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DRM & Key Revocation. By David Coleman. DRM & Key Revocation. Digital Rights Management – A system for controlling the use of content Key Revocation – The ability for content producers to “revoke” the ability of a given device/player to consume the content Important because…

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drm key revocation

DRM & Key Revocation

By

David Coleman

drm key revocation1
DRM & Key Revocation
  • Digital Rights Management – A system for controlling the use of content
  • Key Revocation – The ability for content producers to “revoke” the ability of a given device/player to consume the content
  • Important because…
    • Digital content can be perfectly reproduced
    • CD Audio was a disaster from music studios’ perspective (the need for DRM)
    • DVD-Video wasn’t much better (the need for good DRM)
  • I’ll be discussing 3 systems: CSS (DVD-Video), Microsoft Windows Media DRM, and AACS
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DRM & Key Revocation

Content Scramble System (CSS)

  • Used on DVD-Video discs
  • 40-bit keys using a secret encryption algorithm (2 LFSRs)
  • Keys
    • Disc key – Key that allows (indirectly) decrypting the content
    • Player key – Key that allows player to decrypt disc key
    • 400+ player keys
  • Key block
    • Disc key hashed with CSS hashing algorithm
    • Table containing disc key encrypted with all valid player keys
    • Player would decrypt the disc key and then hash it to compare against hashed value
  • Revocation
    • Removing the player key from the key block
  • Completely broken
    • Player key recovered from Xing software player
    • Remaining player keys were poorly chosen and quickly guessed
    • Encryption was very weak anyway (a few discrepencies allowed for an O(25) attack)
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DRM & Key Revocation

Microsoft Windows Media DRM

  • My motivation for this topic
  • Widely used in online music services (not Apple)
  • Basics
    • StubLib – Certificate that is statically linked in to player
    • Encrypted content file
    • License – Contains key to decrypt the content file. Encrypted with player’s public key (from StubLib).
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DRM & Key Revocation

Microsoft Windows Media DRM

License acquisition

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DRM & Key Revocation

Microsoft Windows Media DRM

  • Revocation
    • Certificate Revocation List (CRL)
    • Microsoft maintains the CRL and license servers are expected to pull and keep local copy current
    • CRL is included in the license
    • Works because API to play is a black box
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DRM & Key Revocation

Advanced Access Control System (AACS)

  • Used on next generation DVD (blue laser)
  • Strong encryption based on published standards (AES-128, SHA-1, etc.)
  • Certificates
    • Not X.509
    • Each player & drive have a certificate
  • Keys
    • Media key – necessary to decrypt content
    • Device keys
      • Each device given a set of keys
      • Sets overlap, but no two devices have the identical set
      • NNL Key Management
      • Keys actually organized in a binary tree where child keys of a node can be computed via a one-way function
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DRM & Key Revocation

AACS

  • Revocation
    • Two methods
    • CRL
      • Every disc has a player CRL and a drive CRL
      • CRLs must be stored after reading
    • Player key revocation
      • Media key is encrypted with the minimal set of keys s.t. no revoked device’s key is used but one of every valid device’s key is used
      • Subset-difference
      • Tree structure helps
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