This paper gives some well-known, little known, and new results on the problem of generating random elements in groups, with particular emphasis on applications to cryptography. The groups of greatest interest are the group of all orthogonal n × n matrices and the group of all permutations of a set. The chief application is to A. D. Wyner's analog scrambling scheme for voice signals.

Encrypting by random rotations

Analog scramblers for speech privacy

Subband signals are formed each corresponding to a portion of an input signal frequency spectrum. The subband signals are partitioned into subband time segments. The subband time segment signals are permuted. The permuted signals are combined to form a scrambled signal.

Time-frequency scrambler

The paper is a review of the subject of analogue encryption. The rank correlation approach to the study of permutations is also reviewed. Some families of permutations that have been used in scrambling systems are described. The problems of analogue encryption, as well as its scope, are analysed.

Overview of analogue signal encryption

Speech is probably the most fundamental form of communication available to us and our society has become highly dependent on our modern, fast and accurate means of transmitting spoken messages. Usually the main aim of communicants is merely to trasmit a message as quickly, accurately and cheaply as possible. There are, however, a number of situations where the information is confidential and where an interceptor might be able to benefit immensely fromt he knowledge gained by monitoring the information circuit. In such situations the communicants must take steps to conceal and protect the content of their spoken message. Of course, the amount of protection will vary. On occasions it is sufficient to prevent a casual ‘listener’ from understanding the message but there are other times when it is crucial that even a determined interceptor must not be able to deduce ti.

Analogue speech security systems

This paper surveys some past and present principles of speech coding. The advent of digital logic systems and integrated circuit components is seen as a major turning point in the implementation of effective coding principles that could not previously be used satisfactorily.Various coding methods are discussed, covering frequency domain, time domain and analogue and digital output systems. The problems of key generator integrity, synchronization, received voice quality and recognizability, and channel bandwidth restrictions, and the security offered by different systems are considered.The author concludes that in the immediate future high security speech encipherment systems will be of the digital output crypto-vocoder type, with audio bandwidth analogue output coders offering an increasingly secure alternative for the majority of applications. Digital output coders will continue to be used effectively in those applications where the communication equipment and transmission path have the necessary characteristics to carry low-error-rate digital signals. Advances in analogue-to-digital speech conversion providing acceptable quality at bit rates below 9.6 k bit/s should eventually enable a simple digital output coder to be as effective as a cryptovocoder for audio band high security applications.

N.R.F. MacKinnon

Papers

The development of speech encipherment