Introduction
Visual cryptography uses the human brain's visual system to execute computations. According to Naor and Shamir, the secret might be a rectangular image, I, made up of only black and white pixels. They took into account a threshold scheme scenario in which the shares for this "secret" are similar images made up of black and white pixels, and reconstructing the secret entails superimposing a subset of shares.
The reconstruction is carried out "visually" by simply stacking the shares if the shares are printed on transparencies. Therefore, a scheme of this kind is referred to as a visual threshold scheme.
Visual Threshold Scheme
Naor and Shamir introduced visual cryptography. With the help of a novel cryptographic paradigm, a hidden image can be divided into n shares, each printed on a transparency. Only some of the n participants are eligible to recover the original image. Therefore the shares are divided among them. The secret image is recreated by stacking k (2 ≤ k ≤ n) of these transparencies from the group of eligible participants. The original image cannot be decoded when fewer than k transparencies are placed.
A (k, n)-visual threshold scheme is the name of the resulting cryptographic scheme. Contrary to conventional cryptography algorithms, where a significant amount of processing is required to reconstruct the plain text, human visual systems perform the reconstruction. Therefore no computations are required during decoding.
Black and white graphics were used in the Naor and Shamir systems. The concept was further developed to include both color and grayscale images. Black and white photos can be encrypted in several effective methods, while grayscale and color images are more difficult to secure. Color photographs are particularly challenging to encrypt for two reasons.
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First, it doesn't appear that a color image can be encrypted using a tiny pixel expansion (a method has a pixel expansion of m if each original image pixel is stored as m subpixels on each transparency).
- The brightness of the colors in the reconstructed image is the second issue with color image encryption. The brightness of the reconstructed image is subpar in every scheme for color visual cryptography that has been put forth so far.
This is because, after stacking the required number of transparencies, the proportion of subpixels that retain the color of the original encoded pixel is relatively minimal.
Since the ratio is:
The number of subpixels that possess the true color / the total number of subpixels.
It requires a different definition because it affects the brightness of the reconstructed image.
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