An Efficient Technique for Sample-Masked Voice Transmission

Introduced here is a new technique for encrypting bandpass signals. The technique linearly filters a sample-masked signal, modifying the encryption component of the cryptogram to place its spectrum within the frequency band of the signal. Sample masking is a recognized means of encrypting baseband signals while containing the spectrum of the cryptogram within the signal bandwidth. A drawback occurs for bandpass signals, owing to the processing required in the mixing procedures used ordinarily to convert the signal from bandpass to baseband and back. In contrast, the linear filter modifies the encryption component of the cryptogram without processing the signal component, reducing the computational burden of encrypting and decrypting by an order of magnitude. When the maximum frequency is an integer multiple of the minimum frequency, the filter can output the same information symbols that appear at the input and replace the redundant symbols with symbols that control the passband of the output signal. For voice bandwidths of 300-3000 Hz sampled at 6000 Hz, redundancy symbols (one in every ten or 600 samples/s) are removed, the information symbols (nine in ten or 5400 samples/s) are encrypted, and the redundancy symbols are replaced by bandwidth-controlling symbols. Experimental results of clear voice transmitred over simulated telephone lines validate the technique.