Cryptanalysis of Keystream Reuse in Stream Ciphered Digitized Speech using HMM based ASR Techniques
Liaqat Ali Khan, Muhammad Shamim Baig · 2007
Abstract — The keystream reuse problem in case of textual data has been the focus of cryptanalysts for quite some time now. This paper presents the use of hidden markov models based speech recognition approach to cryptanalysis of stream ciphered digitized speech in a keystream reuse situation. In this paper, we show that how an adversary can automatically recover the digitized speech signals encrypted under the same keystream. The technique is flexible enough to incorporate all modern speech coding schemes and all languages for which the speech recognition techniques exist. The technique is simple and efficient and can be practically employed with the existing HMM based probabilistic speech recognition techniques with some modification in the training (pre-computation) and/or the maximum likelihood decoding procedure. The simulation experiments, though preliminary, showed promising initial results by recognizing about 80 percent correct phoneme pairs encrypted by the same keystream. Index Terms — cryptanalysis, hidden markov model, keystream reuse, speech recognition, stream cipher. I.