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The Replacement Attack

Billions of dollars allegedly lost to piracy of multimedia have recently triggered the industry to rethink the way music and movies are distributed. As encryption is vulnerable to rerecording, currently all copyright protection mechanisms tend to rely on watermarking. A watermark is an imperceptive secret hidden in a host signal. In this paper, we analyze the security of multimedia copyright protection systems that use watermarks by proposing a new breed of attacks on generic watermarking systems. A typical replacement attack relies upon the observation that multimedia content is often highly repetitive. Thus, the attack procedure replaces each signal block with another, perceptually similar block computed as a combination of other similar blocks found either within the same media clip or within a library of media clips. Assuming the blocks used to compute the replacement are marked with distinct secrets, we show that if the computed replacement block is at some minimal distance from the original marked block, a large portion of the embedded watermark is removed. We describe the logistics of the attack and an exemplary implementation against a spread-spectrum data hiding technology for audio signals.