Digital signal processor: Private communications

Where normal safeguards for message privacy are not adequate, some form of encryption is required. Voice messages, encoded using an adaptive differential pulse-code-modulation encoder such as that described in a companion paper, may be encrypted for privacy (protection against casual eavesdropping) through similar digital signal processor programs with little additional computation. Two methods of implementation are described: The use of U-permutations for temporal scrambling of the transmitted bit stream and the use of bit masking by stored random numbers. The relative merits of each system are discussed, illustrating both the flexibility and limitations of the digital signal processor for such applications.