Information Technology Reference
In-Depth Information
Fig. 2.
Distributions of
R
with different payload
L
3
LHA Steganography
3.1
Steganographic Algorithm
In the basic LSB steganography, a gray value is not altered if its LSB is the same as the
bit to be hidden. Otherwise 2
i
is changed to 2
i
+1 when embedding a 1, or 2
i
+1 changed
to 2
i
when embedding a 0. Swappings between 2
i
and 2
i
−1 or between 2
i
+1 and 2
i
+2
never occur. All the three above-mentioned steganalytic approaches use statistical
parameters exploring this property in detecting the presence of secret data.
To improve security, we introduce a new embedding method that causes least ab-
normality in the histogram for resisting the χ
test. Also, since both
F
1
and
F
−
1
are used
when modifying gray values so that it can withstand RS and GPC steganalyses.
Consider pixels with a gray value
j
, 0 ≤
j
≤ 255, in the host image, among which there
are
h
j
having their LSB different from the corresponding secret bit to be embedded.
Modify these pixels by either +1 or −1 instead of simply replacing the LSBs. In this
way, as in the simple replacement approach, the resulting LSBs will also be identical to
the embedded data. Assume that a total of
x
j
pixels are modified with −1, and (
h
j
−
x
j
)
pixels with +1 in the embedding. The number of newly generated pixels having a gray
level
j
in the stego-image is therefore
2
′
(
)
.
h
=
x
+
h
−
x
(6)
j
j
+
1
j
−
1
j
−
1
At both ends of the gray scale,
h
′
=
x
,
(7)
0
1
h
′
=
x
+
h
,
(8)
1
2
0
(
)
(9)
h
′
=
h
+
h
−
x
,
254
255
253
253
