Information Technology Reference
In-Depth Information
10.4.2 embedding Process
For a grayscale cover image
C
, permute and divide all pixels of the cover image
into many groups, each containing
N
pixels, according to a secret key. The
number of pixel groups is denoted as
NP
. Denote the pixel values in the
n
th
pixel group as
g
(
n
, 1),
g
(
n
, 2), …,
g
(
n
,
N
),
n
= 1, 2, …,
NP
. Suppose that message
ND
-ary symbol
m
∈ {0, 1, …, ND - 1} is to be embedded in each pixel group.
1. For the
n
th pixel group, assign the pixel value
g
(
n
,
i
) by using the fol-
lowing symbol assignment function
t
(
n
,
i
) =
Q
(
g
(
n
,
i
)) =
g
(
n
,
i
) mod 3, 1 ≤
i
≤
N
and obtain the corresponding 3-ary symbols
t
(
n
, 1),
t
(
n
, 2), …,
t
(
n
,
N
).
2. Assign each pixel group
x
= {
t
(
n
, 1),
t
(
n
, 2), …,
t
(
n
,
N
)} to a disk unit by
using DM allocation function DM(
x
).
3. If DM(
x
) =
m
, no modification is made.
4. If DM(
x
) ≠
m
, find the closest stego-vector
y
= {
t
′(
n
, 1),
t
′(
n
, 2), …,
t
′(
n
,
N
)} so that DM(
y
) =
m
and the Hamming distance
H
(
x
,
y
) is as small
as possible where
N
∑
,
,
( )
=
( )
⊕ ′
( )
(
)
H
xy
t ni
tni
,
.
i
=
1
Assume a single embedding change, the symbol
t
(
n
,
j
) gives the change, and
g
(
n
,
j
) is changed into
g
′(
n
,
j
) so that
Q
(
g
′(
n
,
j
)) =
t
′(
n
,
j
) and |
g
′(
n
,
j
) -
g
(
n
,
j
)|
is minimal.
10.4.3 extraction Process
For a grayscale stego-image
S
, permute and divide all pixels of the stego-
image into many groups, each containing
N
pixels, according to a secret key.
The number of pixel groups is denoted as
NP
. Denote the pixel values in the
n
th pixel group as
g
′(
n
, 1),
g
′(
n
, 2), …,
g
′(
n
,
N
),
n
= 1, 2, …,
NP
.
1. For the
n
th pixel group, assign the pixel value
g
′(
n
,
i
) by using the
following symbol assignment function
t
′(
n
,
i
) =
Q
(
g
′(
n
,
i
)) =
g
′(
n
,
i
) mod 3, 1 ≤
i
≤
N
and obtain the corresponding 3-ary symbols
t
′(
n
, 1),
t
′(
n
, 2), …,
t
′(
n
,
N
).
2. Assign each pixel group
y
= {
t
′(
n
, 1),
t
′(
n
, 2), …,
t
′(
n
,
N
)} to a disk unit by
using DM allocation function DM(
y
) to extract message
m
=
DM(
y
).
