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DRM & Key Revocation

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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  1. DRM & Key Revocation By David Coleman

  2. 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

  3. 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)

  4. 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).

  5. DRM & Key Revocation Microsoft Windows Media DRM License acquisition

  6. 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

  7. 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

  8. 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

  9. DRM & Key Revocation Questions?

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