Abstract: A communication system for providing secured communication over a communication channel. The system includes a DES encryption device and a DES decryption device retrofittable into or within a conventional radio unit. The encryption device receives analog input at the transmitter end of the radio and converts the analog signal to digital signal, then encrypts this digital data, modulates the encrypted digital data using modified frequency modulation (MFM) which is then sent over the communication path. During decryption, the apparatus receives MFM data from the communication path, demodulates this digital data, decrypts the data and then outputs this decrypted digital data to the interface unit. The interface unit contains both a transmitter and a receiver which is capable of half duplex operation. The interface unit also contains a voice coder and a voice decoder which are of the sub-band type.