Report: Effective number of keys in a voice privacy system based on permutation scrambling

In a voice privacy system that is based on permutation scrambling of N speech elements, the total number of permutation keys is N!. It is generally acknowledged that this number is an exceedingly optimistic indicator of cryptanalytical strength. Because of the redundancy in the speech signal and the intelligibility of imperfectly inverse-permuted speech, an eavesdropper often needs to try only K << N! random permutations before breaking the speech code.