A Review on Visual Cryptography Schemes for Gray Scale and Color Images

Visual cryptography is a technique which combination of perfect ciphers and secret sharing in cryptography. A binary image can be divided into shares which can be stacked together to approximately recover the original image. Unfortunately, it has not been used much primarily because the decryption process entails a severe degradation in image quality in terms of loss of resolution and contrast. Its usage is also hampered by the lack of proper techniques for handling grayscale and color images, the technique has a unique flexibility which allows a single encryption of a color image but allows three types of decryptions on the same cipher text. The three different types of decryptions allow for the recovery of the image of varying qualities. The physical transparency stacking type of decryption allows for the recovery of the traditional visual cryptography quality image. An enhanced stacking technique allows for the decryption into a halftone quality image. And finally, a computation based decryption scheme makes the perfect recovery of the original image possible. In this paper, we have discussed various approaches for grayscale and color images in visual cryptography from starting to till now.