A conditional access (CA) system comprises a combination of scrambling and encryption to prevent unauthorized reception. Scrambling is the process of rendering the sound, pictures and data unintelligible. Encryption is the process of protecting the secret keys that have to be transmitted with the scrambled signal in order for the descrambler to work. After descrambling, any defects on the sound and pictures should be imperceptible, i.e. the CA system should be transparent. The primary purpose of a CA system for broadcasting is to determine which individual receivers/ set-top decoders shall be able to deliver particular programme services, or individual programmes, to the viewers. The reasons why access may need to be restricted include: to enforce payments by viewers who want access to particular programmes or programme services; - to restrict access to a particular geographical area because of programme-rights considerations (territorial control can be enforced if the receiver has a built-in GPS system); - to facilitate parental control (i.e. to restrict access to certain categories of programme2).