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…  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

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)

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).

DRM & Key Revocation Microsoft Windows Media DRM License acquisition

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

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

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

DRM & Key Revocation Questions?