The Content Scrambling System (CSS) Carlos Garcia Jurado Suarez 03/10/2006

What is CSS? CSS is the mechanism for encrypting data on DVDs. Created and administered by the DVD Copy Protection Association around The need: DVDs contain digital and high quality versions of movies so perfect copies can be made if the data is not protected. CSS is not very effective.

CSS basics Each DVD has a main disk key, which is encrypted with 409 keys (the keys are owned by the CCA). DVD players have a subset of keys embedded in them (licensed from the CCA) The players use one of these keys to decrypt the disk key. The decryption is validated against a hash of the decrypted key that is included in the disk.

The CSS Cipher

Why did CSS fail? Weak cipher. The CSS encryption primitive uses a 40 bit key and is based on LFSRs (a 2 16 attack is possible). Software DVD players. These have to ship keys in software so they are easily obtainable. Decrypted data is present in memory at some point. No revocation mechanism.

DeCSS Software created by three Norwegian programmers than can rip most DVDs to a hard drive. Was created be reverse engineering the Xing player (by Real Networks). Widely available online.

What’s next? At this time all 409 keys are public. New standard being developed: AACS (Advanced Access Content System). –Based on AES. –Watermarking for detection of illegal copies –NNL/Media Key Blocks for key management and revocation.