Content Scramble System for DVD

PeiXian Yan,Bo Zhou,Gang Liu, ZongPeng Liu, Matthew Black December 6,2004

Supervised by Andy Brown

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

Introduction

What is CSS?CSS: Content Scramble System.It is the data scrambling method used to garble the cont

ent 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 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 Cannot play DVD under Linux OP DeCSS introduced. What is DeCSS ?DeCSS is an executable binary utility, written for Micr

osoft 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.

Introduction

‘ *.vob ’ file MPEG-4 file (very large) (much smaller)

MPEG-2file

(protectedBy CSS)

DeCSS FlaskMPEG

How to store the DVD data in to PC

DVD PC

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 Norwegian put it on to the web in late September 1999.

MPAA(The Motion Picture Association of America )’s response.

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All versions of DeCSS currently in release are built around the Xing player key, which reportedly has been revoked.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.

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.

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

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

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)

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)

Generic LFSR

A shift register Tap sequence Certain tap sequences will cycle through all

2^n-1 possible internal states (called maximal length LFSR)

XOR

Output

Feedback Path

1 0 1 1 1 1 0 0 1 0 1 0 1 0 0 1 1

XOR

Output

CSS’ LFSR17

1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 1 1

XOR

Output

CSS’ LFSR17

CSS’ LFSR17

1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 1 1

XOR

Output

1 1

CSS’ LFSR17

1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 1 1

XOR

Output

0

CSS’ LFSR17

1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 10

XOR

Output

1

0

CSS’ LFSR17

1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 10

XOR

Output

10

0

CSS’ LFSR17

1 0 1 1 1 1 0 0 1 0 1 0 1 1 0 10

XOR

Output

1

0

CSS’ LFSR17

1 0 1 1 1 1 0 0 1 0 1 0 1 1 01 0

XOR

Output

1

01

CSS’ LFSR17

1 0 1 1 1 1 0 0 1 0 1 0 1 1 01 0

XOR

Output

1 0

01

CSS’ LFSR17

1 0 1 1 1 1 0 0 1 0 1 0 1 1 01 0

XOR

Output

1

01

CSS’ LFSR17

1 0 1 1 1 1 0 0 1 0 1 0 1 11 1 0

XOR

Output 011

0

CSS’s LFSRs CSS: LFSR17 (2 bytes+1bit seeded in b

it 4) CSS: LFSR25 (3 bytes+1bit seeded in b

it 4) So……CSS uses a 40-bits key Addition between the LFSRs

More on LFSR

Bit-wise Inverter before addition Bit-wise Inverter before addition

1 byte

Output-byte

LFSR-17

LFSR-25

+8-bit add

Optional bit-wise inverter

Optional bit-wise inverter

Carry-out fromthe previous

addition

1 byte

Carry-out

inverter modes

Mode LFSR-17 LFSR-25

Authentication Yes No

Session Key No No

Title key No Yes

Data Yes No

Data Encryption

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 Table-based substitution

XOROutput data bytes

Key Encryption/Decryption

0

Permutation table

+

Permutation table

K0

+ K0

1

1

Permutation table

+

Permutation table

K1

+ K1

2

2

Permutation table

+

Permutation table

K2

+ K2

3

3

Permutation table

+

Permutation table

K3

+ K3

4

4

Permutation table

+

Permutation table

K4

+ K4

5

Bytes of Ciphertext

Bytes of Plaintext

CSS streamcipher used to encrypt/decrypt keys

Play a CSS protected disc

DVD itself Content delivery in between DVD player

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)

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

Mutual Authentication

Host Drive

AGID

Request AGID

Chanllenge(H) (nonce)

Encrypted Chanllenge(H)

Chanllenge(D) (nonce)

Success or Failure

Encrypted(D)

Initialization done

Encrypt Challenge

Decrypt and verify Challenge(D)

Session key is encrypted Challenge(H) + Challenge(H)

Decrypt and verify Challenge(H)

Encrypt Challenge(D)

Session key is encryptedChallenge(H) + Challenge(H)

Initiaization done

Data transfer Decrypt disk key Verify disk key (has

h) Decrypt the title ke

y Data decrypted by t

he XOR of the title key and the sector

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 hash making it feasible in runtime applications

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.

Attack with 6-bytes of LFSR output.

1. For each Guess of the contents of LSFR-17

1. Clock out 4 bits2. Get the output of LSFR-25 by

subtracting3. Workout the contents of LSFR-25

from the output

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.

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

Content Protection Technologies

Copy protection methods integrated within DVDs

Copy Generation Management System (CGMS)

Analog Protection System (APS) Content Scrambling System (CSS)

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)

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

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

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)

The various approaches High-bandwidth Digital Content

Protection (HDCP) Extended Conditional Access (XCA) Advanced Access Content System

(AACS)

CSS CPPM 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 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

CSS vs AACS

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

A Mechanism of encrypt data to DVD disk.

Still been used?