Abstract: An information encryption method particularly intended for a communication system in which bidirectional multiplexed communications of the point-to-multipoint type are derived between a switching exchange and plural terminal stations. The switching exchange transmits first multiplexed words respectively destined for the terminal stations. The terminal stations supply second words which are multiplexed and transmitted towards the switching exchange. In the switching exchange, informations concerning transmission errors detected in the second words are derived and transmitted towards the terminal stations, and the first words are encrypted as a function of second words previously received free of transmission error in the switching exchange. In each of the terminal stations, the informations concerning the transmission errors are detected, and the second words transmitted from the terminal station and received free of transmission error by the switching exchange are memorized.