Grouped k-threshold random grid-based visual cryptography scheme

TL;DR

This paper introduces a new hierarchical threshold visual cryptography scheme that significantly improves contrast, enabling better visual quality in secret image sharing without pixel expansion.

Contribution

It proposes a novel grouped threshold paradigm for random grid-based visual cryptography, achieving the highest contrast values documented to date.

Findings

Achieves hierarchical contrast with optimal intra-group recovery.

Introduces a new contrast calculation formula for the paradigm.

Demonstrates superior contrast through theoretical analysis and experiments.

Abstract

Visual cryptography schemes (VCSs) belong to a category of secret image sharing schemes that do not require cryptographic knowledge for decryption, instead relying directly on the human visual system. Among VCSs, random grid-based VCS (RGVCS) has garnered widespread attention as it avoids pixel expansion while requiring no basic matrices design. Contrast, a core metric for RGVCS, directly determines the visual quality of recovered images, rendering its optimization a critical research objective. However, existing $(k,n)$ RGVCSs still fail to attain theoretical upper bounds on contrast, highlighting the urgent need for higher-contrast constructions. In this paper, we propose a novel sharing paradigm for RGVCS that constructs $(k,n)$-threshold schemes from arbitrary $(k,n')$-threshold schemes $(k \leq n'\leq n)$, termed \emph{$n'$-grouped $(k,n)$ RGVCS}. This paradigm establishes hierarchical contrast characteristics: participants within the same group achieve optimal recovery quality, while inter-group recovery shows a hierarchical contrast. We further introduce a new contrast calculation formula tailored to the new paradigm. Then, we propose a contrast-enhanced $(k,n)$ RGVCS by setting $n'= k$, achieving the highest contrast value documented in the existing literature. Theoretical analysis and experimental results demonstrate the superiority of our proposed scheme in terms of contrast.