1.11 What are the copy protection issues?
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This article is from the DVD Formats FAQ, by jtfrog@usa.net (Jim Taylor) with numerous contributions by others.
1.11 What are the copy protection issues?
There are four forms of copy protection used by DVD:
1) Analog CPS (Macrovision)
Videotape (analog) copying is prevented with a Macrovision 7.0 or similar
circuit in every player. The general term is APS (Analog Protection
System). Computer video cards with composite or s-video (Y/C) output must
also use APS. Macrovision adds a rapidly modulated colorburst signal
("Colorstripe") along with pulses in the vertical blanking signal ("AGC")
to the composite video and s-video outputs. This confuses the
synchronization and automatic-recording-level circuitry in 95% of consumer
VCRs. Unfortunately, it can degrade the picture, especially with old or
nonstandard equipment. Macrovision may show up as stripes of color,
distortion, rolling, black & white picture, and dark/light cycling.
Macrovision creates severe problems for most line doublers. Macrovision is
not present on analog component video output of early players, but is
required on newer players (AGC only, since there is no burst in a component
signal). The discs contain "trigger bits" in the header of each sector
telling the player whether or not to enable Macrovision AGC, with the
optional addition of 2-line or 4-line Colorstripe. This allows fine control
over which sections are copy protected. The producer of the disc decides
what amount of copy protection to enable and then pays Macrovision
royalties accordingly (a few cents per disc). Just as with videotapes, some
DVDs are Macrovision-protected and some aren't. (For a few Macrovision
details see STMicroelectronics' NTSC/PAL video encoder datasheets at
<http://www.st.com/stonline/books/>.)
2) CGMS
Each disc also contains information specifying if the contents can be
copied. This is a "serial" copy generation management system (SCMS)
designed to prevent copies or copies of copies. The CGMS information is
embedded in the outgoing video signal. For CGMS to work, the equipment
making the copy must recognize and respect the CGMS. The analog standard
(CGMS/A) encodes the data on NTSC line 21 (in the XDS service). The digital
standard (CGMS/D) is not yet finalized, but will apply to digital
connections such as IEEE 1394/FireWire. See section 4, below.
3) Content Scrambling System (CSS)
Because of the potential for perfect digital copies, paranoid movie studios
forced a deeper copy protection requirement into the DVD standard. Content
Scrambling System (CSS) is a data encryption and authentication scheme
intended to prevent copying video files directly from the disc. CSS was
developed primarily by Matsushita and Toshiba. Each CSS licensee is given a
key from a master set of 400 keys that are stored on every CSS-encrypted
disc. This allows a license to be revoked by removing its key from future
discs. The CSS decryption algorithm exchanges keys with the drive unit to
generate an encryption key that is then used to obfuscate the exchange of
disc keys and title keys that are needed to decrypt data from the disc. DVD
players have CSS circuitry that decrypts the data before it's decoded and
displayed. On the computer side, DVD decoder hardware and software must
include a CSS decryption module. All DVD-ROM drives have extra firmware to
exchange authentication and decryption keys with the CSS module in the
computer. Beginning in 2000, new DVD-ROM drives are required to support
regional management in conjunction with CSS (see 1.10 and 4.1). Makers of
equipment used to display DVD-Video (drives, decoder chips, decoder
software, display adapters, etc.) must license CSS. There is no charge for
a CSS license, but it's a lengthy process, so it's recommended that
interested parties apply as soon as possible. Near the end of May 1997, CSS
licenses were finally granted for software decoding. The license is
extremely restrictive in an attempt to keep the CSS algorithm and keys
secret. Of course, nothing that's used on millions of players and drives
worldwide could be kept secret for long. In October 1999, the CSS algorithm
was cracked and posted on the Internet, triggering endless controversies
and legal battles (see 4.8).
4) Digital Copy Protection System (DCPS)
In order to provide for digital connections between components without
allowing perfect digital copies, five digital copy protection systems have
been proposed to CEMA. The frontrunner is DTCP (digital transmission
content protection), which focuses on IEEE 1394/FireWire but can be applied
to other protocols. The draft proposal (called 5C, for the five companies
that developed it) was made by Intel, Sony, Hitachi, Matsushita, and
Toshiba in February 1998. Sony released a DTCP chip in mid 1999. Under
DTCP, devices that are digitally connected, such as a DVD player and a
digital TV or a digital VCR, exchange keys and authentication certificates
to establish a secure channel. The DVD player encrypts the encoded
audio/video signal as it sends it to the receiving device, which must
decrypt it. This keeps other connected but unauthenticated devices from
stealing the signal. No encryption is needed for content that is not copy
protected. Security can be "renewed" by new content (such as new discs or
new broadcasts) and new devices that carry updated keys and revocation
lists (to identify unauthorized or compromised devices). A competing
proposal, XCA (extended conditional access), from Zenith and Thomson, is
similar to DTCP but can work with one-way digital interfaces (such as the
EIA-762 RF remodulator standard) and uses smart cards for renewable
security. Other proposals have been made by MRJ Technology, NDS, and
Philips. In all five proposals, content is marked with CGMS-style flags of
"copy freely", "copy once," "don't copy," and sometimes "no more copies".
Digital devices that do nothing more than reproduce audio and video will be
able to receive all data (as long as they can authenticate that they are
playback- only devices). Digital recording devices are only able to receive
data that is marked as copyable, and they must change the flag to "don't
copy" or "no more copies" if the source is marked "copy once." Digital CPS
is designed for the next generation of digital TVs, digital receivers, and
digital video recorders. It will require new DVD players with digital
connectors (such as those on DV equipment). These new products won't appear
until 2000. Since the encryption is done by the player, no changes are
needed to the existing disc format.
The first three forms of copy protection are optional for the producer of a
disc. Movie decryption is also optional for hardware and software playback
manufacturers: a player or computer without decryption capability will only
be able to play unencrypted movies. DCPS is performed by the DVD player,
not by the disc developer.
These copy protection schemes are designed only to guard against casual
copying (which the studios claim causes billions of dollars in lost
revenue). The goal is to "keep the honest people honest." Even the people
who developed the copy protection standards admit that they won't stop
well-equipped pirates. There are inexpensive devices that defeat
Macrovision, although only a few work with the new Colorstripe feature.
These devices go under names such as Video Clarifier, Image Stabilizer, and
CopyMaster.
Movie studios have promoted legislation making it illegal to defeat DVD
copy protection. The result is the World Intellectual Property Organization
(WIPO) Copyright Treaty and the WIPO Performances and Phonograms Treaty
(December 1996) and the compliant U.S. Digital Millennium Copyright Act
(DMCA), passed into law in October 1998. Software intended specifically to
circumvent copy protection is now illegal in the U.S. and many other
countries. A co-chair of the legal group of the DVD copy protection
committee stated, "in the video context, the contemplated legislation
should also provide some specific assurances that certain reasonable and
customary home recording practices will be permitted, in addition to
providing penalties for circumvention." It's not at all clear how this
might be "permitted" by a player or by studios that set the "don't copy"
flag on all their discs.
DVD-ROM drives and computers, including DVD-ROM upgrade kits, are required
to support Macrovision, CGMS, and CSS. PC video cards with TV outputs that
don't support Macrovision will not work with encrypted movies. Computers
with IEEE 1394/FireWire connections must support the final DCPS standard in
order to work with other DCPS devices. Every DVD-ROM drive must include CSS
circuitry to establish a secure connection to the decoder hardware or
software in the computer, although CSS can only be used on DVD-Video
content. Of course, since a DVD-ROM can hold any form of computer data,
other encryption schemes can be implemented. See 4.1 for more information
on DVD-ROM drives.
The Data-Hiding Sub-Group (DHSG) of the industry's Copy Protection
Technical Working Group (CPTWG) is evaluating watermarking proposals. The
original seven watermarking proposals that were merged into three: IBM/NEC,
Hitachi/Pioneer/Sony, and Macrovision/Digimarc/Philips. On February 17,
1999, the first two groups combined to form the "Galaxy Group" and merged
their technologies into a single proposal. Watermarking, which is used for
DVD-Audio and will be added to DVD-Video at some point, permanently marks
each digital audio or video frame with noise that is supposedly
undetectable by human ears or eyes. Watermark signatures can be recognized
by playback and recording equipment to prevent copying, even when the
signal is transmitted via digital or analog connections or is subjected to
video processing. New players and other equipment will be required to
support watermarking, but the DVD Forum intends to make watermarked discs
compatible with existing players. There were reports that the early
watermarking technique used by Divx caused visible "raindrop" or "gunshot"
patterns, but the problem seemed to have been solved for later releases.
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