Cryptography Reference
In-Depth Information
(a) (b)
Fig. 11.8.
The watermarks extracted from the original image using the two-stage
codebooks. (a) Extracted robust watermark, NHS = 1.0. (b) Extracted semi-fragile
watermark, NHS = 0.047.
stage VQ codebook. The fragile watermark cannot be extracted from the
original image. We can decide that there are no watermarks embedded in the
original image. The first problem of algorithm [20] described in Section 11.2.1
is therefore solved.
JPEG Compression Attacks
In this experiment, we perform JPEG compression using different quality
factors (QF) on the watermarked image as shown in Fig. 11.9. Here QF =
100%, 80%, 50% and 30%, respectively. The extracted watermarks and NHS
values are depicted in Fig. 11.10. From these results, we see that the proposed
algorithm is robust to JPEG compression. Where QF is larger than 80%,
the extracted watermarks, both robust and semi-fragile, are similar to the
embedded ones. For all cases, the extracted robust watermarks have relatively
high NHS values. From these results, we see that the VQ indices can, to some
extent, tolerate the incidental distortions produced by high-quality JPEG
compression.
VQ Compression Attacks
We now use four different codebooks to compress the watermarked image.
Codebook 1 is the product codebook used in our method. Codebook 2 with
a size of 8192 and Codebook 3 with a size of 256 are both trained from the
Lena image. The corresponding PSNRs of the encoded Lena images are 37.08
dB and 30.39 dB respectively. Codebook 4 with a size of 4096 is trained from
the Pepper image. The corresponding PSNR of the encoded Pepper image is
33.74 dB. Fig. 11.11 shows the four VQ-attacked watermarked images, and
Fig. 11.12 shows the watermarks extracted from these images. We can see that
from these results the proposed algorithm can extract the same watermarks
as the embedded ones from the VQ compressed watermarked image with the
product codebook. The reason for this is that the watermarked image is not
modified under the VQ compression with the product codebook. For the other
cases, the robust watermark can tolerate the VQ compression, while the semi-
fragile watermark cannot. The higher the codebook performance is, the larger
the NHS value of the semi-fragile watermark.
