Method of controlling descrambling of a plurality of program transport streams, receiver system and portable secure device

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 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 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.