Image analysis – Applications
Reexamination Certificate
1998-05-20
2002-07-02
Mancuso, Joseph (Department: 2721)
Image analysis
Applications
C713S176000
Reexamination Certificate
active
06415040
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and arrangement for detecting a watermark embedded in an information signal. The invention also relates to a method of removing a watermark from an information signal having an embedded watermark.
BACKGROUND OF THE INVENTION
Watermarks are perceptually invisible messages embedded in information signals such as multimedia material, e.g. audio, still pictures, animations or video. Watermarks can be used to identify the copyright ownership of information. They allow a copyright owner to trace illegal copies of his material by inspecting whether his watermark is present in said copies.
Watermarks are embedded in an information signal by modifying data samples of the signal (e.g. audio samples of an audio signal, pixels of an image, transform coefficients of a transform-coded signal, etc.) such that the original is not perceptibly affected. Various methods of watermarking are known in the art. For example, pixels of an original image are slightly incremented or decremented in accordance with corresponding bits of a binary watermark pattern.
In order to detect whether an information signal has an embedded watermark, the signal is subjected to a statistical analysis. The statistical analysis yields a parameter, hereinafter referred to as “decision variable”, which indicates to which extent the watermark is present in the signal. For example, if an image signal is watermarked by incrementing or decrementing its pixels in accordance with a watermark pattern, the decision variable may be the amount of correlation between the signal and an applied reference copy of the watermark. If an image is watermarked by modifying selected pixels, a prediction for said pixels is calculated from temporally or spatially adjacent pixels. The decision variable may then be the number of pixels being sufficiently different from their prediction.
Prior art watermark detectors generate a binary output signal indicating “watermark found” or “no watermark found”. This is achieved by comparing the decision variable with a predetermined threshold. If the value of the decision variable exceeds the threshold, the watermark is considered to be present in the signal. In consumer products such as home recorders, the watermark detector will generally be implemented as a tamperproof box, so that an attacker can neither reversely engineer the detection algorithm nor its implementation parameters. It has been found, however, that an attacker can nevertheless remove a watermark by observing the detector's binary output signal under various input signal conditions.
OBJECT AND SUMMARY OF THE INVENTION
It is an object of the invention to provide a method and arrangement for detecting a watermark which is less vulnerable to attacks.
To this end, the method in accordance with the invention is characterized by the step of randomly generating said output signal for decision values below said threshold. Preferably, the random output signal is generated for a range of decision values between said threshold and a further predetermined threshold.
The invention is based on the recognition that the prior art watermark detectors exhibit a sharp transition between the decisions “watermark found” and “no watermark found”. This property allows an attacker to iteratively modify an input signal and observe the detector's output until he has found an input signal which causes the detector to operate in the vicinity of its threshold. Having thus found the transition point, it is not difficult to generate an input signal which closely resembles the watermarked signal but is not recognized as being watermarked. By randomizing the transition point of the detector, the attacker acquires less (or at least less reliable) information from each signal modification.
Further advantageous embodiments of the invention are defined in the dependent claims.
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patent: 5748783 (1998-05-01), Rhoads
patent: 5933798 (1999-08-01), Linnartz
patent: 6128401 (2000-10-01), Suzuki et al.
patent: 6131161 (2000-10-01), Linnartz
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patent: 0766468 (1997-04-01), None
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T. Kalker et al., “Watermark estimation through detector analysis”, 1998 International Conference on Image Processing, Oct. 1998.*
Harold Stone, “Analysis on Image Watermarks with Randomized Coefficients”, NEC Research Institute, 1996.*
T. Kalker, “A security risk of publicly available watermark detectors”, Benelux Information theory Symposium, May 1998.*
“Analysis of the Sensitivity Attack Against Electronic Watermarks in Images”, Jean-Paul M.G. Linnartz and Marten Van Dijk, IHW '98 Proceedings of the International Information . . . , vol., Apr. 1998 (Oregon, U.S.A.).
Depovere Geert F. G.
Kalker Antonius A. C. M.
Linnartz Johan P. M. G.
Maes Maurice J. J. J-B.
Rongen Peter M. J.
Belk Michael E.
Koninklijke Philips Electronics , N.V.
Mancuso Joseph
Miller Martin E
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