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Visual Cryptography Schemes with Optimal Pixel Expansion
Authors: Carlo Blundo, Stelvio Cimato, Alfredo De SantisAbstract:
A visual cryptography scheme encodes a black&white secret image
into n shadow images called shares which are distributed to the n
participants. Such shares are such that only qualified subsets of
participants can ``visually’’ recover the secret image.
Usually, the reconstructed image will be darker than the background of
the image itself. In this paper we consider visual cryptography schemes
satisfying the model introduced by Tzeng and Hu (Designs, Codes and
Cryptography, Vol. 27, No. 3, pp. 207-227, 2002). In such a model
the recovered secret image can be darker or lighter than the background.
We prove a lower bound on the pixel expansion of the scheme and,
for (2,n)-threshold visual cryptography schemes, we provide schemes
achieving the bound. Our schemes improve on the ones proposed by
Tzeng and Hu.
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