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CGMS & CSS & CPPM & CPRM

1) CGMS
Each disc contains information specifying if the contents can be copied. This is a serial copy generation management system (SCMS) designed to prevent initial copies or generational copies (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 information. The analog standard (CGMS-A) encodes the data on NTSC line 21 (in the XDS service) or line 20. CGMS-A is recognized by most digital camcorders and by some computer video capture cards (they will flash a message such as “recording inhibited”). Professional time-base correctors (TBCs) that regenerate lines 20 and 21 will remove CGMS-A information from an analog signal. The digital standard (CGMS-D) is included in DTCP and HDMI for digital connections such as IEEE 1394/FireWire. See subsections 6 and 7 below.

2) Content Scramble System (CSS)
Because of the potential for perfect digital copies, paranoid movie studios forced a deeper copy protection requirement into the DVD standard. Content Scramble System (CSS) is a data encryption and authentication scheme intended to prevent copying video files directly from DVD-Video discs. CSS was developed primarily by Matsushita and Toshiba. Each CSS player licensee is given a key from a master set of 409 keys stored on every CSS-encrypted disc. The theory was to allow 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, and computer 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. As of 2000 DVD-ROM drives are required to support regional management in conjunction with CSS . Makers of equipment used to display DVD-Video (drives, decoder chips, decoder software, display adapters, etc.) must license CSS. There is an annual $15,000 fee for the CSS license, and qualification is a lengthy process, so it’s recommended that interested parties apply early. CSS is administered by the DVD Copy Control Association (DVD CCA). 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.

3) Content Protection for Prerecorded Media (CPPM)

CPPM is used only for DVD-Audio. It was developed as an improvement on CSS. Keys are stored in the lead-in area, but unlike CSS no title keys are placed in the sector headers. Each volume has a 56-bit album identifier, similar to a CSS disc key, stored in the control area. Each disc contains a media key block, stored in a file in the clear on the disc. The media key block data is logically ordered in rows and columns that are used during the authentication process to generate a decryption key from a specific set of player keys (device keys). As with CSS, the media key block can be updated to revoke the use of compromised player keys. If the device key is revoked, the media key block processing step will result in an invalid key value. The authentication mechanism is the same as for CSS, so no changes are required to existing drives. A disc may contain both CSS and CPPM content if it is a hybrid DVD-Video/DVD-Audio disc.

4) Content Protection for Recordable Media (CPRM)
CPRM is a mechanism that ties a recording to the media on which it is recorded. It is supported by some DVD recorders, but not by many DVD players. Each blank recordable DVD has a unique 64-bit media ID etched in the BCA. When protected content is recorded onto the disc, it can be encrypted with a 56-bit C2 (Cryptomeria) cipher derived from the media ID. During playback, the ID is read from the BCA and used to generate a key to decrypt the contents of the disc. If the contents of the disc are copied to other media, the ID will be absent or wrong and the data will not be decryptable.

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