Novel Visual Cryptography Schemes Without Pixel Expansion for Halftone Images

Visual cryptography (VC) is a secret-sharing scheme which uses images distributed as shares such that, when the shares are superimposed, a hidden secret image is revealed. While VC was developed for application to binary images, and is inherently based on binary logical operations, it can be applied to grayscale images through their halftone representations. In this paper, we present a new method for processing halftone images that improves the quality of recovered secret images in a VC scheme. Notably, our approach mitigates the two traditional problems in VC of pixel expansion and loss of contrast. Based on this processing stage, this paper proposes and demonstrates the results of two applications of VC on halftone images, one application in multiple VC, and the second in extended VC. Both applications function without pixel expansion, with enhanced visual quality of the recovered secret image over earlier approaches in the literature, while also maintaining perfect security of the original VC approach.