Information Technology Reference
In-Depth Information
Table 3.
Differential cryptanalysis for CAA and Comparative speed of CAA and MD5
(in WindowsNT 4.00-1381,IBM)
Input
Avg.Std.Devn of XOR
CPU Time in Seconds
size of
Distribution for
SACA
(%)
file in
key-length 128
key-length 256
MD5 p=1 p=2 p=4 p=8
bytes p=1 p=2 p=4 p=8 p=8 p=16
method n=128 n=64 n=32 n=16
1608 9.110 8.950 7.881 5.899 5.660 4.883
0.0549 0.055 0.050 0.040 0.040
35860 14.821 12.111 8.458 6.134 6.123 5.123
0.165 0.147 0.105 0.105 0.087
65780 8.989 7.813 6.657 5.034 5.002 4.986
0.193 0.166 0.129 0.110 0.091
142164 6.824 6.771 5.998 4.823 4.989 5.024
0.2198 0.2053 0.1650 0.118 0.081
259120 14.100 11.783 10.213 7.982 6.102 4.033
0.299 0.271 0.267 0.210 0.200
852984 13.015 12.443 7.893 4.342 3.032 4.003
0.330 0.294 0.252 0.205 0.205
secret key to verify the authenticity, integrity and ownership ofan image. Ifthe
user performs the watermark extraction with an incorrect key or an image which
is not watermarked, the user obtains an Image that resembles noise.
Recent systems apply sophisticated embedding mechanisms, including the
use ofcryptographic hash unctions to detect changes to a watermarked im-
age. This section reports a watermarking scheme that employs CAA based hash
functions.
Let the original grey-scale image be X. A bi-level watermark 'A' will be
inserted in it and again will be extracted from it for authentication. X and A
are divided into some equal blocks ofsize
n × n
and say, each block ofX is
termed as
X
r
andAas
A
r
.
Insertion
Let, Image block
X
r
=
255 128
108 11
11111111 10000000
01101100 00001011
or,
and
10
11
Watermark block
A
r
=
.
1111111
0
1000000
0
0110110
0
0000101
0
-
Set all LSBs of
X
r
to 0,
X
r
=
is obtained.
11
10
- Hash
X
r
using CAA
and the hash output
H
r
=
.
-
Perform pixel by pixel ex-or operation between
H
r
and
A
r
,(
H
r
⊕A
r
=
C
r
)
and obtain
C
r
=
01
01
.
-
Insert values of
C
r
into all LSBs of
X
r
. The resulting watermarked block
1111111
0
1000000
1
0110110
0
0000101
1
254 129
108 11
X
r
w
=
or,
.
Extraction
254 129
108 11
11111110 10000001
01101100 00001011
-
Let,
Y
r
=
or,
be the watermarked image
block.
