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Attacks on Watermarks

The possible attacks against watermarks are wide and varied. Cox et al [CL97] present a number of such attacks, including:

• Image modification attacks. These use image transformations such as those listed above.
• Bit-level attacks. If the attacker has access to a watermark presence detector, the contents and location of the watermark can be derived. This also makes it much easier to remove a watermark.
• Watermark-insertion attacks. If the attacker has access to a watermark insertion device, and the watermarking process is not a one-way function, it is possible to recover the original, unwatermarked image, by pre-distorting the copy, and rewatermarking it.
• Statistical averaging attacks. The attacker uses multiple watermarked images to estimate the watermark, and then subtracts this from the image. This is especially a problem with video.
• Scrambling attacks. By inserting a scrambler before the watermark detector, and a de-scrambler after it, detection of the watermarking can be avoided.

As we can see from this list, even with a perfect watermarking method, there are various system-level attacks that can frustrate the secure use of watermarks in a copyright scheme.