1. A security module for an audio/video digital data processing unit for conditional access, said audio/video digital data being encrypted within an encoded data stream by control words, said security module comprising a processor programmed to: process a security message included within a control message stream and containing at least one cryptogram relative to a control word and one instruction relative to the control word; download at least two micro programs to the security module within said security messages; receive the security message and extract the cryptogram and the instruction; select one micro program among the micro programs according to a value of the instruction; execute the selected micro program with at least the extracted cryptogram as an entry variable of the selected micro program; in response to executing the selected micro program, calculate the control word; and transmit the calculated control word to a decoding device to decrypt the encrypted audio/video digital data; and a memory programmed to store said at least two micro programs after being downloaded to the security module.