In a method of descrambling a scrambled content data object, at least a section of the scrambled content data object is descrambled by applying at least one decryption operation under a key at least partly derivable from a content descrambling key. At least one content descrambling key is obtained from a message received from a conditional access sub-system over a data communication channel. At least one cryptogram of data obtainable from at least one content descrambling key in the message, each of which cryptograms are carried in the message, is decrypted under an associated channel key. A first key is used to establish each channel key. At least the section of the scrambled content data object is descrambled by applying a further decryption operation under a key at least partly derivable from the first key.