From the introduction of voice encryption to today, encryption techniques have evolved drastically. Digital technology has effectively replaced old analog methods of voice encryption and by using complex algorithms, voice encryption has become much more secure and efficient. One relatively modern voice encryption method is Sub-band coding. With Sub-band Coding, the voice signal is split into multiple frequency bands, using multiple bandpass filters that cover specific frequency ranges of interest. The output signals from the bandpass filters are then lowpass translated to reduce the bandwidth, which reduces the sampling rate. The lowpass signals are then quantized and encoded using special techniques like, pulse-code modulation (PCM). After the encoding stage, the signals are multiplexed and sent out along the communication network. When the signal reaches the receiver, the inverse operations are applied to the signal to get it back to its original state. A speech scrambling system was developed at Bell Laboratories in the 1970s by Subhash Kak and Nikil Jayant.Kak, S. and Jayant, N.S., Speech encryption using waveform scrambling. Bell System Technical Journal, vol. 56, pp. 781–808, May–June 1977. In this system permutation matrices were used to scramble coded representations (such as pulse-code modulation and variants) of the speech data. Motorola developed a voice encryption system called Digital Voice Protection (DVP) as part of their first generation of voice encryption techniques. DVP uses a self-synchronizing encryption technique known as cipher feedback (CFB). The extremely high number of possible keys associated with the early DVP algorithm, makes the algorithm very robust and gives a high level of security. As with other symmetric keyed encryption systems, the encryption key is required to decrypt the signal with a special decryption algorithm.
