On speech encryption using waveform scrambling

This paper presents a study of speech scramblers based on permutations of samples within an N-block. It has been found that a new family of “uniform” (U) permutations (defined by the address mapping i → ki modulo N; k prime to N) is as effective as pseudorandom (pr) permutations in destroying speech intelligibility. Analytical results show the relation between input and scrambled-signal spectra, while computer simulations compare the effects of scrambling on pam samples and on codes based on adm, apcm, and adpcm. Scrambling is increasingly effective in that order, and encoding delays in adpcm can be as low as 1 to 2 ms. Finally, scrambling has been compared with frequency inversion, which corresponds to sign-inversion in every other Nyquist-rate waveform sample.