Research And Implementation Of Regional Incremental Visual Cryptography

A (k, n) image secret sharing scheme (ISSS) with threshold divides an secret image into n share shadows for n participants, whereby any k or more than k shadows can recover the original secret image, but any k-1 shadows can get nothing about the secret. Visual cryptography scheme (VCS) plays a very important role in image secret sharing scheme. In reconstructing the secret, participants could identify the secret by human visual system without other device or knowledge of cryptography. The model of visual cryptography is equivalent to one-time pad (OTP), and all shadows are indistinguishable. Therefore, the security of secret image is guaranteed.Visual cryptography is a developing secret sharing technique that has being extensively and deeply studied in last decade. Many papers about visual cryptography have been published in reputable international conferences and journals, such as the three top cryptography conferences, IEEE Transactions on Information Forensics and Security、 Journal of Cryptology、Designs, codes and cryptography.Recently, Wang proposed a novel binary region incrementing visual cryptography scheme (RIVCS) which can gradually reveal the secrets hidden in multiple secrecy regions of an image by human visual system. The property of region incrementing provides a beautiful application in visual secret sharing and makes that more shadows can be used to reconstruct more sub-secrets. This paper presents two new region incrementing visual cryptography schemes based on the analysis and research on the existing region incrementing visual cryptography schemes. The main works are as follows.1. Yang constructed a binary (k, n) region incrementing visual cryptography scheme. In his scheme, the contrasts of multiple secrecy regions reconstructed are not the same, which is not compatible with the traditional visual cryptographic scheme with the same contrast, and which may make the participants think that the secrets in different regions are of different importance. To make it compatible with the traditional VCS, we propose a (k, n)-RIVCS taking the advantage of linear programming, which has same contrast in all secrecy regions. Our solution addresses the problem of different contrast and that of reverse color. We prove that the scheme meets the requirement of security and region incrementing. Test shows that our scheme has the same contrasts which make the participants feel that the different secrecy regions are of the same importance, and there is no negative contrast region.2. The existing region incrementing visual cryptography schemes suffer from the larger pixel expansion and the reverse color problem (negative contrast). To reduce further the pixel expansion, we use integer linear programming to design a binary RIVCS with optimal pixel expansion. Compared with the previous region incrementing visual cryptography schemes, the proposed solution has smaller pixel expansion and higher contrast, but also overcome the reverse problem.