Efficient Visual Cryptography

Abstract

Visual cryptography scheme (VCS) is a secret- sharing scheme which allows the encryption of a secret image into n shares that are distributed to n participants. The beauty of such a scheme is that, the decryption of the secret image requires neither the knowledge of cryptography nor complex computation. Colour visual cryptography becomes an interesting research topic after the formal introduction of visual cryptography by Naor and Shamir in 1995. It is a powerful technique which combines the notions of perfect ciphers and secret sharing in cryptography with that of raster graphics. 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. In this paper, I have developed a novel technique which enables visual cryptography of color as well as grayscale images. 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. Based on this basic scheme, I have then established a progressive mechanism to share color images at multiple resolutions. I extracted shares from each resolution layer to construct a hierarchical structure; the images of different resolutions can then be restored by stacking the different shared images together. I have implemented our technique and present results.