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.