Abstract
Abstract Visual cryptography is a cryptographic technique that allows visual information to be encrypted so that the human optical system can perform the decryption without any cryptographic computation. The halftone visual cryptography scheme (HVCS) is a type of visual cryptography (VC) that encodes the secret image into halftone images to produce secure and meaningful shares. However, the HVC scheme has many unsolved problems, such as pixel expansion, low contrast, cross-interference problem, and difficulty in managing share images. This article aims to enhance the visual quality and avoid the problems of cross-interference and pixel expansion of the share images. It introduces a novel optimization of color halftone visual cryptography (OCHVC) scheme by using two proposed techniques: hash codebook and construction techniques. The new techniques distribute the information pixels of a secret image into a halftone cover image randomly based on a bat optimization algorithm. The results show that these techniques have enhanced security levels and make the proposed OCHVC scheme more robust against different attacks. The OCHVC scheme achieves mean squared error (MSE) of 95.0%, peak signal-to-noise ratio (PSNR) of 28.3%, normalized cross correlation (NCC) of 99.4%, and universal quality index (UQI) of 99.3% on average for the six shares. Subsequently, the experiment results based on image quality metrics show improvement in size, visual quality, and security for retrieved secret images and meaningful share images of the OCHVC scheme. Comparing the proposed OCHVC with some related works shows that the OCHVC scheme is more effective and secure.
Highlights
The rapid growth of the Internet and the transmission channels enable access to visual information much easier for an unauthorized individual [1,2]
Test results for each of them are shown based on image quality metrics: mean squared error (MSE), peak signal-to-noise ratio (PSNR), normalized cross correlation (NCC), and universal quality index (UQI)
The results show that the proposed OCHVC scheme achieves high performance, but it cannot distinguish some of the changes in the shared image
Summary
The rapid growth of the Internet and the transmission channels enable access to visual information (image, video, etc.) much easier for an unauthorized individual [1,2]. The visual cryptography scheme (VCS) is a perfect method to encrypt visual information without complicated calculations on the receiver’s side to protect this information’s confidentiality [3,4]. When Naor and Shamir explained the concept of “How to share a secret” by dividing D data into n pieces in such a way that simple reconstruction from any k pieces can recover the secret, but not any (k − 1) pieces. Based on this principle, the concept of VCS presented by Naor and Shamir [2] emerges to encrypt digital images. Codebook technique is a pattern or a mask that decides various secret pixels’ possibilities, as illustrated in Figure 1 [5]
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