Abstract: This relates to a cryptographically secure communication system employed with an analog input, an analog output and a transmission medium each having a given low frequency pass band with a predetermined bandwidth. The input analog signal is sampled and then quantized to have an amplitude equal to one of a predetermined number of discrete amplitude levels. This quantized signal is applied together with a first sequence of signal representing randomly and with substantially equal probability the discrete amplitude levels to a modulo - M adder to encrypt the sample pulses. These encrypted sample pulses are converted to an encrypted analog signal for propagation through the transmission medium. The encrypted analog signal is received from the medium and then sampled and quantized to provide the encrypted amplitude samples at the receiver.