Cipher-text only attack on hopping window time domain scramblers

Wireless channels are highly prone to eavesdropping. To mitigate this problem encryption systems are used. Analog scrambling systems achieve confidentiality through modification of analog signals. Unfortunately, current literatures lack a thorough security analysis of these systems. In this paper security of hopping window time domain scrambler is investigated. It is shown that cipher-text of these systems is highly redundant. Based on this observation, a cryptanalysis system is proposed. Firstly, audio signal is mapped into a set of puzzles. Later, scrambling keys are found by solving these pictorial puzzles. Proposed method is compared with FWLSD which, to our best knowledge, is the only existing cryptanalysis method for these systems. Objective and subjective tests show that voices descrambled by the proposed method achieve intelligibility scores of 0.739 and 0.82 respectively. These scores are 0.37 and 0.25 higher than scores achieved by FWLSD method.