.png on the second layer (layer 1).
Second, set the opacity of B.png (Layer 1) to 50%.
 
Next, select the 2 layers and change the mode from “Normal” to “Difference”

Lastly, set the B.png (Layer 1) to 100% opacity to read the hidden message.

Visual cryptography is a cryptographic technique which allow visual information (pictures, text, etc.) to be encrypted in such a way that decryption becomes the job of the person to decrypt via sight reading.One of the best-known techniques has been credited to Moni Naor and Adi Shamir, who developed it in 1994.[1] They demonstrated a visual secret sharing scheme, where an image was broken up into n shares so that only someone with all n shares could decrypt the image, while any n -1 shares revealed no information about the original image.

Your flag,
VolgaCTF{Classic_secret_sharing_scheme}

 

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