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On the Contrast in Visual Cryptography Schemes
Authors: Carlo Blundo, Alfredo De Santis, Douglas R. StinsonAbstract:
A visual cryptography scheme is a method to encode a secret image SI into
shadow images called shares such that certain qualified subsets of shares
visual'' recovery of the secret image. The visual’’ recovery consists of xeroxing the shares onto transparencies,
and then stacking them. The shares of a qualified set will reveal the secret
image without any cryptographic computation.
In this paper we analyze the contrast of the reconstructed image
in k out of n visual cryptography schemes. (In such a scheme
any k shares will reveal the image, but no set of k-1 shares
gives any information about the image.)
In the case of 2 out of n threshold schemes we give a complete
characterization of schemes having optimal contrast and minimum
pixel expansion in terms of certain balanced incomplete block designs.
In the case of k out of n threshold schemes with k>2 we obtain
upper and lower bounds on the optimal contrast.
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