Most visual cryptography schemes (VCSs) are condition-oriented which implies their designs focus on satisfying the contrast and security conditions in VCS. In this paper, we explore a new architecture of VCS: contrast-oriented region-based progressive probabilistic VCS (CRP2-VCS). The term contrast-oriented indicates the optimality of multi-contrast is taken into consideration when producing shadows. First of all, new requirements for CRP2-VCS, described by probabilities, are introduced. As a non-interference requirement is proposed, the secret interference problem in existing region-based progressive VCS can be avoided. Then, a construction of CRP2-VCS based on a multi-contrast-maximizing model is provided. The multi-contrast-maximizing problem is essentially a probabilistic VCS model that fuses region-based sharing, multi-contrast optimization, and general access structure (GAS) together. Finally, a Max-Min based technique is adopted to solve the multi-objective optimization problem. Moreover, to further boost the visual quality, the proposed method is extended to allow employing XOR operation for image recovery. Experimental results and comparisons are demonstrated to show the effectiveness and advantages, such as optimal visual quality, non-expansible shadow and GAS sharing policy, are provided by the proposed technique.

In this article, a secret image sharing (SIS) scheme for general access structures (GAS) is designed for distributed multi-user scenario. In the proposed distributed multi-user SIS (DM-SIS), multiple secret images are encoded into shadows which are then distributed to the corresponding storage nodes of a network. By collecting the shadows from nodes, each user is capable of decrypting the corresponding secret image. Fundamentally, we utilize an invertible target matrix, which is initially obtained from the GAS and a base matrix with Vandermonte coordinates, to construct shadows. To deal with the case of non-invertible target matrix, three matrix-adjusting procedures are further introduced. Theoretical analysis, numerical examples, and experiments are provided to verify the feasibility of the proposed technique. When compared to previous methods, the proposed approach can implement GAS sharing strategy in distributed multi-user environment. Meantime, significant improvements on storage overhead and sharing capacity are also achieved.

A systematic investigation of evolving visual cryptography scheme (EVCS) is carried out in this article. The evolving scheme, denoted as \((k,\infty)\) , differs from the \((k,n)\) threshold in that it permits an arbitrary and perhaps unlimited number of participants. More importantly, the access structure can be updated dynamically by adding new users. First of all, a preliminary implementation strategy for the \((2,\infty)\) EVCS is introduced. Then, by employing the \((2,2)\) VCS recursively with the \((2,\infty)\) EVCS, a \((k,\infty)\) EVCS is created. In order to enhance the performance, an improved scheme is constructed based on the multi-secret VCS (MVCS) and a series of EVCS schemes with thresholds of \((1,\infty)\) , \(\cdots\) , \((k-1,\infty)\) . Moreover, Boolean XOR operation is adopted for secret recovery to further improve the visual quality. To facilitate the XOR decryption, a novel access structure partition algorithm is presented. Additionally, the proposed partition method can successfully solve the security issue in existing multi-secret XOR-based VCS (MXVCS). By integrating the more secure MXVCS into the improved scheme, XOR decryption is provided. The two proposed methods are shown to be effective and advantageous through extensive experiments and comparisons.