Cryptography Reference
In-Depth Information
In
Bn
10...1
x1
100101...
100110...
Bn
01...0
x2
...
...
Cn=Comp(Bn)
x128
y1
110010...
110100...
00...1
11...1
Hn
Hn=Hash(In)
...
...
y128
110101...
11...0
13 bits
Cn
Hn
100...
Wn=Cn+Hn+1000...
Fig. 6.15.
The procedure of watermark generation.
2) To ensure the induced distortions not exceed the tolerance τ , choose the
last 13 bit-plane of x and y coordinates as the positions used for em-
bedding watermark. This can be denoted as B
n
=b
i
∈0, 1i =1, 2,
, 128132. Compress the selected bit-plane without loss. Then de-
note the compressed bit-plane as C
n
= Comp(B
n
). Here Comp()isthe
lossless compression function.
3) Calculate the hash digest of the original map as H
n
=Hash(I
n
). Then the
watermark data can be constructed as W
n
= H
n
+ C
n
. The length of W
n
is
extended to equal the length of B
n
by appending an ending mark followed
by enough zeros to complete the row to the end of W
n
. The procedure of
watermark generation is shown in Fig. 6.15.
4) Replace the selected bit-plane of I
n
with the watermark W
n
to get a wa-
termarked segment I
wn
. Repeat the procedure for all segments. Then the
original map is protected by watermarking.
2. Extracting and Authenticating Procedure
1) Divide the suspect map into the same number of segments I
′
wn
.
2) For each I
′
wn
, extract the watermark W
′
n
. Then separate W
′
into the Hash
n
Digest H
′
n
and the Compressed Bit-Planes C
′
n
. Decompress C
′
to B
′
n
.
n
3) Replace the selected bit-plane with B
′
n
and calculate the hash digest H
′′
n
of the recovered map.
4) If the two hashes H
′
n
and H
′′
n
exactly match, the map data in current
segment is deemed authentic and the original data has been recovered
as part of the process. If the two hashes differ, then the segment under
consideration has been tampered with.
