Analysis of the Multiple Excess-S Modulo K (MSK) Coding Scheme
Nadine Fea
Department of Computer Science
University of Canterbury
Abstract
The coding scheme known as MSK, an extension to an early commercial Excess-3 coding scheme, has been proposed by ML and S Keshariya as a cryptographic function suitable for improving the strength of stream ciphers. The authors make the claim that the scheme “exponentially enhances” the security of stream ciphers. Four schemes for applying MSK are proposed, from the simplest form whereby MSK is applied to the keystream, to the most complex incorporating two MSK modules and a non-linear combining function. This claim is investigated through empirical testing. The formal test regimes available for assessing randomness are considered and the Monobit, Maurer’s Universal and the Approximate Entropy tests are selected to determine the extent of improvement achieved by using MSK. A set of arbitrary pseudorandom number generators are created based on linear feedback shift registers and these are used for initial verification of the MSK claim. The GSM system’s A5/1 algorithm is also used for verification of real world usefulness of MSK. The results in this the report will show that the author’s claim cannot be sustained and that the MSK coding scheme does not make any significant difference to the security, that is an improvement in randomness, of a stream cipher.
