Digital Video Broadcasting Conditional Access Architecture

A Presentation for SJSU-CS265 (Fall 2002): Security Engineering by Tong Ho (tong_ho@yahoo.com)
Full report in PDF

Introduction

• Digital Video Broadcasting (DVB) -- A one-to-many data network for digital TV
• Conditional Access (CA) -- Management of end-users' rights to access contents

Functional Partitions

• Three elements to protect contents:

• Data-Scrambling

• Encryption of channel contents

• Subscriber Authorization System (SAS)

• Enforcement of access control
• Three distributed components: CA-Host, CA-Client, and CA-Module

• Subscriber Management System (SMS)

• Keep a database of subscribers
• Determine access rights by monitoring subscription level and payment standing

System Architecture

• Distributed components
• Same data network for content delivery and access control

• Scrambler and Descrambler

• Scrambler in broadcast center; Descrambler in every receiver
• DVB Common Scrambling Algorithm; secret cipher; 64-bit key (control words)

• CA Messages - from CA-Host to CA Modules

• Entitlement Control Messages (ECM) -- channel specific: control words, access criteria
• Entitlement Management Message (EMM) -- subscriber specific: access rights

• CA Descriptors - from CA-Host to CA-Client

• Describe association between a channel and its ECM stream -- channel specific

• CA-Host -- the SAS Command-and-Control

• Encrypt and deliver channel-specific control words and access criteria as ECMs
• Encrypt and deliver subscriber-specific rights as EMMs

• CA-Client -- the SAS Access Coordinator

• Receive and pass ECMs and EMMs to CA-Module
• Deliver control words from CA-Module to descrambler

• CA-Module -- the SAS Gate Keeper

• CAM-ID, uniquely assigned to each subscriber
• Authenticate and decrypt EMMs for rights; result never leaves the CAM
• Authenticate and decrypt ECMs for control words and access criteria
• Return control words only if stored rights matches channel's access criteria
• Temper-evident and temper-resistant
• Renewable -- cheaply replaced when damaged or compromised

• Subscriber Management System

• Decide each subscriber's rights of channel access
• Update subscriber entitlements by giving CA-Host the (CAM-ID, rights) doublets.

Network Integration

• DVB-CA fits into the MPEG-2 Transport Architecture

Summary

• Manage end-users' access rights to a DVB digital TV network's content
• Use the same broadcast network for content delivery and access protocol
• Rely on secret cryptography and temper-resistant hardware
• Contain damage of security compromise by being renewable
• Adopted by many DTV networks for real deployment