[WM]: Intentional attacks on QIM watermark

[Eugene Kurbatov <e_kurbatov@yahoo.com>]

  Hello all,
   
  I was going through the famous paper B. Chen and G. Wornell,  "Quantization index modulation: a class of provably good methods for
digital watermarking and information embedding," but one thing still remains completely unclear for me (may be it is easy question
for you, but it is really blocking point for my further studying of QIM watermarking).
   In case of dither modulation when watermark is embedded in the following way y=q(x+d(m))-d(m) one can absolutely destroy WM just
by changing y to y+a/2 ,where a is step size with the probability 1/2 and keeping y with the probability 1/2. The second attack is
requantization of watermarked signal as y'=q(y). These attacks result in "break of channel" and length-L pseudo random sequence is
useless to detect WM.
   Both of these attacks do not destroy the watermarked message drastically.
   Could you write please how does it overcome for practical systems?
   Why in the most of articles about QIM these type of attacks did not take into a consideration? I have found only the following
article: Matthias Salge "Quantization Watermarking" LNT I Seminar, WS 98/99, which deals with this type of intentional attack.
   If QIM is so easy to delete, why this approach is so popular now? It seems to me that spread-spectrum watermark is more resistant
to attacks. Especially, such its modification as Improved Spread Spectrum embedding.
   But may be I just understand this technology (QIM) completely wrong?
   Could you please explain it to me. I look forward to your suggestion and opinion.
   Many thanks in advance!
   
   Evgeny Kurbatov,
   Ph.D student
   St-Petersburg State University of Telecommunications.



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