We will now discuss and show graphically results of subjecting the smoking lady image to a variety of non-malicious and malicious attacks. The method of attack will be described and possible reasons for why one algorithm achieved stronger correlation over the other will try to be discovered.
Testing the Cox and Xie watermarking algorithms
weaknesses and strengths were uncovered. Cox was found to be stronger
of the two algorithms tested. Both Cox's and Xie's watermark was vulnerable
against cropping and rotation. Cox's algorithm faired better than Xie's
against the lower bit level attacks such as Linear Geometric Transformation,
row and column removal and NxN median filtering. Cox's algorithm was found
to be more robust, one reason being that it stores the watermark information
in significant regions of the DCT were as Xie's manipulates a sliding
window of bits in the DWT that can easily be changed. Another weakness
of Xie's algorithm found was that a picture only has to be cropped 1%
to completely destroy watermark recovery process. This is true because
cropping the Xie watermarked image slightly destroys the watermark reconstruction
point that is needed to extract the watermark correctly. Cox's watermark
was affected by cropping when most of the watermark had been cropped out.
Even though Cox's algorithm was one of the first robust watermarking algorithms
developed it was determined to be superior to Xie's algorithm.
The un-attacked watermarked images correlations differed between the Xie and Cox algorithm. Parameters for the algorithms were adjusted to achieve the best-correlated and unambiguous watermark possible. The embedding strength of Xie was set to 0.7 and the watermark length was set to 800. When embedding strength was set to above 0.7, say 0.8, the watermarked image was very blotchy and the watermark was definitely noticeable. The un-attacked extracted watermark had a correlation of 0.899999. The embedding strength of the Cox watermark was set at 0.5 and length of the watermark was 800. When trying to increase length of watermark or embedding strength led to a blotchier watermarked image. The un-attacked extracted watermark had a correlation of 0.976267.
NxN Median F
Figure #2: Cox NxN Median Filtering
Figure #3: Unfiltered and 11x11 Median Filtered Images