and transmits it to the device 13 of the security element moreover the device 11 transmits the random number to the device 12 the latter encrypts the random number al under the action of a key k 1 so as to give the decryption key k in the same way the device 13 of the security element encrypts the random number under the action of a key k 1 and produces the encryption key k according to a particular embodiment of the invention described in fig3 the encryption algorithm used by the devices 12 and 13 can be replaced by a u201c one way u201d function with key k 1 such a function is for example described in the european patent application filed under number 96401336 1 2209 advantageously the devices 12 and 13 prevent any pirate from discovering the encryption decryption key k solely through the data item al which travels between the decoder and the security element according to another particular embodiment of the invention the key k 1 used by the devices 13 and 12 can be specific to the security element decoder pair thus exhibiting the advantages mentioned earlier according to another particular advantageous embodiment of the invention the procedure for generating the encryption and decryption key k can be.