Introduced by Moni Naor and Adi Shamir at EUROCRYPT 1994, Visual cryptography is a cryptographic technique which allows visual information (pictures, text, etc.) to be encrypted in such a way that decryption becomes a mechanical operation that may or may not require a computer. It is used to encrypt written material (printed text, handwritten notes, pictures, etc) in a perfectly secure way. The decoding is done by the human visual system directly, without any computation cost.
Three OR – Visual Cryptography Techniques:
- Divide image into two parts
- Separately, they are random noise
- Combination reveals an image
Some common mechanisms are explained below.
k out of n sharing problem:
For a set P of n participants, a secret image S is encoded into n shadow images called shares (shadows), where each participant in P receives one share. The original message is visible if any k or more of them are stacked together, but totally invisible if fewer than k transparencies are stacked together.
k out of n example (k=3,n=4):
General k out of k Scheme:
In k out of k, the image is visible only if all the shares are stacked together. If any share in k is lost, and remaining shares are stacked together, it will not form the image. Thus, in k out of k, all the shares are important to constructing the image.
k out of k example (k=n,n=3):
- Pixels are split
2. n shares per pixel
(Note: You will find more mechanisms in the project report)
Future Use and Applications:
- Remote Electronic Voting
- Anti-Spam Bot Safeguard
- Banking Customer Identification
- Message Concealment
- Key Management