Efficient Visual Cryptography for General Access Structures with Stamping and Synthesizing

Visual cryptography is a popular solution for image encryption. The encryption procedure encrypts a image into the so-called shares which are noise-like secure images which can be transmitted or distributed over an untrusted communication channel. Using the properties of the human visual system to force the recognition of a secret message from overlapping shares, the image is decrypted without additional computations and any knowledge of cryptography. Visual cryptographic solutions operate on binary or binarized inputs. The natural (continuous-tone) images must be first converted into halftone images by using the density of the net dots to simulate the original gray or color levels in the target binary representation. Then, the halftone version of the input image is used instead of the original image to produce the shares. The decrypted image is obtained by stacking the shares together. Due to the nature of the algorithm, the decrypted image is darker, contains a number of visual impairments, and most of visual cryptography solutions increase the spatial resolution of the image. In addition, the requirement for inputs of the binary or dithered nature only limits the applicability of visual cryptography.