A (n, n) Extended Visual Cryptography Technique to Share Grayscale Image

Abstract

Visual Cryptography (VC) is used for sharing secret images among various participants. Different VC techniques are available to share binary, grayscale and colour images. Basic advantage of VC is that the secret image can be revealed without any computation. Now-a-days due to evolution of digital world the internet and computing devices are available everywhere. So, for revealing the secret image, computing devices can be used instead of printing the share on transparencies. Also, it is easy to maintain the transparencies in digital form rather than printed transparencies. In this paper, we have developed a (n, n) visual cryptography technique, where n ≤ 8, to share grayscale images. Shares generated are meaningful and hence it avoids suspicion. The recovered secret image is totally lossless. Each pixel of grayscale secret image is decomposed into 8 binary bits and these bits are encrypted in the shares. Every pixel of secret image is converted into a block of 3×3 pixels, hence the dimensions of cover image are increased by three times. There is no pixel expansion in this technique, it means the dimensions of original secret image and recovered secret image are same.