Retrofittable encryption/decryption apparatus using modified frequency modulation

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. The interface unit further contains a method for selecting from either a digital data transceiver or a voice coded digital data transceiver. Still further, the apparatus contains a method for selectively controlling the authorized reception for digital data by a single receiver or a plurality of receivers. Also, a method is available for selecting one of a plurality of master keys, which are used for the encryption process initialization. Still further, the present system allows for encrypting and decrypting the digital data by exclusive-oring the digital data with the operative digital encryption device which is implemented in DES.