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Consumer Video DVD Content Protection In 1987, the appearance of Digital Audio Tape
(DAT) recorders, as the first consumer digital-recording technology, raised
the bar of content protection by enabling professional bootleggers to create multiple generations of
perfect copies. Alarmed by the potential for enormous losses, the recording
industry did its best to kill the medium, just as Hollywood had attempted to
outlaw VCRs a decade earlier. It also exempted manufacturers from prosecution for infringement in exchange for collecting a royalty on recorders and blank media that would compensate copyright holders for presumed piracy losses. Surprisingly, this provision did little to hobble large-scale bootlegging operations by excusing high-end pro-quality DAT recorders from the SCMS requirement. Despite its attempt to effect a compromise, the AHRA ultimately gave the recording industry everything it wanted. Although DAT had once been eagerly anticipated by both consumers and audiophiles, years of delaying tactics and public disdain for copy-protection eventually ended its chances as a consumer format. Similar constraints helped put nails in the coffins of subsequent digital audio-recording technologies, such as the Philips Digital Compact Cassette and Sony's original MiniDisc. Unlike the compact disc, which was created by only two companies, the DVD is a committee compromise. Virtually all the current DVD specifications are the result of hard-fought compromises pounded out by the computer, consumer-electronics and content industries. Motivated by the potential for enormous royalties, powerful multinationals competed fiercely at every stage of the effort to promote technologies that incorporate their patents. Even more contentious was the question of copy protection. Hardware manufacturers would have been happy to simply ignore the issue, but the DAT (digital audiotape) debacle had taught them the folly of butting heads with the content industry. The major labels and studios, which had always been spooked by the idea of perfect digital copying, were close to panic over the ease with which CDs could be ripped by anyone with an off-the-shelf disc recorder. They blackballed all DVD proposals that didn't include robust security measures, and threatened to do anything in their power to kill formats that didn't adequately protect content. The argument became so ferocious that the DVD Forum, which had been struggling to define a definitive set of DVD formats, finally handed off the problem to an ad hoc organization called the Copy Protection Technical Working Group. The CPTWG assumed the task of evaluating copy-protection technologies and, although its goal was to merely stimulate informal debate, it managed to resolve issues that would otherwise have plagued the industry for years. This division of labor also helped the DVD Forum avoid encumbering its core DVD specifications with politically charged content-protection schemes. All CPTWG recommendations exist as independent technologies that are maintained and licensed by third-parties. Accommodating them required the Forum to make only minor revisions to its own specs. When the smoke cleared, DVDs had been crammed with more layers of copy protection than any other consumer format.
Many of these technologies were circumvented shortly after their release, but that hasn't ended calls to add even more anti-piracy features. The justification for these continued efforts - a copy-protection scheme doesn't necessarily become ineffective when it's defeated by a determined hacker. Most are intended primarily to limit "casual copying," in which a consumer cavalierly runs off a quick duplication for a friend. One justification for packing so many levels of copy protection into DVDs was to make circumvention tools too complicated, specialized, and intimidating for mainstream consumers. The downside of all this technology is a DVD content-protection system so complex that it requires a formal architecture to coordinate its components. This framework, called the Content Protection System Architecture (CPSA), is an outgrowth of a 1999 proposal by IBM, Intel, Matsushita and Toshiba (the so-called "4C Entity"), a group that also helped develop technologies like CPPM and CPRM. CPSA consists of a list of 11 rules (or "axioms") that define standard ways to implement access and recording controls specified by content providers. These rules describe cross-platform procedures and codes of conduct that help integrate discrete content-protection modules into a cohesive system free from conflicts, redundancies, inconsistencies, and omissions. CPSA is applicable to both computer and consumer-electronics devices, as well as to audio and video content in either analog or digital formats. Compliant technologies include most DVD-specific copy-protection schemes, such as CSS, CPPM, CPRM, and Verance's DVD-Audio Watermarking. CPSA guidelines also apply to technologies that protect streaming content, such as DTCP, HDCP, and Conditional Access, which safeguards pay-per-view programming distributed via satellite or cable. CPSA-compliant technologies can incorporate two types of content-protection mechanisms: watermarking and encryption. Watermarking embeds copy-protection information (known as a "watermark") directly into an audio or video stream. Watermarks usually contain usage rules known as Content Management Information (CMI), which specify how the content's owner will allow his property to be accessed, played, or copied. This embedded data is intended to be transparent to the consumer, but is detected by the compliant hardware or software modules that enforce the system. If a content owner doesn't want to risk degrading content with a watermark, CPSA also allows CMI to be packaged as a discrete piece of data that accompanies, but is not embedded into, the content. The lock that enforces such a system in the digital domain is encryption, which scrambles content until it is decrypted by a compliant device, according to the rules specified by the CMI. In the case of DVD content protected by CPPM or CPRM, encryption is performed by the C2 (Cryptomeria Cipher) function, a mathematical operation designed specifically to protect multimedia content. Other technologies, such as CSS, HDCP and DTCP, each use their own types of encryption functions. The Cryptomeria Cipher (C2) is a Feistel network-based block cipher
designed for use in the area of digital entertainment content protection. The cipher was designed for cryptographic
robustness, efficiency when implemented in software, and small size when implemented in hardware. CPRM technology provides the following features:
CPRM is similar to CPPM in using a MKB. All media used with CPRM must include a BCA (Burst Cutting Area) which can be used to record a unique media ID as part of the manufacturing process. This will then prevent the data from being copied to another disc because it will not have the same unique media ID and therefore the data cannot be decrypted. CPRM was developed by 4C (comprising IBM, Intel, MEI and Toshiba) and uses 56-bit keys and C2 encryption for content encryption. It allows for a hacked playback device to be revoked using a Media Key Block (MKB). The MKB is stored as a file on the disc and contains a very large number of keys. Each licensed recorder has assigned to it a set of unique device keys that allow it to obtain the Media Key from the MKB and decrypt the audio content.
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