Information Technology Reference
In-Depth Information
images in the JPEG format have a property that the coefficient's occurrence frequency
decreases with increasing magnitude, and the data insertion must not change this
feature. To hide data in JPEG images, F5 has been proposed [11], which is developed
from its predecessors F3 and F4 using a
matrix encoding
technique.
Without secret data
Original LSB steg
LHA algorithm
0.4
0.3
0.2
0.1
0
-0.1
0
10
20
30
40
50
60
70
80
90
100
Fig. 6.
Values of
R
−
M
−
R
M
calculated from 100 stego-images by using the LSB-replacement
method and the LHA techniques, and from the same 100 images without secret data
Tes t images
120
Original
By LHA
100
80
60
40
20
0
0.95
0.96
0.97
0.98
0.99
1
1.01
1.02
1.03
1.04
1.05
R=NE/NO
Fig. 7.
Distributions of
R
of original images and that of LHA based stego-images
In F3, nonzero coefficients are used, whose magnitudes are decremented by 1 when
the LSB does not match. This is for keeping the above-mentioned features and pre-
venting the occurrence frequencies of 2
i
and 2
i
+1 from being close to each other, a
signature detectable by χ
test. However, since the magnitude decrements generate 0s
from the original +1 or −1, the receiver is unable to distinguish a steganographic 0 from
an original 0, and the data-hider must repeatedly embed the affected bits when zeros are
produced. Thus, F3 produces more even coefficients than odd ones resulting in an
abnormal histogram, therefore becomes vulnerable to steganalysis.
2
