A proposal is presented for a system for the encryption of the signal resulting from the time division multiplexing ofNchannels carrying digital voice signals. The channel synchronization principle is such that no extra sync-signal is transmitted but synchronization is extracted from the information signal. Furthermore, it provides a first echelon encryption of the signal because there is a continuous and random shift of the position of each channel in time. The main encryption of the signal is done on a bit-by-bit basis, by modulo-2 adding to the signal a sequence that is derived from a non-linear combination of several past bits of the encrypted signal. This is achieved by using an e.p.r.o.m. in conjunction with a r.a.m. In an alternative system, this sequence is derived from a pseudo-random sequence generator that is also controlled by some past bits of the encrypted signal. In both cases synchronization and encryption are based on the transmitted signal itself.