Perfect Secrecy Encryption of Analog Signals

Any scheme that encrypts an analog message with perfect secrecy using a finite-size digital key must inevitably degrade the quality of the recovered message. With the constraints of perfect secrecy and a finite key size, no analog encryption system can ever achieve less degradation than is achievable by optimal digitization of the message (by block source coding or vector quantization) followed by digital encryption. Optimal analog encryption with perfect secrecy can be implemented in such a way that the bandwidth of the encrypted signal is not greater than the bandwidth of the original analog signal; this can be done without altering the key size and without increasing the degradation incurred in the recovered message.