Analog speech cryptology

This thesis presents an investigation of analog speech encryption algorithms. It focuses specifically on schemes which are able to be used over narrowband communication channels, such as the Public Switched Telephone Network. The performance of the devices studied is evaluated according to three criteria. The scrambled speech derived from the encryption algorithm must have a low residual intelligibility. The encryption and decryption processes should have a minimal effect on the quality of the speech recovered by the intended recipient. Finally, the scrambler must be secure from cryptanalytic attack. Time domain, frequency domain and combined time and frequency domain scrambling procedures are considered initially. A review of their respective advantages and disadvantages is included which aims to facilitate the development of improved speech security systems. Original methods for the cryptanalysis of time element and frequency domain bandsplitting scramblers are presented. These attacks are applicable when the cryptanalyst has access to the scrambled speech only. In particular a novel cryptanalytic procedure is introduced which exploits the redundancy of speech using a frequency domain vector codebook. Attacks are also described which utilise plaintext with corresponding ciphertext to recover the encryption permutations used in the encryption process. It is shown that the two scramblers examined, are extremely vulnerable to the proposed cryptanalytic techniques