DMCA Crypto Software

Nomen Nescio nobody at dizum.com
Thu Apr 17 19:50:04 EDT 2003


Cryptography Research's content protection system is summarized at
http://www.cryptography.com/resources/whitepapers/SelfProtectingContent_sum.pdf
and apparently described in more detail in the patent application at
http://cryptome.org/kocher-etal.htm.  The basic idea is to include some
executable code with the content which would run in a VM in every content
player.  This code would then make use of pre-existing low-level crypto
functions and keys built into the players to decrypt and play the content.

The main advantage of this approach is that as various content protection
schemes are cracked, or keys extracted, future releases of content
can be updated to use new schemes and to invalidate the stolen keys.
Unlike current approaches which build the technology into firmware in
players, this method would be much more flexible and adaptable in the
face of ongoing attacks.

Note that there would still be limits on what could be done in terms of
revising protection schemes, based on the cryptographic primitives which
are built into players.  If some future design for content protection
were invented which relied on the Weil pairing, for example, it would
not be implementable if the necessary primitives are not supported on
widely used devices.

A disadvantage is that each player must be powerful enough to run this
VM program at speed.  It's not clear how complex the programs will be
or how fast they will have to run.  Providing general purpose computing
functionality may be more expensive and difficult than implementing a
special-purpose standard in firmware and hardware, especially for small
portable devices.  The CR report estimates that a 1 MIPS processor would
be adequate, but they don't offer any justification.

All cryptography-based protection schemes have a fundamental flaw, which
is that keys "scraped" out of authorized devices can be used to unlock
data and then release it in unprotected form.  CR proposes to address this
by watermarking the data.  For this to work, the watermark must inherently
reveal the key which was used for the decryption.  This involves the
technique broadly known as traitor tracing.  The problem is that if the
bad guys have access to a considerable number of stolen keys it may be
intractable to devise a traitor tracing scheme that can identify them.

CR advocates "forensic watermarking".  In the longer report (available by
email request) they describe this as a system where there are two versions
of selected portions of the content - for example, two alternate versions
of a particular movie frame.  There would be multiple such "polymorphs"
throughout the content, and each device would have keys such that for each
polymorph it would see only one version.  By randomizing and encrypting
the frames it can be arranged that the devices can't even tell which
frames are polymorphic.  The set of keys assigned to a playback device
implicitly identifies the device itself, so that if an unprotected version
of the movie is released, the specific versions of the polymorphs that
are present will reveal which device did the decryption.

The obvious attack is to combine the output from multiple devices
from which keys have been scraped, but this does not work (up to a
point) because even when multiple devices are used, there is still
enough information in the output to identify which specific devices
were involved.  CR gives an example of a 90 minute movie, 30 frames per
second, with 1% of the frames being polymorphic - 1620 frames.  Even if
an adversary breaks into 4 playback devices and gets their keys in order
to identify the polymorph frames, the manufacturer can identify those four
devices with an error probability, according to the formula derived by the
CR report, of less than 4 x 10^(-10), an extremely good detection rate.

But what happens if you use the CR formula with the assumption that
the attacker cracks one more device for a total of 5?  Suddenly the
system doesn't work so well, and there are over 10^20 possible sets
of 5 devices that could produce the combined output!  We go from
4 x 10^(-10) to 10^20 with just one more device.  This kind of exponential
explosion is common to many traitor tracing schemes.  The attackers
have an inherent mathematical advantage which is very hard to address.
All this is glossed over in the CR analysis.

And this is leaving aside the question of whether we can create
polymorphic frames at all - 2 different versions of a movie frame, either
of which is equally plausible as an intermediate frame among the others
in the sequence.  It must be done so that looking at one version of the
movie or the other will not call attention in any way to the frames which
have been altered like this.  And it has to be doable in a completely
(or at least largely) automated way, due to the volume of content that
would have to be protected.

It also would be desirable to create content that can run on all future
players, at a time when many of them may not yet have been built or even
designed.  For example, pressing a CD today, it should still play on CD
players built 10 years from now.  Could the content be unlocked by devices
whose keys have not yet been chosen, but still be watermarked so as to
reveal those keys?  Or would we need a centralized key-creation agency
which pre-creates all of the keys which will ever be used in a CD player
for the rest of time, and then assigns them to manufacturers who enter
the CD business?  Most of these traitor tracing and broadcast-encryption
schemes have been implemented in closed environments like satellite TV
broadcasts, but the complexity of a multi-vendor world and an open-ended
future may push this technology beyond its current limitations.

Even in the most optimistic assessment, what we would expect to see
is a constant struggle between "protectors" and "unlockers".  Each
success by the unlockers will be thwarted in the next generation of
content released by the protectors.  But the old content will still be
vulnerable.  So we would expect that this approach would yield a time
window for protection.  At the time it was released, new content would
use the latest techniques and be relatively secure.  Over time, breaks
would occur to which content released in the past would be vulnerable.
So content in general would have a limited time in which it was protected.
This would be the window in which profitability was possible.  Content
older than some threshold - weeks?  months?  years? - would be freely
available.

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