Cryptography Reference
In-Depth Information
Watermarking
Position key 3
W F
Encoded
Indices
Index
Constrained
VQ Encoder
Input
Division
X 1
Vectors
X
Codebook C 2
+
X
Ouput
+
Normal VQ
Decoder
Composition
+
X W
Vectors
Fig. 11.3. The semi-fragile watermark embedding process in the second stage.
image and can be described as follows. First, perform the inverse permuta-
tion operation with key 4 on codebook C u to obtain the product codebook
C. Secondly, the watermarked image X W is divided into blocks or vectors.
Thirdly, the normal VQ encoder then does the Nearest Neighbor Codeword
Search on all input vectors to obtain the encoded overall indices. Fourthly,
according to the two stage codebook sizes, each overall index is segmented
into two indices. One is for robust watermark extraction. The other is for
semi-fragile watermark extraction. Finally, the robust and semi-fragile water-
marks are extracted independently. For the robust watermark extraction, we
first compute the polarities P from the indices of Stage 1. Then we do the
XOR operation between P and key 2 to obtain the extracted permuted robust
watermark W EPR . Finally do the inverse permutation operation with key 1 to
obtain the extracted robust watermark W ER . For the semi-fragile watermark
extraction, we check the key 3 -th bit of each index of Stage 2 to obtain the
extracted watermark bit. Here key 3 is the watermarking position. We then
piece all the extracted bits together to form the extracted semi-fragile water-
mark W EF . From the above we can see that the advantages of using ICVQ
in the semi-fragile watermarking are as follows: (1) the embedding and the
extraction processes are very simple; (2) the extraction process is blind; (3)
the embedded position can be controlled by a key for more security.
In Section 11.2.1, we mention two problems with the robust embedding
technique [20]. In our algorithm, these two problems are automatically solved.
Detecting the existence of the semi-fragile watermark in the original image
provides a solution to the first problem. Two-stage codebooks are not used.
The equivalent product codebook is used however to extract the watermarks
and it also provides a solution to the second problem. From Fig. 11.4 we can
see that the extraction time is determined by the codebook size of C.IfN is
very large, then the full search VQ encoding is a time-consuming process. As a
consequence the fast codeword search algorithm [23] is used in this algorithm.
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