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報告人:葉瑞群
日期:2011/11/29
出處:IEEE Transactions on Knowledge and Data Engineering
 1.
Introduction
 2.
Review of Conventional Matrix-Based VC and
Probabilistic-VC
 3.
Proposed Scheme
 4.Simulation
 5.
Results
Conclusion
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 Visual
Cryptography (VC) owns the unique decoding
mode which discloses the secret by inspecting the
superimposed shares using the naked eye without any
computation.
 It
is a method for fulfilling secret sharing activities in
the environments with insufficient computing power.
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 Tagged
Visual Cryptography(TVC) is that an extra tag
can be revealed by folding up each share, which
provides users with supplementary information such
as augmented message or distinguishable patterns to
identify the shares.
 The
proposed TVC (t,n), 2 ≤ t ≤ n.
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 Illustrates
a computer simulation result of a basic (2, 2)
VC sharing example,where panel (a) is the secret
image with text “VC,” and panels (b) and (c) are the
two generated shares. Each share consists of noisy
black dots, and is two times the size of the original
image.
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Some extended VC schemes were proposed for
constructing meaningful shares. These methods encode
the secret image in natural-look shares to decrease the
suspicious of the track about the secret.
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 The
conventional matrix-based (t,n) VC scheme
depicts the method of sharing a binary image which
consists of a collection of black and white pixels.
 Each
original pixel is handled separately and encoded
in n shares, with each share contains m black and
white pixels.
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 The
construction of a conventional (t,n) VC scheme is
usually characterized by two n*m basis matrices: a
white pixel basis matrix B0 and a black pixel basis
matrix B1. Let C0 denote the set of all matrices
obtained by permuting the columns of B0, and C1
denote the set of all matrices obtained by permuting
the columns of B1.
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 B0
= black、0 = black
 B1 = white、1= white
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 That
is, 1) no secret information is revealed by
stacking any t-1 or fewer shares, 2) the secret message
can be revealed by superimposing any or more shares,
and 3) folding up a share will display the tag
associated with it.
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 Note
that the base-shares generated by the base-VC
scheme should conform to the contrast and security
requirements of an effective VC scheme.
 In
the n base-shares Bi, and obtains the n taggedshares{S1,S2,...,Sn}. Each tagged-share Si is
constructed in such a way that the tag patterns on Ti
can be revealed by folding up it.
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 1)
Fetch a pixel p from T sequentially in scan order,
i.e., from left to right and from top to bottom, and
then proceeds with one of the following two encoding
rules.
 2a)
If the color of p is black ( p = 1 ) and the colors of
the two corresponding pixels in B are both white, one
of the two corresponding pixels is selected with equal
probabilities, and the color of the selected pixel is set
to black.
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 2b)
If the color of p is white ( p = 0 ), and the color of
a corresponding pixel is white, set the color of the
corresponding pixel to black in a probability p0/2,
where p0 is the probability of white pixel in the baseshare .
 3)
Repeat Steps 1 and 2 until all of the pixels in T are
processed.The tagged-share is obtained.
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 For
example, it can display fake message to establish
a cheating mechanism to unauthorized inspectors, or
the tag pattern can exhibit unique symbol associated
with each sharing instance, and provide a userfriendly environment for users to distinguish among
and manage to the numerous shares.
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END
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