A conditional access system is provided wherein digitized data are received in successive data packets at least some of which are scrambled. The digitized data contain content data and access control data. The system has a module with descrambler circuitry and a security device such as a smart card adapted to be coupled to the module to provide descrambling control data to the module as derived from the access control data. Selected ones of the data packets are encrypted in addition to being scrambled, and the access control data include decryption control data.