A method of controlling descrambling of a plurality of program transport
streams received by a receiver system comprises receiving a sequence of
messages in a conditional access sub-system (9,10) comprised in said
receiver system, each message being associated with one of a number of
scrambled program transport streams and representing a request for
returning information enabling the associated scrambled transport stream
to be descrambled by at least one descrambler module (12) in the receiver
system, determining whether messages received within a certain interval
are associated with a number of different scrambled program transport
streams, and denying at least one of the requests represented by the
messages received in the certain interval, if the number of different
scrambled program transport streams with which the messages are
associated exceeds a pre-determined number.