Cryptography Reference
In-Depth Information
height of HI are W and H. Moreover, each pixel in halftone image
HI contains only 1 bit.
Step 2:
Divide halftone image HI into Z nonoverlapping 6-bit blocks,
Z = b
H
6
c. HI
z
(z = 1,2,...,Z) is the z-th block of HI and the
value of HI
z
is denoted as P
z
illustrated in Figure 16.19.
FIGURE 16.19
The z-th block of GI and the corresponding HI
z
,P
z
.
Step 3:
This is the basic process for secret image sharing with P
z
as the
secret, z = 1, 2 ,..., Z, Z = b
H
6
c . This procedure contains five
substeps, which are performed as follows.
(a) Take the value Xi
i
formed from 2-MSBs (Most Significant
Bits) of the four pixels of each cover CBi
i
as the value x
specified in Equation 16.16, i = 1, 2 , ..., n, as shown in
F(x
i
) = (s + a
1
x
i
+ a
2
x
i
+ ::: + a
k1
x
k
i
) mod 2
6
(16.16)
(b) Take the secret P
z
as the value s specified in Equation
16.16.
(c) With a random number generator, generate a set of k 1
integers a
1
;a
2
;:::;a
k1
in Equation 16.16, where k n.
(d) For each X
i
, compute the corresponding value of F(Xi)
i
) by
Equation 16.16 to form a secret share (Xi,
i
, F(X
i
)) for each
participant in the secret sharing group, F(Xi)
i
) = f
5
f
4
f
3
f
2
f
1
f
0
.
(e) Hide the six data bits of F(Xi)
i
) in the Least Significant Bits
of the four pixels Ci,
i
, D
i
, G
i
, and H
i
of the corresponding
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