Content Scramble System (CSS) is a Digital Rights Management (DRM) and encryption system employed on almost all commercially produced DVD-Video discs. Content Scrambling System utilizes a proprietary 40-bit stream cipher algorithm. Content Scrambling System has been superseded by newer DRM schemes such as Content Protection for Recordable Media (CPRM) or by Advanced Encryption Standard (AES) in Advanced Access Content System (AACS) DRM scheme used by HD DVD and Blu-ray Disc, which have 56-bit and 128-bit key sizes providing a much higher level security than the 40-bit key size of Content Scrambling System.

Advantages of Content Scrambling System

  • Content Scrambling System prevents byte-for-byte copies of an MPEG stream for being playable since such copies do not include the keys that are hidden on the lead-in area of the restricted DVD.
  • Content Scrambling System provides a reason for manufacturers to make their devices complaint with an industry-controlled standard, since CSS scrambled discs cannot in principle be played on noncompliant devices, anyone wishing to build complaint devices must obtain a license, which contains the requirement that rest of the DRM system be implemented.

Keys used in Content Scrambling System

  • Authentication Key – The secret is used as a part of mutual Authentication process.
  • Session key ( also called as Bus Key) – This key is negotiated during authentication and is used to encrypt the title and disk keys before sending them over the unprotected bus. The encryption is necessary to prevent eavesdropping.
  • Player Key – This key is licensed by the DVD Copy Control Association to the manufacturer of a DVD player. It is stored within the player. It is used to establish the trustworthiness of the player. It is used to decrypt the disk key.
  • Disk Key – This key is used to encrypt title key. It is decrypted using the player key.
  • Sector Key – Each sector has a 128-byte plain-text header. Bytes 80-84 of each sector’s header contain an additional key used to encode the data within the sector.
  • Title Key – This key is XORed with a per-sector key to encrypt the data within the sector.

Process Involved in Content Scrambling System

  • Mutual Authentication – The host and the drive use a challenge-response system to establish their trustworthiness to each other. In this process, they negotiate session key.
  • Decoding disk – The DVD player tries each of several player keys until it can decode the disk key. The disk key is a disk-wide secret.
  • Send Disk and Title keys – The title and bus keys are sent from the player to the host. The session key is used to encrypt the title and disk keys in transit to prevent a man-in-the-middle attack.
  • The DVD player sends a sector to the host.
  • The host decodes the title key using disk key.
  • The host decodes the sector using the title key and a sector key in the sector’s header.

Content Scrambling System (CSS) includes both player-host mutual authentication and data encryption, is used to protect the content of DVDs from piracy and to enforce region-based viewing restrictions.