Page 9

Disc Recognition

The active protection mechanisms employed by XCP and MediaMax regulate access to raw CD audio,

blocking access to the audio tracks on albums protected with a particular scheme while allowing access to
all other titles.

To accomplish this, the schemes install a background process that interposes itself between applications

and the original CD driver. In MediaMax, this process is a kernel-mode driver called

sbcphid.sys

XCP uses a pair of filter drivers called

crater.sys

and

cor.sys

that attach to the CD-ROM and IDE

devices [29]. In both schemes, the active protection drivers examine each disc that is inserted into the
computer to see whether access to it should be restricted. If the disc is recognized as copy protected, the
drivers monitor for attempts to read the audio tracks, as would occur during a playback, rip, or disc copy
operation, and corrupt the audio returned by the drive to degrade the listening experience. MediaMax
introduces a large amount of random jitter, making the ripped audio sound like it has come from a badly
scratched or damaged CD; XCP replaces the audio with random noise.

Each scheme's active protection software interferes with attempts to rip or copy any disc that is protected

by the same scheme, not merely the disc from which the software was installed. This requires some mecha-
nism for identifying discs that are to be protected. This section discusses the security requirements for such
a recognition system, describes the design and limitations of the actual recognition mechanism employed by
the MediaMax scheme, and presents an improved design that better satisfies the requirements.

5.1

Recognition Requirements

Any disc recognition system detects some distinctive feature on discs protected by a particular copy protec-

tion scheme. Ideally, such a feature would satisfy these requirements:

1. Uniqueness. The feature should identify protected discs without accidentally triggering the copy

protection on unprotected titles.

2. Detectability. It should be possible for the active protection drivers running on client systems to

reliably and quickly detect the feature in protected discs. In practice, this limits the amount of audio
that can be read from the disc before deciding whether to apply protection.

3. Indelibility. The feature should be hard to remove without substantially degrading the quality of the

audio; that is, it should be difficult to make a copy of the copy protected disc that does not itself trigger
the protection.

4. Unforgeability. It should be difficult to apply the feature to an unprotected album without the cooper-

ation of the protection vendor, even if the adversary has access to protected discs.

This last requirement stems from the DRM vendor's platform building strategy, which tries to put the

DRM software on as many computers as possible and to have the software control access to all marked discs.
If the vendor's identifying mark is forgeable, then a record label could mark its discs without the vendor's
permission, thereby taking advantage of the vendor's platform without paying.

There are advantages and disadvantages for an entity placing unauthorized marks. Copyright would

prohibit rogue publishers from distributing an installer for the active protection software, though they might
depend on the existing installed base if the software was included on many widely sold titles. They would

Forging a mark is probably not copyright infringement. Unlike the musical work in which it is embedded, the mark itself is

functional and contains little or no expression, and therefore seems unlikely to qualify for copyright protection. In principle, the
mark recognition process could be covered by a patent, but we are unaware of any such patent relating to XCP or MediaMax. Even
if the vendor does have a legal remedy, it seems worthwhile to design the mark to prevent forgery if the cost of doing so is low.