Cipher text only attack on speech time scrambling systems using correction of audio spectrogram

Abstract

Recently permutation multimedia ciphers were broken in a chosen-plaintext scenario. That attack models a very resourceful adversary which may not always be the case. To show insecurity of these ciphers, we present a cipher-text only attack on speech permutation ciphers. We show inherent redundancies of speech can pave the path for a successful cipher-text only attack. To that end, regularities of speech are extracted in time and frequency using short time Fourier transform. We show that spectrograms of cipher-texts are in fact scrambled puzzles. Then, different techniques including estimation, image processing, and graph theory are fused together in order to create and solve these puzzles. Conducted tests show that the proposed method achieves accuracy of 87.8% and intelligibility of 92.9%. These scores are 50.9% and 34.6%, respectively, higher than scores of previous method. Finally a novel method, based on moving spectrogram distance, is proposed that can give accurate estimation of segment length of the scrambler system.