Cryptography Reference
In-Depth Information
7
Visual Cryp
tography and Random Grids
Shyong Jian Shyu
Ming Chuan University, Taiwan
CONTENTS
7.1
Introduction :::::::::::::::::::::::::::::::::::::::::::::::::::::: 185
7.2
Random Grids :::::::::::::::::::::::::::::::::::::::::::::::::::: 186
7.2.1
Random Pixel, Random Grid, and Average Light
Transmission ::::::::::::::::::::::::::::::::::::::::::::: 186
7.2.2
Superimposition of Random Grids :::::::::::::::::::::: 187
7.3
Visual Cryptograms of Random Grids ::::::::::::::::::::::::::: 189
7.3.1
Denition of VCRG ::::::::::::::::::::::::::::::::::::: 189
7.3.2
(2, 2)-VCRG Algorithms for Binary Images ::::::::::: 190
7.3.3
Experiments for (2, 2)-VCRG ::::::::::::::::::::::::::: 193
7.3.4
Definition of Light Contrast and Performance
Evaluation ::::::::::::::::::::::::::::::::::::::::::::::: 194
7.3.5
Algorithms of (n;n)-VCRG for Binary Images ::::::::: 196
7.3.6
Algorithms of (n;n)-VCRG for Gray-Level and Color
Images ::::::::::::::::::::::::::::::::::::::::::::::::::: 206
7.3.7
Experiments for (n;n)-VCRG ::::::::::::::::::::::::::: 207
7.4
Concluding Remarks ::::::::::::::::::::::::::::::::::::::::::::: 219
7.1 Introduction
A random grid was defined by Kafri and Keren in 1987 [7] as a transparency
comprising a two-dimensional array of pixels. Each pixel is either fully trans-
parent or simply opaque and the choice between the alternatives is made by
a coin-flip procedure. Thus, there is no correlation between the values of dif-
ferent pixels in the array.
We could encrypt binary pictures or shapes into two random grids such
that only the areas containing information in the two grids are intercorrelated,
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