Drm key revocation
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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

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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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DRM & Key Revocation

Questions?


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