Extended visual cryptography for natural images

Abstract

Extended Visual Cryptography is a type of cryptography which encodes a number of images in the way that when the images on transparencies are stacked together, the hidden message appears without a trace of original images. The decryption is done directly by the human visual system with no special cryptographic calculations. This paper presents a system which takes three pictures as an input and generates two images which correspond to two of the three input pictures. The third picture is reconstructed by printing the two output images onto transparencies and stacking them together. While the previous researches basically handle only binary images, this paper establishes the extended visual cryptography scheme suitable for natural images. Generally, visual cryptography suffers from the deterioration of the image quality. This paper also describes the method to improve the quality of the output images. The trade-off between the image quality and the security are discussed and assessed by observing the actual results of this method. Furthermore, the optimization of the image quality is discussed.