Content Scramble System for DVD Pei Xian Yan

Content Scramble System for DVD Pei. Xian Yan, Bo Zhou, Gang Liu, Zong. Peng Liu, Matthew Black December 6, 2004 Supervised by Andy Brown

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

Introduction n 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.

Introduction n 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.

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

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

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

Introduction n How does De. CSS work ? De. CSS 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 De. CSS currently in release are built around the Xing player key, which reportedly has been revoked.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; De. CSS will not work on these DVDs.

Introduction n 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.

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.

CSS Overview n n Protection from piracy Client-host authentication Enforce region-based codes Stream encryption

Keys for in CSS n n n n 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)

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

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

CSS’ LFSR 17 1 0 1 1 0 0 1 0 1 0 0 1 1 XOR Output

CSS’ LFSR 17 1 0 1 1 0 0 1 0 1 1 XOR Output

CSS’ LFSR 17 1 0 1 1 0 0 1 0 1 1 1 1 XOR Output

CSS’ LFSR 17 1 0 1 1 0 0 1 0 1 1 0 XOR Output

CSS’ LFSR 17 0 1 1 1 1 0 0 1 0 1 1 XOR Output 0

CSS’ LFSR 17 0 1 1 1 1 0 0 1 0 1 1 0 XOR Output 0

CSS’ LFSR 17 0 1 1 1 1 0 0 1 0 1 1 XOR Output 0

CSS’ LFSR 17 1 0 1 1 0 0 1 0 1 XOR Output 01

CSS’ LFSR 17 1 0 1 1 0 0 1 0 1 1 0 XOR Output 01

CSS’ LFSR 17 1 0 1 1 0 0 1 0 1 1 0 XOR 1 Output 01

CSS’ LFSR 17 1 1 0 0 1 0 1 1 0 XOR Output 011

CSS’s LFSRs n n CSS: LFSR 17 (2 bytes+1 bit seeded in bit 4) CSS: LFSR 25 (3 bytes+1 bit seeded in bit 4) So……CSS uses a 40 -bits key Addition between the LFSRs

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

inverter modes Mode LFSR-17 LFSR-25 Authentication Yes No Session Key No No Title key No Yes Data Yes No

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

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

Key Encryption/Decryption CSS streamcipher used to encrypt/decrypt keys Bytes of Ciphertext 0 1 2 3 4 Permutation table Permutation table + K 0 Permutation table + Bytes of Plaintext 1 + K 1 Permutation table K 0 + 2 + K 2 Permutation table K 1 + 3 + K 3 Permutation table K 2 + 4 + K 4 Permutation table K 3 + 5 K 4

Play a CSS protected disc n n n DVD itself Content delivery in between DVD player

DVD and DVD player n n n 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)

Mutual Authentication n n 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

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

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

Brute Force attack on disk keys n n n 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 hash making it feasible in runtime applications

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

Attack with 6 -bytes of LFSR output. 1. For each Guess of the contents of LSFR-17 1. 2. 3. Clock out 4 bits Get the output of LSFR-25 by subtracting Workout the contents of LSFR-25 from the output

Attack with 5 -bytes of LFSR output. n n Much more practical here For each guess of contents of LSFR-17 n n 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.

CSS Mangling n n n 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

Content Protection Technologies

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

CGMS n n n 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)

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

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

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

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

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

CSS vs AACS n n 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

CSS vs AACS n n AACS uses advanced Media Key Block (MKB) to manage and revoke keys AACS would potentially allow people to store copies of a movie on home computers and watch it on other devices connected to a network—or even transfer it to a portable movie player

Conclusion n n A Mechanism of encrypt data to DVD disk. Still been used?