Introduction
Digital
Video Broadcasting (DVB) is a standard defining a one-to-many unidirectional
data network for sending digital TV programs over satellite, cable, and other
topologies. The standard enables the broadcasting industry to offer hundreds of
pay-TV channels to consumers. The expanded capacities make the broadcast
signals more valuable and attractive to signal thefts. To protect a DVB
data-network, the DVB standard integrates into its broadcasting infrastructure
an access control mechanism, commonly known as Conditional Access, or CA
for short.
This
report is an overview of the DVB-CA architecture. The approach is to provide a
full picture of a typical DVB-CA deployment in operation. First, each of the
major components is individually described. Then, an operational walk-through
is given to show how the different components work together. Throughout the
report, the architecture’s enabling technologies are mentioned. However, the
technical details are not elaborated but instead deferred to references.
Functional Partitions
The
DVB-CA architecture manages end-users ‘access to protected contents with three
elements: data scrambling, a subscriber authorization system (SAS),
and a subscriber management system
(SMS). Together,
they form three layers around the protected contents:
Data
scrambling encrypts the digital-TV contents at the center. The subscriber
authorization system controls the data-scrambling element by handling the
secured distribution of descrambling keys to authorized subscribers. Knowing
which subscribers entitle to what contents, the subscriber management system
delivers access permissions to the SAS for enforcement.
The protection scope of the DVB-CA
architecture ends at the boundary where protected contents are legitimately
descrambled. Thus, DVB-CA offers no protection when a legitimate subscriber wires
up a receiver to tap out the descrambled contents.
Data-Scrambling
The data-scrambling element is the encryption of TV
contents. To avoid confusion, the DVB-CA specification uses the terms scrambling
and descrambling to mean the encrypting and decrypting of TV contents,
differentiating other uses of cryptography in the broader DVB infrastructure.
The broadcast center does the scrambling, and receivers perform the
descrambling.
Subscriber
Authorization System
The
SAS element implements the access-control protocol. It enforces end-users
‘access rights by allowing only authorized subscribers to descramble the
contents. SAS uses cryptography extensively, and the system is designed to be
renewable inexpensively as a strategy to contain damage from being compromised.
Subscriber Management
System
The
SMS element grants access rights. Operating from the business operation center,
the SMS maintains a database of subscribers. For each subscriber, the SMS
database records subscription level, payment standing, and a unique ID inside
the subscriber’s smart card. The SMS uses the information to decide which TV
channels a subscriber is entitled to view, and the access permissions are given
to the subscriber authorization system for enforcement.
System Architecture
The next picture
depicts the major components of the DVB-CA architecture and their relations:
The scrambler and descrambler implement
the data-scrambling element, and control words are the cipher keys. CA-Host,
CA-Client, and CA-Module are the three distributed components of
the SAS element, and they use CA descriptors and CA messages (EMM
and ECM) for communication.
Scrambler and
Descrambler
The data-scrambling cipher is called DVB Common Scrambling
Algorithm (DVB-CSA). The algorithm is a combination of 64-bit block cipher
followed by a stream cipher, with a key-size of 64 bits. However, the detail is
kept secret and disclosed to equipment manufacturers under non-disclosure
agreement. For performance and obscurity, the algorithm is implemented in
hardware.
At
the broadcast center, the scrambler generates control words to scramble the
contents, and it passes the control words to the CA-Host for secured
distribution to descramblers via ECM CA messages. Control words change about
every ten seconds, and the scrambler synchronizes the descrambler to key
switching using a specific bit in data-packet headers. As a defense strategy,
different TV channels are scrambled with different stream of control words.
CA Messages
CA messages are encrypted command-and-control
communications from CA-Host to CA-Modules. The DVB-CA architecture categorizes
CA messages into Entitlement Control Messages (ECM) and Entitlement Management
Messages (EMM). ECMs carry channel-specific access-control list and control
words. EMMs deliver subscriber-specific entitlement parameters. As a strategy
of defense in depth, a secret cipher different from data scrambling is used,
and the details on the message formats are closely guarded secrets.
CA Descriptor
CA
descriptors are data records associating a protected channel to its ECMs. Since
different stream of control words are used to scramble different channels,
there is no need to keep the relations secret. Thus, the CA descriptors are
sent in clear via the electronic channel guide, which is transmitted
continuously in the broadcast traffic.
CA-Host
The
CA-Host is the control center of the access protection. It is responsible for
encrypting all CA messages to CA-Modules and securely distributing CA messages'
cipher keys to CA-Modules.
CA-Client
A
CA-Client is the access-control coordinator at a receiver. It passes CA
messages from the CA-Host to its CA-Module. It delivers the control words from
the CA-Module to the descrambler. When the viewer selects a channel, the
CA-Client uses the channel’s CA descriptor to filter the associated ECMs and
passes them to the CA-Module. If the channel is a pay-per-view, the CA-Client
also walks the viewer through GUI dialogs to confirm purchases.
CA-Module
A
CA-Module (CAM) is the access-control guard at a receiver. Each CA-Module has a
unique CAM-ID for identifying the subscriber. The CA-Module authenticates and
decrypts EMMs to establish a subscriber’s entitlement parameters, which are
stored in the CAM’s non-volatile and secured memory and never leave the CAM.
The CA-Module also authenticates and decrypts ECMs to receive a channel’s
control words and access parameters from the CA-Host. If the access parameter
in an ECM is consistent with the entitlement parameters stored in the
CA-Module, the CA-Module returns the control word to the CA-Client for setting
up the descrambler.
Since
it is important for CA-Modules to be temper-resistant and easily replaced when
damaged or compromised, they are often implemented as smart cards.
Subscriber Management
System
The
SMS is the business manager determining each subscriber’s rights of channel
access. It uses CAM-IDs to link subscribers to the subscriber authorization
system.As
a subscriber’s subscription level and payment standing change, the
SMS modifies the access rights by instructing the CA-Host to send new EMMs to
the CA-Module having the subscriber’s CAM-ID.
Network Integration
The
next picture shows where the components of the DVB-CA architecture are
integrated into a DVB data network:
To
illustrate the interaction between the components in the DVB-CA architecture, a
walk-through of the operations behind the following access scenario is
presented next.
A
subscriber is currently entitled to only basic services, without access to
premium sports channels. In an evening, the subscriber browses through the
on-screen channel guide and decides to watch a boxing event. Tuning to the
channel, the subscriber is presented an on-screen message instructing the
viewer to call the customer service center to upgrade subscription level. After
going through the conversation of service upgrade, the customer representative
confirms the subscriber’s request to upgrade. Within a few seconds, the
on-screen message is replaced by the boxing show.
Behind the scene, the CA-Client receives the
channel-tuning request from the GUI. From the channel number of the boxing
event, the CA-Client looks up the parameters from the channel-guide to set up
the data receiver and packet filter for receiving the show’s digital audio and
video streams. More importantly, the CA-Client looks up the CA descriptor and
extracts parameters to set up the packet filter for receiving ECM packets
associated with the channel. When the ECMs arrive, the CA-Client passes them to
the CA-Module and waits for response.
When
the CA-Module receives an ECM, it authenticates and decrypts the ECM to extract
the control word and access parameter of the tuned channel. Comparing the
access parameter to the stored entitlement parameter, CA-Module finds that the
service belongs to a subscription level higher than that of the subscriber.
Thus, the CA-Module returns a status code of “below service level” to the
CA-Client. The CA-Module continues to return the same status code for every ECM
passed from the CA-Client since the subscriber’s entitlement remains unchanged.
Receiving
a response status of “below service level”, the CA-Client displays a message on
the TV screen, asking the subscriber to call the customer service center for
service upgrade. The message remains
on screen for as long as the CA-Module returns the same response to all ECMs
passed from the CA-Client.
As
instructed by the on-screen message, the subscriber calls the customer service
center to request service upgrade. After confirming the request and obtaining
online credit approval, the customer representative enters the new subscription
level to the SMS. Upon receiving the upgrade, the SMS provides the new
subscription level and the subscriber’s CAM-ID to the CA-Host. The CA-Host
encapsulates the new subscription level into an EMM, tags it with the
subscriber’s CAM-ID, signs and encrypts it, and inserts the EMM into the
broadcast traffic.
Back at the receiver site, the CA-Client receives
the EMM tagged with the subscriber’s CAM-ID and passes it to the CA-Module.
After authenticating and decrypting the EMM, the CA-Module stores the new
subscription level into its internal secured storage. When a subsequent ECM
from the boxing channel comes along, the CA-Module finds that the subscriber is
entitled to the channel. Consequently, CA-Module returns the control word.
Receiving
a valid control word from the CA-Module, the CA-Client sets up the descrambler,
and the digital audio and video data are descrambled, decoded, and shown on the
TV set. Seeing the boxing event, the subscriber is happy and ends the
conversation with the customer representative.
From
this point on, the CA-Client continuously feeds the channel’s ECMs to the
CA-Module, which returns new control words as they change.
