Colored Visual Cryptography Without Color Darkening

Abstract

In a visual cryptography scheme a secret image is encoded into n shares, in the form of transparencies. The shares are then distributed to n participants. Qualified subsets of participants can recover the secret image by superimposing their transparencies. Non-qualified subsets of participants have no information about the secret image.In this paper we consider the case when the secret image is a colored image. Most of the 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 not be distinguishable from a black pixel.In this paper we propose a model where the reconstruction has to guarantee that the reconstructed secret pixel has the exact same color of the original one and not a darker version of it. We consider (k, n)-threshold schemes where a qualified set of participants consists of any k participants. We provide a general construction for any k, 2 ≤ k ≤ n and a construction for the special case k = 2.We also prove lower bounds on the pixel expansion (which is a measure of the goodness of the scheme) for the cases k = 2 and k = n. The lower bounds match the pixel expansion of the schemes provided in these two cases, thus proving that our schemes are optimal with respect to the pixel expansion. We also provide an upper bound on the contrast of (k, n)-threshold schemes and (2, n)-threshold schemes with optimal contrast.

Content based authentication of visual cryptography

“…In particular, the first method is to directly use the colours appeared in original secret image (Cimato, De Prisco & De Santis, 2007;Rijmen & Preneel, 1996). This CVCS method can also be classified into two approaches of concealing unused colours on VC shares by black colour and increasing or decreasing the colour depth to imitate different colours.…”

High-Entropy Visual Identification for Touch Screen Devices

“…Since secure erasure of long-term data (which includes challenges but not the user's responses thereto) may be impossible, such acts may compromise all past challenges. 7 In the case of a remote authentication system, the adversary may be able to prompt the challenger to provide a challenge at any time and then abort the protocol. Our system ensures that challenges do not leak data about their interpretation, even in aggregate.…”

“…The original Naor and Shamir paper [22] discusses k-outof-n threshold schemes more general than the 2-out-of-2 we used here. Visual cryptography has been extended to work with full-color images [14,16,7], with "meaningful" (i.e., non-random) cover images [6,27,28], and general access structures both without [4] and with [5] meaningful cover images. This prior work has tended (the identification schemes of [23] aside) to focus on the act of secret splitting itself, rather than its potential application to authentication.…”

Dual Visual Cryptography Using the Interference Color of Birefringent Material

“…Many types of visual cryptography have been proposed [8]- [19]. Conventional visual cryptography reduces the contrast of the secret image because its encryption is based on a spatial coding that requires multiple subpixels to modulate light intensity.…”