In the previous article, we presented the coding techniques of the voice and showed how the latter was represented and processed. We will now consider analog voice encryption techniques. The latter consist in structurally modifying the signal using various methods, but without modifying its underlying nature, unlike digital techniques. Very effective, however, they require extreme art on the part of the engineer to ensure that no residual intelligibility will allow an attacker to recover all or part of the original voice signal. The main difficulty lies in the fact that the validation of such techniques ultimately resides on human hearing tests, subjective by nature, even if the theory guarantees the optimality of the processes implemented and the parameters used. This is the reason why, because of the importance of the know-how of the engineer in the development of an analog cryptophony system, there is very little information on these analog techniques cisco voice jobs.
Contrary to popular belief, the protection of speech was very quickly imposed on the military, during the Second World War. Alan Turing, the architect of the victory against the famous Enigma machine, also worked on the protection of voice communications. From November 1942 to March 1943, he worked on this subject at Bell Laboratories in New York - there he notably met Claude Shannon, the father of information theory. Then in 1944, in Hanslope Park, in an English secret service laboratory, he developed the Dalila machine to encrypt the voice. This machine will be used to encrypt Churchill's victory speech. And the first techniques implemented are of an analog nature: complex jamming, permutations of frequencies, time division multiplexing… The human voice is nothing more than a set of physical frequencies controlled electronically.
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