Progressive color visual cryptography

. Visual cryptography,is a powerful,technique,that com- bines the notions of perfect ciphers and secret sharing in cryptogra- phy 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 im- age quality in terms of loss of resolution and contrast. Its usage,is also hampered,by the lack of proper techniques,for handling,gray- scale 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 recov- ery 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 es- tablish a progressive,mechanism,to share color images,at multiple resolutions. We extract shares,from each,resolution layer to con- struct a hierarchical structure; the images,of different resolutions can,then be restored,by stacking,the different shared,images,to- gether. Thus, our technique enables flexible decryption. We imple- ment,our,technique,and,present,results. ©,2005 SPIE and IS&T. DOI: 10.1117/1.1993625