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UBC Theses and Dissertations

An asynchronous telephone speech scrambler with a new key generation method Woo, Raymond W. M.


The design of an asynchronous telephone speech scrambler is proposed and its performance experimentally studied. The scrambler transforms the speech signal into a scrambled form by permuting the frequency coefficients of the speech signal. These coefficients are permuted in pairs instead of individually. Subjective tests indicate that such an arrangement results in an improvement in descrambled speech quality but no increase in the residual intelligibility of the scrambled speech signal. A new criterion for selecting good scrambling permutations known as the derangement criterion is introduced. Subjective tests based on the number test confirm that keys satisfying this criterion are able to produce scrambled speech with virtually no residual intelligibility. A new derangement generation algorithm is introduced. This algorithm maps an integer g, in the range of 0 ≤ g < (n - 1)!, uniquely to a derangement of n elements.

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