Four discrete orthogonal transforms have been evaluated for their suitability for use in transform-based analog speech encryption. Subjective as well as objective tests were conducted to compare the residual intelligibility and the recovered speech quality under channel conditions. The cryptanalytic strengths of the schemes were then compared by applying a novel cryptanalytic attack which exploits the redundancy of speech using a spectral vector codebook. The results indicate that the discrete cosine transform (DCT) is the best transform to use in transform-based encryption. A modification of the DCT-based scheme which significantly improves the security of the scrambler is proposed.>

The performances of five discrete orthogonal transforms in speech encryption systems are compared. The transforms considered are the discrete Fourier transform, discrete cosine transform, Walsh-Hadamard transform, Karhunen-Loeve transform, and discrete prolate spheroidal transform. Four objective measures are used to grade the encryption systems with respect to residual intelligibility and recovered voice quality. A figure of merit based on all the four objective measures is formed. It gives good correlation to the subjective results of residual intelligibility and recovered speech quality.>