Progressive color visual cryptography

Visual cryptography is a powerful technique that combines the notions of perfect ciphers and secret sharing in cryptography with that of raster graphics. A binary image can be divided into shares that 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. We develop a novel technique that enables visual cryptography of color as well as gray-scale images. With the use of halftoning and a novel microblock encoding scheme, the technique has a unique flexibility that enables a single encryption of a color image but enables three types of decryptions on the same ciphertext. The three different types of decryptions enable the recovery of the image of varying qualities. The physical transparency stacking type of decryption enables the recovery of the traditional visual cryptography quality image. An enhanced stacking technique enables the decryption into a halftone quality image. Finally, a computation-based decryption scheme makes the perfect recovery of the original image possible. Based on this basic scheme, we establish 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 enables flexible decryption. We implement our technique and present results. (c) 2005 SPIE and IS&T.