On the Contrast in Visual Cryptography Schemes |
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| Authors: | Carlo Blundo Alfredo De Santis Douglas R Stinson |
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| Institution: | (1) Dipartimento di Informatica ed Applicazioni, Università di Salerno, 84081 Baronissi (SA), Italy carblu@dia.unisa.it, adslu@dia.unisa.it http://www.unisa.it/~carbluads/, http://www.unisa.it/~ads/, IT;(2) Department of Combinatorics and Optimization, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 dstinson@cacr.math.uwaterloo.ca http://cacr.math.uwaterloo.ca/~dstinson, CA |
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| 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 we obtain upper and lower bounds on the optimal contrast.
Received 27 September 1996 and revised 13 February 1998 |
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| Keywords: | , Secret sharing scheme, Visual cryptography, Combinatorics designs, |
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