1. A security Module for an audio/video digital data processing unit for conditional access 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, wherein it includes means to receive at least two micro programs by security messages, said micro programs being executable by the security module, said security module comprising means to store at least two micro programs and means to receive the instruction contained in the security message, for selecting the micro program indicated by the instruction, to execute 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.