Secret Image Sharing using Fractal Codes

Abstract

Secret sharing is a well known problem of cryptography. A secret has to be shared among a group of N people, in such a way that only when at least K of them use their shares, the secret is revealed. A special case of this problem is secret image sharing, where the secret is an image. Prior work on image sharing has predominantly been visual secret sharing on binary images, where decoding is done by stacking the shares and viewing them. Some of the works address gray scale and color image sharing too. We target secret image sharing for gray scale images and propose a novel solution based on fractal encoding and decoding. The main advantage of our method as compared to the other schemes is that the space complexity of the shares is O(1). That is the sum total of space required for all the shares is independent of N. We have not seen this property in any of the previous works of image sharing. We achieve this by avoiding the use of images as shares, and using partial and modified fractal codes instead. Moreover, our method implicitly does compression along with image sharing. The proposed algorithm has decoding time complexity O(N), which is the state of the art. Experiments have shown that the proposed method is robust to security attacks.