The aim of the present invention is to limit the impact of security breaches, which are the emulators of the security module. This aim is reached by a processing unit of audio/video digital conditional access data, encrypted by control words, responsible for processing security messages containing at least one cryptogram relative to a control word and one instruction relative to the control word, characterized in that it includes means to receive at least two micro programs by security messages, executable by the security module, said security module comprising means to store at least two micro programs and means to receive an instruction contained in the security message, for selecting the micro program indicated by the instruction, for executing the said micro program with at least the cryptogram as a parameter of execution, this execution allowing the calculation of the control word to be sent back to the audio/video processing unit.