18.11 Conditional Access

Figure 18.34 illustrates a CA system; the transaction supported is the exchange of programmes for funds, i.e. payTV. The system to support this requires a database to manage the subscribers, their addresses and programme requirements which, provided the subscriber is still in credit, will be sent to the receiver using an appropriately structured message the entitlement management message (EMM). Such a database is known as the subscriber management system (SMS). The CA system timing and synchronisation, together with the current encryption key, is sent in a separate message the entitlement control message (ECM). These terms apply to the system adopted by the DVB which has also defined a scrambling algorithm and receiver interface to allow an open-system approach to the normally proprietary CA system architecture. The conclusion of such agreement among highly competitive CA system vendors is a significant achievement and one which is essential to the future commercial success of digital broadcasting systems.

Figure 18.34: Conceptual Conditional Access System

CA systems comprise two stages: scrambling and encryption. The former is required to render the transmitted signal unintelligible to the receiver unequipped with the means to descramble the transmission. The means to enable the receiver to perform the descrambling is conveyed to the receiver in the form of a key or secret digital number and an encryption process is needed to make this key secret. Key generation and management are the essential features of the encryption process as are the service entitlements of the viewer. Thus, the encryption process...