Regardless of how well the unencrypted digital content is protected within
the computer, it is usually possible to perform an analog capture of the content
after it leaves the computer. Of course, this incurs a generation loss, which
reduces the quality of the content.
The Windows Media Rights Manager
The Windows Media Rights Manger is an attempt to create a centralized, OS-
level digital rights management infrastructure that provides secure playback
and handling of copyrighted content. The basic idea is to separate the media
(which is of course encrypted) from the license file, which is essentially the
encryption key required to decrypt and playback the media file.
The basic approach involves the separation of the media file from the play-
back license, which is also the decryption key for the media file. When a user
requests a specific media file the content provider is sent a Key ID that
uniquely identifies the user’s system or player. This Key ID is used as a seed to
create the key that will be used for encrypting the file. This is important—the
file is encrypted on the spot using the user’s specific encryption key. The user
then receives the encrypted file from the content provider.
When the user’s system tries to play back the file, the playback software
contacts a license issuer, which must then issue a license file that determines
exactly what can be done with the media file. It is the license file that carries
the decryption key.
It is important to realize that if the user distributes the content file, the recipi-
ents will not be able to use it because the license issuer would recognize that the
player attempting to play back the file does not have the same Key ID as the orig-
inal player that purchased the license, and would simply not issue a valid license.
Decrypting the file would not be possible without a valid decryption key.
Secure Audio Path
The Secure Audio Path Model attempts to control the flow of copyrighted,
unencrypted audio within Windows. The problem is that anyone can write a
simulated audio device driver that would just steal the decrypted content
while the media playback software is sending it to the sound card. The Secure
Audio Path ensures that the copyrighted audio remains in the kernel and is
only transmitted to audio drivers that were signed by a trusted authority.
Watermarking
Watermarking is the process of adding an additional “channel” of impercepti-
ble data alongside a visible stream of data. Think of an image or audio file. A
watermark is an invisible (or inaudible in the case of audio) data stream that is
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