Content Scramble System for DVD PeiXian Yan,Bo Zhou,Gang Liu, ZongPeng Liu, Matthew Black

1. Content Scramble System for DVD PeiXian Yan,Bo Zhou,Gang Liu, ZongPeng Liu, Matthew Black December 6,2004 Supervised by Andy Brown

2. Content Scramble System • Introduction to CSS and DeCSS • Encryption on the DVD in CSS • How a DVD player plays DVD • Cryptanalysis of CSS • Comparison with other techniques • Conclusion

3. Introduction • What is CSS? CSS: Content Scramble System. It is the data scrambling method used to garble the content of a DVD disc. Data on DVD is protected by CSS,DVD can not be copied. Only be usable with licensed DVD playback mechanisms. Windows and MAC have CSS licence. Linux does not.

4. Introduction • How does CSS work? Every DVD player on the market today is coded with a small set of "player keys" Every DVD disc on the market today is coded with a "disk key", identifying that disc. When a DVD player attempts to read a DVD, the player uses it's player key and proceeds down the list of encrypted disk keys on the disc .

5. Introduction • Cannot play DVD under Linux OP • DeCSS introduced. • What is DeCSS ? DeCSS is an executable binary utility, written for Microsoft Windows. Unscrambled MPEG-2 video files can be copied to the user's hard drive by DeCSS. MPEG-4 video files can be made from DVD very easily,which is very easy to transfer through the web.

6. Introduction How to store the DVD data in to PC DVD PC MPEG-2 file (protected By CSS) ‘ *.vob ’ file MPEG-4 file (very large) (much smaller) DeCSS FlaskMPEG

7. Introduction • Where does DeCSS come from? An anonymous German hacker from MoRE(master of reverse engineering) was respons for writing the code. Jon Johanson, a 16-year-old Norwegianput it on to the web in late September 1999. MPAA(The Motion Picture Association of America )’s response.

8. Introduction • How does DeCSS work ? DeCSS operates much as any other DVD player operates - it uses a player key to unscramble the scrambled contents of a DVD to make playable MPEG-2 video files. All versions of DeCSS currently in release are built around the Xing player key, which reportedly has been revoked. If this is true, no newly-released DVDs can be descrambled with this player key; DeCSS will not work on these DVDs.

9. Introduction • Why was CSS made so weak? CSS uses a 40-bit key. Even if the scrambling algorithm is well-designed, the short key length means that a brute-force search will quickly find the key ! Since at the time (in 1996) the U.S. export regulations banned export of strong encryption technologies.

10. Introduction CSS is different from other examples of cryptography such as encrypted e-mail. Unlike encrypted e-mail where the objective of the encryption is to maintain privacy, CSS has nothing to do with maintaining privacy or secrecy of the video. Anyone who buys a DVD containing a CSS "encrypted" movie can view that movie by placing it in a DVD player. This is totally unlike encrypted mail which only the intended recipients can read.

11. CSS Overview • Protection from piracy • Client-host authentication • Enforce region-based codes • Stream encryption

12. Keys for in CSS • Region key • Authentication key • Session key • Player key • Disk key • Title key • Sector Key- in bytes 80-84 of a sector (a logical or physical group of bytes recorded on the disc)

13. Encryption in CSS • System’s security depends entirely on the insides of the keystream generator. (APPLIED CRYPTOGRAPHY, BRUCE SCHNEIER) • So……what keystream we need? • Pseudo-random bit stream • Generates unpredictable key-stream (at least in any reasonable amount of time, harder time to break it)

14. Generic LFSR • A shift register • Tap sequence • Certain tap sequences will cycle through all 2^n-1 possible internal states (called maximal length LFSR) Output XOR Feedback Path

15. 1 0 1 1 1 1 0 0 1 0 1 0 1 0 0 1 1 XOR Output CSS’ LFSR17

16. 1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 1 1 XOR Output CSS’ LFSR17

17. 1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 1 1 1 1 XOR Output CSS’ LFSR17

18. 1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 1 1 0 XOR Output CSS’ LFSR17

19. 0 1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 1 1 XOR Output 0 CSS’ LFSR17

20. 0 1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 1 1 0 XOR Output 0 CSS’ LFSR17

21. 0 1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 1 1 XOR Output 0 CSS’ LFSR17

22. 1 0 1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 1 XOR Output 01 CSS’ LFSR17

23. 1 0 1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 1 0 XOR Output 01 CSS’ LFSR17

24. 1 0 1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 XOR 1 Output 01 CSS’ LFSR17

25. 1 1 0 1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 XOR Output 011 CSS’ LFSR17

26. CSS’s LFSRs • CSS: LFSR17 (2 bytes+1bit seeded in bit 4) • CSS: LFSR25 (3 bytes+1bit seeded in bit 4) • So……CSS uses a 40-bits key • Addition between the LFSRs

27. LFSR-17 Optional bit-wise inverter +8-bit add 1 byte LFSR-25 Optional bit-wise inverter Carry-out Carry-out from the previous addition More on LFSR • Bit-wise Inverter before addition 1 byte Output-byte • Bit-wise Inverter before addition

28. inverter modes

29. Data Encryption • LFSRs are seeded • Generates pseudo-random bit stream • Substitution on Video data byte • XORed the bitstream and Substitution

30. Data Encryption XOR Output byte from LFSRs Output data bytes Input data byte Table-based substitution

31. Key Encryption/Decryption CSS streamcipher used to encrypt/decrypt keys Bytes of Ciphertext 0 1 2 3 4 Permutation table Permutation table Permutation table Permutation table Permutation table + K0 + K1 + K2 + K3 + K4 Permutation table Permutation table Permutation table Permutation table Permutation table + + + + + K0 K1 K2 K3 K4 Bytes of Plaintext 1 2 3 4 5

32. Play a CSS protected disc • DVD itself • Content delivery in between • DVD player

33. DVD and DVD player • Encrypted content (hidden area) • A table of encrypted disk keys, disk hash • Player keys (used to decrypt the disk key) • Region code( identifies in where player should be used) • Another secret (used for authentication)

34. Mutual Authentication • Between the Host and the Player. • With the authenticated device (licensed by the DVD Copy Control Association) • Verifies both sender and receiver are licensed to use the system • A session key is agreed on to prevent eavesdropping

35. Mutual Authentication Request AGID Drive Host AGID Initiaization done Initialization done Chanllenge(H) (nonce) Encrypted Chanllenge(H) Encrypt Challenge Decrypt and verify Challenge(H) Chanllenge(D) (nonce) Encrypt Challenge(D) Encrypted(D) Success or Failure Decrypt and verify Challenge(D) Session key is encrypted Challenge(H) + Challenge(H) Session key is encrypted Challenge(H) + Challenge(H)

36. Decrypt disk key Verify disk key (hash) Decrypt the title key Data decrypted by the XOR of the title key and the sector Data transfer

37. Brute Force attack on disk keys • CSS only uses 40 bit keys • Possible to find disk key by looking at 240 possible disk keys. • This attack is in fact possible with a complexity of 225 by attacking the hashmaking it feasible in runtime applications

38. Attack with 6-bytes of LFSR output. • Not a terribly useful attack, we don’t normally have 6-bits lying around • Provides a 216 attack on the algorithm • Allows us to find 16(plus 1) bit register • Find input of LFSRS • Hence we have the key.

39. Attack with 6-bytes of LFSR output. • For each Guess of the contents of LSFR-17 • Clock out 4 bits • Get the output of LSFR-25 by subtracting • Workout the contents of LSFR-25 from the output

40. Attack with 5-bytes of LFSR output. • Much more practical here • For each guess of contents of LSFR-17 • Clock out 3 bytes from LSFR • Determine corresponding bytes from LSFR-25 • Reveals all but highest order bit from LSFR-25 • Attempt to verify each final bit.

41. CSS Mangling • When used to encrypt keys an additional mangling step takes place • By trying all 256 possibilities • Possible to recover 5 output bytes from LSFRS and hence find key from above attack

42. Content Protection Technologies

43. Copy protection methods integrated within DVDs • Copy Generation Management System (CGMS) • Analog Protection System (APS) • Content Scrambling System (CSS)

44. CGMS • Each sector of a DVD disc includes CGMS that defines how many times the data can be copied. • Three copying “states”: --copy enable, copy one generation, copy never • Two formats: --analog(i.e., CGMS-A), digital(i.e., CGMS-D)

45. APS A method of forcing copies to be degraded or inhibited when copies are made of video signals containing the Macrovision signals. Two separate technologies: Automatic Gain Control (AGC) Color Stripe

46. CSS • A data encryption and authentication scheme intended to prevent copying video files directly from the disc.

47. The various approaches • Content Protection for Recordable Media (CPRM) • Content Protection for Pre-recorded Media (CPPM) • Content Protection System Architecture (CPSA) • Digital Transmission Content Protection (DTCP)

48. The various approaches • High-bandwidth Digital Content Protection (HDCP) • Extended Conditional Access (XCA) • Advanced Access Content System (AACS)

49. Protects video content distributed on DVD Uses 40-bit key Weak key management Common weakness Protect pre-recorded DVD audio content Uses 56-bit key Better key management Common weakness CSS CPPM

50. CSS vs AACS • CSS uses a 40-bit key. ----brute force attack can be carried out with a complexity of 240 • AACS uses AES-128 ----brute force attack can be carried out with a complexity of 2128