written by Peter Gutmann, Department of Computer Science, University of Auckland
Last updated 27 December 2006
Executive Summary
Windows Vista includes an extensive reworking of core OS elements in order to provide content protection for so-called "premium content", typically HD data from Blu-Ray and HD-DVD sources. Providing this protection incurs considerable costs in terms of system performance, system stability, technical support overhead, and hardware and software cost. These issues affect not only users of Vista but the entire PC industry, since the effects of the protection measures extend to cover all hardware and software that will ever come into contact with Vista, even if it's not used directly with Vista (for example hardware in a Macintosh computer or on a Linux server). This document analyses the cost involved in Vista's content protection, and the collateral damage that this incurs throughout the computer industry.
Executive Executive Summary
The Vista Content Protection specification could very well constitute the longest suicide note in history.
Introduction
This document looks purely at the cost of the technical portions of Vista's content protection [Note A]. The political issues (under the heading of DRM) have been examined in exhaustive detail elsewhere and won't be commented on further, unless it's relevant to the cost analysis. However, one important point that must be kept in mind when reading this document is that in order to work, Vista's content protection must be able to violate the laws of physics, something that's unlikely to happen no matter how much the content industry wishes it were possible. This conundrum is displayed over and over again in the Windows content-protection specs, with manufacturers being given no hard-and-fast guidelines but instead being instructed that they need to display as much dedication as possible to the party line. The documentation is peppered with sentences like:
"It is recommended that a graphics manufacturer go beyond the strict letter of the specification and provide additional content-protection features, because this demonstrates their strong intent to protect premium content".
This is an exceedingly strange way to write technical specifications, but is dictated by the fact that what the spec is trying to achieve is fundamentally impossible. Readers should keep this requirement to display appropriate levels of dedication in mind when reading the following analysis [Note B].
Disabling of Functionality
Vista's content protection mechanism only allows protected content to be sent over interfaces that also have content-protection facilities built in. Currently the most common high-end audio output interface is S/PDIF (Sony/Philips Digital Interface Format). Most newer audio cards, for example, feature TOSlink digital optical output for high-quality sound reproduction, and even the latest crop of motherboards with integrated audio provide at least coax (and often optical) digital output.
Since S/PDIF doesn't provide any content protection, Vista requires that it be disabled when playing protected content. In other words if you've invested a pile of money into a high-end audio setup fed from a digital output, you won't be able to use it with protected content. Similarly, component (YPbPr) video will be disabled by Vista's content protection, so the same applies to a high-end video setup fed from component video.
Indirect Disabling of Functionality
As well as overt disabling of functionality, there's also covert disabling of functionality. For example PC voice communications rely on automatic echo cancellation (AEC) in order to work. AEC requires feeding back a sample of the audio mix into the echo cancellation subsystem, but with Vista's content protection this isn't permitted any more because this might allow access to premium content. What is permitted is a highly-degraded form of feedback that might possibly still sort-of be enough for some sort of minimal echo cancellation purposes.
The requirement to disable audio and video output plays havoc with standard system operations, because the security policy used is a so-called "system high" policy: The overall sensitivity level is that of the most sensitive data present in the system. So the instant any audio derived from premium content appears on your system, signal degradation and disabling of outputs will occur.
What makes this particularly entertaining is the fact that the downgrading/disabling is dynamic, so if the premium-content signal is intermittent or varies (for example music that fades out), various outputs and output quality will fade in and out, or turn on and off, in sync. Normally this behaviour would be a trigger for reinstalling device drivers or even a warranty return of the affected hardware, but in this case it's just a signal that everything is functioning as intended.
