Abstract
The appearance of shares encoded by traditional visual cryptography is not easy to distinguish and meaningless. It means the shares cannot be managed efficiently nor user-friendly. A tagged visual cryptography scheme (TVCS) provides additional tag images for the shares and decodes the tag images by folding up a single tagged share. A TVCS allows users to efficiently manage shares and greatly increases the user-friendliness of the shares. Previous researches have focused on codebook-based and random-grid-based TVCS approaches, but the visual qualities of recovered secret images and tag images can still be improved. Therefore, this study aims to provide better visual quality of recovered images by improving on previous methods. Moreover, the proposed approach supports adjustable visual qualities of recovered secret images and tag images. In this study, we theoretically analyze the performances of the proposed approach and then demonstrate implementation of the approach.
Highlights
The threshold visual cryptography scheme, denoted (k, n)-VCS (2 ≤ k ≤ n), for the sharing of binary images is known as a threshold visual secret sharing scheme ((k, n)-visual secret sharing schemes (VSSSs))
In this paper, we proposed a two-phase tagged visual cryptography construction method, the (k, n)-tagged visual cryptography scheme (TVCS), that adopts an optimized probabilistic visual cryptography scheme (VCS) to encode base shares and hide tag images
The proposed approach provides a feature for tuning the contrasts of recovered secret images and tag images by a parameter so that users can adjust visual qualities according to their requirements
Summary
The threshold visual cryptography scheme, denoted (k, n)-VCS (2 ≤ k ≤ n), for the sharing of binary images is known as a threshold visual secret sharing scheme ((k, n)-VSSS). The visual cryptography scheme (VCS) features one-time-pad encryption so that it can securely protect secret images [1]–[6]. In 2001, Ateniese et al pointed out this issue [7] and, to address it, proposed the construction of the extended visual cryptography scheme (EVCS), and added stego-images, or cover images into noise-like shares, to make identifiable meaningful shares. The TVCSs generate noise-like shares that same as traditional VCSs, but tagged visual cryptography conceals the tag pattern in each share. FVCSs usually consider residual traces problem in encryption approaches to improve visual quality. Compared with the original VCS, the FVCSs and the TVCSs have lower visual quality of recovered secret images. Improving the quality of recovered secret images and tag images are crucial issues in TVCS research.
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