DVD Demystified

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Chapter 4
DVD-Audio content may use both CSS and CPPM. CPPM is planned for
use on other prerecorded media such as SD memory cards.
CPPM has some similarities to CSS but is more robust and sophisticated.
It has no title keys, and the disc key is replaced by an album
identifier. The role of the album identifier is to provide a key that cannot be
duplicated on recordable media, since it is stored in the control area of the
lead-in, which is not accessible on writable discs. Each player has a set of 16
device keys. Key sets may be either unique to each device or shared by multiple
devices. Device keys are highly confidential. Rather than secretly storing
the set of all known device keys on the disc, CPPM stores a media key
block (MKB) in the DVDAUDIO.MKB file on the disc. The media key block
is provided by the 4C entity to disc replicators. The player performs a series
of decryptions and mathematical transforms on the media key block with
its device key. The resulting media key is used with the album identifier to
decode the encrypted portions of the disc. If a device key is revoked, the
media key block is changed on future discs. Revoked players will then generate
an invalid media key that will not decrypt the disc. 11
As with CSS, only audio, video, subpicture, and still picture sectors are
encrypted. Other sectors containing navigation, highlight, and real-time
information are not encrypted.
Content Protection for Recordable Media (CPRM) CPRM is a
mechanism that ties a recording to the medium on which it is recorded. It
was developed by the same 4C group that created CPPM and shares many
features. CPRM is supported by all DVD recorders released after 1999. The
goal of CPRM is to enable a recording to be made and played on different
devices while ensuring that copies of the recording will not be playable.
CPRM is defined for writable DVD formats and for SD memory cards. It is
intended to be used for other recordable media such as compact flash cards
and microdrives.
Each blank recordable DVD disc has a unique 64-bit media identifier
(media ID) etched in the burst cutting area (BCA; see Chapter 5). This
means that each disc can be uniquely distinguished from all other recordable
discs. The media ID does not have to be secret but must be an unalterable
value tied to the medium. When protected content is recorded onto the
11 This feature is called renewability by the creators of the process, but it does not renew anything.
On the contrary, it can make formerly functioning devices cease to work. Perhaps they were
thinking of “renew” as used in Logan’s Run. A problem with revocable devices is that if keys are
stolen or the system is cracked so that legitimate keys can be used in unauthorized devices, the
keys cannot be revoked without making hundreds or thousands of genuine players stop working.