watermark in an audio file. This uses similar
watermark in an audio file. This uses similar auditory masking processes to those exploited by audio compression schemes. Automatic web spiders continuously can search the web looking for open (unencrypted) content that carries invisible watermarks. A typical example is the MarcSpider from DigiMarc. Persistent watermarks usually should be able to survive copying to be effective. The copies may be compressed using codecs like JPEG or MPEG. There may be intermediate analogue copies. The watermark should survive all this signal processing. There are other watermarking schemes where the mark deliberately is made very fragile. It should not survive the copying process, so if it is missing, that content can be identified as a copy. This is very much the analogue of the bank note. The lack of the watermark would indicate a counterfeit. Unlike data encryption, watermarks can be embedded in the waveform rather than the data. This can be used for images, audio, and video. Spread spectrum techniques can be used to add data in the time domain. These can be thought of as time-delayed echoes. These can be recovered by cepstrum analysis. If video and audio are compressed by a codec like MPEG, the coding thresholds can be modulated with the watermark data. Watermarks are no panacea. They can be defeated, just as encryption can be broken. They form part of an environment where the theft of digital assets is made more difficult and the culprits can be traced more easily. Security Before setting up a secure media distribution system, it is a good idea to look at your goals. What are you trying to protect and how much do you want to spend on security? The security analysis splits into three areas: 1. The content 2. The monetary transaction 3. The server infrastructure DRM gives protection of the content and often includes the monetary transaction. The transaction can use mature technologies from the e-commerce arena. The third area, the server infrastructure, is covered by normal enterprise level computer security (outside the scope of this book). The infrastructure should be protected for several reasons. One is the value of the content asset library; another is that attack could compromise the brand of the publisher, aggregator, or retailer. This attack could be from denial of service or loss of data integrity. Rights management 279
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