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covert communication in a hostile environment. Nonetheless the technique can be
used in watermarking applications for copyright protection with a high payload and
sufficient robustness.
It should be noted that the dithering attack described in this paper differs in an im-
portant aspect from the generally referred attacks on watermarks such as JPEG com-
pression and conventional signal processing. QIM-embedded watermarks are highly
robust against these attacks. For instance, watermarks in an image can still be ex-
tracted without error when PSNR becomes as low as 20dB after JPEG coding or 28dB
with interference of additive Gaussian noise [5]. By using the dither attack, in con-
trast, the hidden data are completely erased with negligible, or even without, addi-
tional degradation to the image.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No.
60072030) and Key Disciplinary Development Program of Shanghai (2001-44). The
authors are grateful to the anonymous reviewers for their valuable comments and
recommendations.
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