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Conditional access for satellite broadcasting

Satellite digital TV is probably the best well  known application for satellite broadcasting.   A key role in the success of digital TV has been played by the standardisation process undertaken in Europe by the DVB Project. The DVB Project (Digital Video Broadcasting, DVB), for example, started the development of a system for digital television broadcasting via satellite (DVB-S) in 1992 and finalised the specification in 1993.  Since that year, many geostationary satellites worldwide have started the transmission of DVB-S signals.  Currently, about 250 organisations are actively co-operating to identify the strategies for the introduction of digital television via satellite, cable and terrestrial channels.  The DVB system has been designed with a modular structure, based on independent sub-systems, so that a variety of DVB systems (such as DVB-S: satellite, DVB-T: terrestrial, DVB-C: cable) could maintain a high level of interoperability.  In fact the security system in DVB, called Conditional Access (CA), is the same for all one-way DVB variants (DVB-S, DVB-T and DVB-C).

Conditional Access (CA) is a service that allows broadcasters to restrict certain programming products to certain viewers, by encrypting the broadcaster's programmes.  Consequently, the programs must be decrypted at the receiving end before they can be decoded for viewing.  CA offers capabilities such as pay-per-view (PPV), interactive features such as video-on-demand (VOD) and games, the ability to restrict access to certain material (sports or film channels, for example) and the ability to direct messages to specific set-top boxes (perhaps based on geographic region). Conditional Access used in the DVB system  includes three main functions: scrambling / descrambling, entitlement checking and entitlement management:

• The scrambling/descrambling function aims to make the service incomprehensible to unauthorised users.  Descrambling can be achieved by any receiver having an appropriate descrambler and holding a secret Control Word (CW).  Scrambling can be applied to service components, either using a common Control Word or using separate Control Words for each component.
• The entitlement checking function consists of broadcasting the conditions required to access a service, together with encrypted secret codes (the Control Word(s)) to enable the descrambling for authorised receivers. These codes are sent inside dedicated messages (DVB tables) called Entitlement Control Messages (ECMs) and these are carried in the ensemble.
• The entitlement management function consists of distributing entitlements to receivers. There are several kinds of entitlements that match different means of subscribing to a service: subscription per theme, level or class, pre-booked pay-per-programme or impulse pay-per-programme, per service or per time. This information is sent inside dedicated messages called Entitlement Management Messages (EMMs) and these may be carried in the same ensemble as the scrambled services or by some other means.

For more information on this topic see ETSI web pages , DVB Common Scrambling Algorithm and the security report published by ETSI working group called Broadband Satellite Multimedia (BSM) and follow the SES and BSM links) .