Speech Encryption by Manipulations of LPC Parameters

Encryption can be an important requirement in speech communication systems. Conventionally, encryption has largely been accomplished by signal manipulations in the frequency domain; for example, by means of spectrum inversion techniques. 1 With the increased popularity of digital codes for speech transmission, time-domain encryption techniques have received increased attention. Typically the time-domain encryption technique consists of temporal rearrangement of samples within a time block. For the scrambling of PCM bits in speech waveform coding, a block-length that is at least a pitch period long is usually adequate to provide a nonspeech-like output waveform. Similarly, the scrambling of differential PCM and delta-modulation bits can also produce a nonspeech-like output waveform provided that the time-block is sufficiently long. For example, in a 24-kb/s speech code, this constraint implies approximately a block length of 64 samples for an adequate scrambling of the coded bits.2 The temporal scrambling of speech samples within millisecondlength blocks generally provides what may be referred to as casual encryption. This means that a noncasual 'eavesdropper' can break the 1373