Visual cryptography

About: Visual cryptography is a research topic. Over the lifetime, 1724 publications have been published within this topic receiving 25300 citations.

Colored visual cryptography without color darkening

Abstract: Visual cryptography schemes allow the encoding of a secret image into shares, in the form of transparencies, which are distributed to the participants. The shares are such that only qualified subsets of participants can visually recover the secret image by superimposing the transparencies. In this paper we study colored visual cryptography schemes. Most of previous work on colored visual cryptography allows the superposition of pixels having the same color assuming that the resulting pixel still has the same color. This is not what happens in reality since when superimposing two pixels of the same color one gets a darker version of that color, which effectively is a different color. Superimposing many pixels of the same color might result in a so dark version of the color that the resulting pixel might be not distinguishable from a black pixel. Thus we propose a model where the reconstruction has to guarantee that the reconstructed secret pixel has the same color of the original one and not a darker version of it. We give a construction of c-color (k,n)-threshold visual cryptography schemes. Since we have to guarantee the reconstruction of the exact original color, in many cases our schemes have a bigger pixel expansion than previous ones. However, for the case of k = n, we get a smaller pixel expansion when compared with schemes that to do not guarantee the exact reconstruction of the original color. We also prove that, in the model introduced in this paper, our schemes for k = n have optimal pixel expansion.

The ringed shadow image technology of visual cryptography by applying diverse rotating angles to hide the secret sharing

Abstract: The visual cryptography, which differs from conventional visual cryptography that is restricted to four specific angles (0/spl deg/, 90/spl deg/, 180/spl deg/, 270/spl deg/) , can hide two sets of confidential data into the ringed shadow images and present them by applying diverse rotating angles. Due to the purpose of reaching arbitrary rotating angles, we have presented a new coding measure to demonstrate two sets of confidential data from the correspondent two ringed shadow images via rotating angle easily. This angle is located among the entire 360/spl deg/ circles arbitrarily. The measure raised in this thesis has not only overwritten the angle restriction of conventional visual cryptography, it also has made a significant modification on the method of displaying the two sets of data embedding in the encrypted imagines. The preceded study relevant to this topic is to present diverse confidential data by using various areas, which makes a serious degradation of resolution for partial encrypted images in regard of the area's location. Therefore, this thesis of study finds a way to present a clear and complete confidential data regardless the area is located at.

An Extended Visual Cryptography Algorithm for General Access Structures

Abstract: Conventional visual secret sharing schemes generate noise-like random pixels on shares to hide secret images. It suffers a management problem, because of which dealers cannot visually identify each share. This problem is solved by the extended visual cryptography scheme (EVCS), which adds a meaningful cover image in each share. However, the previous approaches involving the EVCS for general access structures suffer from a pixel expansion problem. In addition, the visual cryptography (VC)-based approach needs a sophisticated codebook design for various schemes. In this paper, we propose a general approach to solve the above- mentioned problems; the approach can be used for binary secret images in noncomputer-aided decryption environments. The pro- posed approach consists of two phases. In the first phase, based on a given access structure, we construct meaningless shares using an optimization technique and the construction for conventional VC schemes. In the second phase, cover images are added in each share directly by a stamping algorithm. The experimental results indicate that a solution to the pixel expansion problem of the EVCS for GASs is achieved. Moreover, the display quality of the recovered image is very close to that obtained using conventional VC schemes.