Progressive Color Visual Cryptography ( Final version of the manuscript ID : JEI 03158 )

Visual cryptography 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, we have developed a novel technique which enables visual cryptography of color as well as grayscale images. With the use of halftoning and a novel microblock encoding scheme, the technique has a unique flexibility which allows a single encryption of a color image but allows three types of decryptions on the same ciphertext. 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. Based on this basic scheme, we have then established a progressive mechanism to share color images at multiple resolutions. We extract 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. Thus, our technique allows for flexible decryption. We have implemented our technique and present results.