Steganographic File Systems

The Movie Industry

Copy Protection for DVD Video

DVD players were introduced into the consumer market segment in late 1996. The capacity of these disks is significantly larger than audio CDs, 4.7 Gb per side versus 650 Mb. Prerecorded movies use the MPEG-2 compression scheme and are subsequently encrypted prior to being stored on DVD.

Unlike a DVD copy, a copy of a VHS tape looks inferior to the original tape. Thus as second-generation DVD players with digital video recording capabilities continue to be introduced into the marketplace, there is a pressing need to provide several levels of copy protection.

The Supreme Court has held that personal, home videotape recording of a television broadcast for time-shifting purposes is a fair use and therefore does not constitute copyright infringement.

The DVD copy-protection system is designed to support a copy generation management system (CGMS). CGMS/A is used to control the amount of legal copies allowed. The CGMS/A information is embedded in the outgoing video signal. For CGMS/A to work, the equipment making the copy must recognize and respect the CGMS. This requires at least two bits of information to be associated with a piece of video, indicating one of the following copy states:

• "copy_never"

• "copy_once"

• "copy_no_more"

• "copy_freely"

These metadata are problematic in two respects:

1. Getting the metadata out is expensive and the computer industry does not want the responsibility.

2. Copy control could survive analog-to-digital-to-analog conversions, which is where watermarking comes in, and that means that watermarking hardware will need to be in every DVD player.

This leads to the Copy Protection Technical Working Group, which follows two major principles:

1. The copy protection system is not mandatory; devices are either compliant or noncompliant.

2. The system is cost effective.

At present, there are two components that are already being built into consumer devices, the content scrambling system and the analog protection system.

Content Scrambling System (CSS)

• Scrambles MPEG 2.

• Two keys, one to the disk, the other to the MPEG file.

• Keys are stored in the lead area of the disk.

• Usually only works in compliant drives.

• Prevents byte-for-byte copies of a MPEG data stream.

• Encourages the manufacture of compliant drives.

Analog Protection System (APS)

This modifies NTSC/PAL signals so they can be viewed on a TV but not recorded by a VCR. DVDs are not NTSC/PAL encoded, so the encoder must be in the DVD player.