Cryptology ePrint Archive: Report 1996/013
On the Contrast in Visual Cryptography Schemes
Carlo Blundo, Alfredo De Santis, and Douglas R. Stinson
Abstract: 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
enable the ``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.
Category / Keywords:
Publication Info: Appeared in the THEORY OF CRYPTOGRAPHY LIBRARY and has been included in the ePrint Archive.
Date: received September 25, 1996.
Contact author: carblu at udsab dia unisa it
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