Cryptography Reference
In-Depth Information
2) The semi-fragile and robust watermarks are extracted independently and
blindly.
3) Different codebooks are used for the embedding and the extraction pro-
cesses. The first uses the two stage codebook and the second the product
codebook. The final product codebook can be made public for users, so
that the extraction process sometimes can be performed publicly for special
applications.
4) Because the embedded position in the VQ index for the semi-fragile wa-
termark is secret, the two stage codebooks used in the embedding process
are secret. The product codebook used for extraction is encrypted and
not the same as that for users, it is hard for the attacker to forge a valid
semi-fragile watermark in the tampered watermarked image.
5) The proposed algorithm can be extended to meet three purposes. These
are (a) A digital fingerprint, (b) copyright protection, and (c) image au-
thentication, by embedding three watermarks in a three-stage VQ.
6) The VQ-based watermarking algorithm can also reduce the amount of the
data needed or transmitted.
There are some disadvantages or problems remaining which need to be solved
in the future. These are as follows:
1) The quality of the watermarked image obtained by use of the existing VQ-
based watermarking algorithms is not high enough for copyright protection.
We may use other kinds of vector quantizers or combine it with DWT or
DCT to improve the image quality.
2) The semi-fragile watermark used in the proposed algorithm does not know
the kind of attack the watermarked image is under.
3) Human visual characteristics are not adopted in the VQ-based watermark-
ing systems.
4) The extraction process is based on the product codebook. It is therefore
time-consuming if we use the full-search encoding algorithm.
5) From the experimental results, we see that in the proposed algorithm, the
robust watermark should be embedded in the first stage to obtain a high
quality. The method of analyzing the embedding order in general sense is
a problem yet to be solved.
11.3 DCT Based Multipurpose Watermarking Technique
for Image Copyright Notification and Protection
A visible watermark can act as an advertisement while the invisible watermark
can be used for copyright protection or content authentication. Relatively,
only a few researches [24, 25, 26, 27, 28] have focused on visible watermark-
ing. This may be due to the fact that the visible watermark will reduce the
commercial value of the watermarked content. It can also reduce the inten-
tion of the intruder to illicitly possess the visibly watermarked content. The
