Cryptography Reference
In-Depth Information
Later, both the parity check bits and the shared pixels are embedded into
the corresponding four-pixel blocks in the cover image. For simplicity, assume
that I
ij
is the ij-th pixel in the secret image and B
ij
;B
ij
;:::;B
ij
are the n
cover blocks in the cover image, where B
ij
is the block with position (i, j )
in the k-th cover for 1 k n, which contains four pixels, Y
ij
, V
ij
, W
ij
, and
Z
ij
, as shown in Figure 16.1.
FIGURE 16.1
The ij-th block B
ij
of the k-th cover.
Here, the binary representations of the pixels Y
ij
;V
ij
;W
ij
and Z
ij
, are (y
1
y
2
...y
8
), (v
1
v
2
...v
8
), (w
1
w
2
...w
8
) and (z
1
z
2
...z
8
), respectively. The steps
in this scheme are as follows.
Step 1:
Set the modulus p used in the polynomial in 16.1 to 251. If the pixel
I
ij
250, it is directly truncated to 250. Apparently, this operation
can distort the secret image.
Step 2:
Take the pixel I
ij
as the coecient c
0
and choose k 1 inte-
gers as other coecients c
1
;c
2
;:::;c
k1
randomly to construct a
polynomial in Equation 16.1. Then takes the pixel value Y
ij
for
1 k n as the input x of the polynomial to compute the shared
pixel F(Y
ij
) = S
ij
= (s
1
s
2
:::s
8
), where (s
1
s
2
...s
8
) means eight bits
of the shared pixel S
ij
.
Embed the shared pixel S
ij
into the block B
ij
by replacing the eight
16.2.
Step 3:
Search WWH ::
Custom Search