Architectures for Forensic Watermarking

In interpreting bandwidth, however, it is important to distinguish between the original message and an encoded message. Forensic watermarking implementers may apply multiple layers of error control coding (ECC) to the message, to compensate for the "noise" in the channel. Such coding can expand the message several fold, thereby reducing the effective bandwidth of the watermarking technique by the same factor. It is also common to embed several copies of the message into the content. For a robust implementation, the bandwidth requirement is many times that implied by the message length.

Robustness - Robustness is the degree to which the watermark can remain viable against the various transformations the content undergoes before reaching the recovery process. An effective forensic watermarking system must be robust against operations such as rescaling, resampling, recompression, cropping, rotation, resolution changes, deinterlacing, gamma changes, and temporal averaging, all of which may occur in the course of pirating the content. Additionally, a pirate may undertake attacks to directly suppress the watermark by filtering, noise addition, collusion or other signal processing techniques.

Although no watermarking technique can by unconditionally robust, the most effective techniques should require the adversary either to apply an unreasonable amount of effort or to unacceptably degrade the content, in order to successfully suppress the watermark. Viewed as a signal processing problem, robustness tends to increase with the energy of the watermark signal, which is dependent upon the watermark's intensity, size, and duration. Paradoxically, however, if the signal intensity level reaches the threshold of perceptibility, its nature and location become apparent to the attacker, thereby compromising robustness. Consequently, the watermark strength must be carefully calibrated to achieve maximum robustness.

As mentioned previously, error control coding is important to attaining robustness. Alterations to the content may erase or distort significant portions of the watermark signal. Effective recovery must include mechanisms to compensate for missing or erroneous signal segments. In signal processing terms, the watermark system constitutes an extremely noisy channel, requiring aggressive error control.

Renewability - Content pirates have unfailingly adapted to media security technology. Watermarking will not be spared. As watermarking technology is deployed, adversaries will build tools to suppress the watermark. As such tools become widely available, the targeted watermark technology is rendered ineffective. Thus the ability to evolve to more robust watermarking techniques, and to vary the watermark signal, is a hallmark of an effective system.

Flexibility - Analogous to renewability, flexibility describes the ease of adapting watermarking to the needs presented by specific content items. The universe of content includes a broad spectrum of exposures to piracy, as well as sensitivity to quality. Thus content providers may favor watermark perceptibility-robustness tradeoffs that differ by content item and according to content provider policy. Ideally, a watermark system should provide a means of control, to effect the content provider's preferences.