December 05, 2006
WATERMARK RECOVERYArchitectures for Forensic WatermarkingOf course, a watermarking technique is only effective insofar as the forensic message can be recovered from a subject content instance. In general, recovery consists of searching the content instance for the watermark signal(s), extracting the forensic message codeword, and applying the inverse error control algorithms, if used, to estimate the original forensic message. Ideally, the recovery process provides a confidence estimate for the accuracy of the recovered forensic message. This conceptually simple model is shown in Figure 7:
 Informed vs. Blind Recovery - Recovery scenarios are typically classified according to the availability of the original unmarked content to the demodulation process. In the blind scenario, the demodulator has no knowledge of the original content, and thus must be able to separate a weak watermark signal from the strong host signal. In communications terminology, the demodulator must handle very poor signal-to-noise ratios (SNR). In the Informed recovery scenario, the demodulator greatly improves the SNR by removing the original (unmarked) host signal from the subject content instance, ideally leaving a residual containing only the watermark signal. The only noise arises from the errors made in matching the original host signal to the recovered content. A weaker watermark signal, relative to the blind recovery scenario, can thereby produce the same degree of robustness. Consequently, watermarking for the informed recovery scenario permits less perceptible watermarks and corresponding image quality improvements. Certain implementations of the replacement model carry informed recovery a step further by using the previously described metadata to anticipate the location and exact content of each watermark. With this additional information, a very subtle watermark can be used, without sacrificing robustness.
 In practice, the subject content instance will have often undergone some decidedly unhelpful transformations. These transformations can be modeled at a high level as shown in Figure 8. The content may be altered during capture, particularly if the pirate is not able to (or chooses not to) directly record the digital representation of the content. Resampling of analog signals typically changes quantization, resolution, and alignment. Camcorder captured video content may exhibit substantial rotation, keystone distortions, pincushion, and jitter. The pirate may also attempt to attack the watermark directly, to disguise the origin of the content. Filters can be designed to obscure watermarks which are confined to narrow frequency bands. Collusion attacks attempt to obscure watermarks by combining several separately marked copies of the content in some manner. The pirate may also edit the content for his intended distribution channel by changing frame rate, aspect ratio, interlace, resolution, gamma, and color mapping. All of these transformations distort both the watermark signal and the host signal. In the informed recovery scenario, the demodulator must closely match the recovered instance to the original content, in order to accurately remove the host signal. The demodulator can also use the original content to accurately model the transformations that distort the watermark and host signals, and then invert these transformations before searching for the watermark. In the blind recovery scenario, some model of the transformations must also be constructed, so as to arrive at an estimate of the target watermark signal. Much of the art in an effective watermarking system is in the design of a watermark signal that is resistant to the channel transformations, while facilitating successful recovery. Watermark literature also defines a semi-informed recovery scenario, where the recovery process is provided some embedding information, but not the entire original source. Such information may consist of keys or equivalent secret information which aids the recovery process in determining the location of watermarks or interpreting their meaning. Semi-informed recovery scenarios can improve robustness by depriving the attacker of essential information about the watermarks. It is important to note that the recovery process must include a means of determining the nature of the watermarks in the subject content. This determination may become problematic in either of the architectures described. The relatively static single-ended embedding architecture is straightforward, but only if the distribution system can ensure that all rendering devices have equivalent watermarking technology. The roll-out of an upgrade may also introduce ambiguity, since the many recovery scenarios would lack a priori knowledge of whether the subject content was rendered before or after the upgrade took effect on the particular device. In either case, if the watermarking technology is not known to the recovery process, recovery must search through a repertoire of techniques. Since most techniques are proprietary, several software implementations may have to be tried to achieve a successful recovery. The problem takes on a different form in the replacement scenarios. Since the nature of the watermark is determined prior to distribution, all subject instances associated with the distribution will have been watermarked consistently. If, however, the distributor chooses to vary the nature of the watermark between content titles, or separate distributions of the same content, it may become necessary to track what sort of watermark was supplied in the metadata for each title or distribution.
CONCLUSIONS
Footnotes:
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2 WATERMARKING CONCEPTS
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3 ARCHITECTURAL ADVANTAGES
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4 SINGLE-ENDED EMBEDDING ARCHITECTURE
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5 THE REPLACEMENT MODEL
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6 WATERMARK RECOVERY & CONCLUSIONS
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