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Public watermarks allow embedded signals to be extracted from audio and video content for a variety of purposes. One application is for copyright control, where it is envisaged that digital video recorders will not permit the recording of content that is watermarked as "never copy". In such a scenario, it is important that the watermark survive both normal signal transformations and attempts to remove the watermark so that an illegal copy can be made. In this paper, we discuss to what extent a public watermark can be resistant to tampering and describe a variety of possible attacks.