MaximumPC 2008 04

Hollywood math:

HDCP + AACS = PITA

2

r & d BREAKING DOWN TECH —PRESENT AND FUTURE

W

e’re philosophically opposed to any

technology that impairs our fair-use

rights, but the Hollywood studios insist on

infesting their products with annoyances

such as HDCP (High-bandwidth Digital

Content Protection) and AACS (Advanced

Access Content System) which do just that.

Since we also believe knowledge is power,

here’s a primer on how those systems work.

But before we dive into the details,

allow us to point out the absurdity of digi-

tal rights management—in all its many

forms. DRM treats those of us who pur-

chase movies and music, and then make

backup copies or transfer that content

to our PCs and digital media players,

as criminals. We are not pirates; we are

Hollywood’s best customers. And we

understand that purchasing content is a

prerequisite to claiming fair-use rights.

HDCP and AACS add to the complexi-

ty and the cost of every PC, every HD DVD

and Blu-ray drive and disc, every monitor,

every commercial operating system, most

every A/V receiver, and every HD television

on the market—and all it takes to defeat

this supposedly invincible technology

is an easily obtained program: SlySoft’s

AnyDVD HD ( http://www.slysoft.com ).

THE CRYPTONOMICON

AACS is a cryptographic system designed

to limit the output and recording of pro-

tected content to methods the content

owner approves of. Unlike the simpler CCS

(Content Scramble System) used to encrypt

commercial DVDs, AACS uses a set of keys

that can be tied to a specifi c player—much

like a serial number.

The AACS process results in a “play”

or “don’t play” validation. A Blu-ray or HD

DVD disc includes a Media Key Block that is

combined with a playback device’s unique

set of keys to generate a Media Key, which is

then combined with another piece from the

disc—the Volume ID key. (Only a commercial

burner can reach the hidden section of that

disc to write the Volume ID key, a technique

designed to thwart disc-to-disc copies on

consumer-grade equipment.) The combina-

tion of aforementioned keys culminates in

the Volume Unique Key. This key is used

to decrypt the disc’s title keys and then—

fi nally—the movie itself. At this point, the

content moves into the HDCP chain, where

it is re-encrypted until it reaches the display

(we’ll explain HDCP in more detail shortly).

If an AACS device is compromised,

newly pressed content can be published

with an updated Media Key Block that is

incompatible with the compromised device’s

keys. This, in essence, revokes the known

cracked keys and disables the associated

players. The new Media Key Block, however,

will continue to function with any older keys

that remain secure.

SECRET HANDSHAKES

HDCP is an interconnect system that works

hand-in-glove with AACS. Designed to

protect the transmission of high-bandwidth

audiovisual streams—at rates up to 5GB/s—

HDCP has been deployed in all three of the

major digital display interfaces: DVI, HDMI,

and DisplayPort. It is also supported in the

less-common UDI (Unifi ed Display Interface)

and GVIF (Gigabit Video Interface). Since

it must be everywhere in the display chain,

you’ll also fi nd HDCP in videocards, Blu-ray

and HDVD DVD drives, HDTVs, A/V receiv-

ers, and CableCARD digital CATV tuners.

The technology can also be incorporated

into repeaters (which can be essential for

long cable runs) and switches (crucial for

connecting more than one player to a dis-

play with a single digital-video input).

The HDCP authentication process is the

same for every content player: Software on

the PC or set-top box generates a playback

signal, which initiates a handshake process.

Using a temporary random number, unique

keys built into each HDCP transmit device

(e.g., a videocard) and receive device (e.g.,

an HDTV) are used to generate a checksum.

If the checksums are identical at each stage,

the content is green-lit for playback.

This process is repeated every few

seconds, and the signal is reauthenticated

BY ZACK STERN

68 MAXIMUMPC | APR 08 | http://www.maximumpc.com

White Paper: HD Video Encryption

A replicator encrypts content on a Blu-ray or HD DVD disc using a Media Key and Media Key Block provided by the AACS

Licensing Authority (AACS LA). The AACS LA provides the media player (either a stand-alone device or a PC application)

with a corresponding Device Key. The media player combines its Device Key with the Media Key Block to calculate the

Media Key. If the calculated Media Key matches the Media Key on the disc, the content can be decrypted.

HOW IT WORKS

PRERECORDED VIDEO

CONTENT OWNER

LICENSING ENTITY

LICENSED REPLICATOR

TITLE(S)

ENCRYPTED

TITLE(S) TITLE(S)

TITLE KEY(S) (AUTHORIZED

USE)

ENCRYPTED

TITLE KEY(S)

MEDIA KEY

MEDIA KEY

DEVICE KEY

DEVICE KEY

USAGE

RULES

USAGE

RULES

VOLUME

IDENTIFIER

VOLUME

IDENTIFIER

MKB MKB

MKB

USAGE

RULES

ENCRYPT DECRYPT

ENCRYPT DECRYPT

HASH

HASH

MEDIA KEY

HASH

HASH

PROCESS

MKB

TITLE(S)

TITLE KEY(S)

LICENSED PLAYER

Copy protecting a Blu-ray or HD DVD movie