Automated Cryptanalysis of Plaintext XORs of Waveform Encoded Speech
Liaqat Ali Khan, Muhammad Shamim Baig · 2008
Abstract — Keystream reuse also known as the “two time pad” problem in case of stream ciphered data has been the focus of cryptanalysts for several decades. All heuristics presented so far assume the underlying plaintext to be uncompressed text based data encoded through conventional encoding mechanisms such as ASCII Coding. This paper presents the use of hidden Markov model (HMM) based automatic speech recognition (ASR) approach to cryptanalysis of stream-ciphered waveform-encoded speech in a keystream reuse situation. We present that an adversary can automatically recover the digitized speech signals from their plaintext XORs obtained from two different speech signals stream ciphered with the same keystream. The proposed technique can be practically employed with the existing HMM based probabilistic speech recognition techniques with some modification in the selection of HMMs, their training and the maximum likelihood decoding procedures. Simulation experiments using such modified speech recognition tools have been presented. Index Terms — cryptanalysis, keystream reuse, speech coding, stream cipher, two time pad. I.