(k,n)-visual secret sharing scheme ((k,n)-VSSS) is a method to divide a secret image into n images called shares that enable us to restore the original image by only stacking at least k of them without any complicated computations. In this paper, we consider (2,2)-VSSS to share two secret images at the same time only by two shares, and investigate the methods to improve the quality of decoded images. More precisely, we consider (2,2)-VSSS in which the first secret image is decoded by stacking those two shares in the usual way, while the second one is done by stacking those two shares in the way that one of them is used reversibly. Since the shares must have some subpixels that inconsistently correspond to pixels of the secret images, the decoded pixels do not agree with the corresponding pixels of the secret images, which causes serious degradation of the quality of decoded images. To reduce such degradation, we propose several methods to construct shares that utilize 8-neighbor Laplacian filter and halftoning. Then we show that the proposed methods can effectively improve the quality of decoded images. Moreover, we demonstrate that the proposed methods can be naturally extended to (2,2)-VSSS for RGB images.

In this paper, a method is proposed to construct a visual secret sharing (VSS) scheme for multiple secret images in which each share can be rotated with 180 degrees in decryption. The proposed VSS scheme can encrypt more number of secret images compared with the normal VSS schemes. Furthermore, the proposed technique can be applied to the VSS scheme that allows to turn over some shares in decryption. From the theoretical point of view, it is interesting to note that such VSS schemes cannot be obtained from so-called basis matrices straightforwardly.

Visual secret sharing (VSS) scheme is a perfect secure method that protects a secret image by breaking it into shadows. Unlike other secret sharing schemes, the VSS scheme can be easily decoded by the human visual sight when staking the shadows. We replace a pixel in the secret image by m sub pixels in the shadow image and the value m is called as pixel expansion. In general, most papers are dedicated to find the minimum m for a VSS scheme, i.e. a smaller shadow size. However, it seems that no one studies how to trade the shadow size for the contrast. In this paper, we take the lead in studying size-adjustable VSS schemes such that one can choose appropriate shadow size and the recovered image contrast for practical use.

We propose a method for reducing the size of a share in visual secret sharing schemes. The proposed method does not cause the leakage and the loss of the original image. The quality of the recovered image is almost same as that of previous schemes.

Combinatorial designs are normally used to construct visual cryptographic schemes. For such schemes two parameters are very important viz. pixel expansion and contrast. Optimizing both is a very hard problem. The schemes having optimal contrast tend to use a high pixel expansion. The focus of the paper is to construct schemes for which pixel expansion is modest and the contrast is close to optimality. Here the tool is latin squares that haven't been used earlier for this purpose.

In this paper, a new method is proposed to construct a visual secret sharing scheme with a general access structure for plural secret images. Although the proposed scheme can be considered as an extension of Droste's method that can encode only black-white images, it can encode plural gray-scale and/or color secret images.

In this letter we propose a new visual secret sharing scheme (VSSS) applicable to color images containing many colors such as photographs. In the proposed VSSS we can perceive a concealed secret image appearing on a reproduced image, which is obtained by stacking certain shares, according to the principle called the meanvalue-color mixing (MCM). First, we mathematically formulate the MCM and define a new parameter that determines the minimum quality of the reproduced secret image. Then, we explicitly construct the VSSS based on the MCM under general access structures. The construction is proved to be realistic by experiment under the (2,2)-threshold access structure.

In this paper, a method is proposed to construct an n-out-of-n visual secret sharing scheme for gray-scale images, for short an (n,n)-VSS-GS scheme, which is optimal in the sense of contrast and pixel expansion, i.e., resolution. It is shown that any (n,n)-VSS-GS scheme can be constructed based on the so-called polynomial representation of basis matrices treated in [15],[16]. Furthermore, it is proved that such construction can attain the optimal (n,n)-VSS-GS scheme.

In this paper, we treat visual secret sharing scheme (VSSS) for color images. We first evaluate the brightness of the decrypted color image under certain conditions on the mixture of colors. We obtain a general formula for the construction of VSSS using mixture of colors. We second propose an iterative algorithm for constructing VSSS in a practical situation. If we use the iterative construction, we have only to solve partial differential equations with small n even if n is actually large, where n denotes the number of participants. This iterative construction has never discussed in the both cases under the original images are black-white images and color images. Finally, we propose the way to embed a color image on each share for the case that the original image is color.