On selection of optimal parameters for the WG stream cipher family

In this paper, we first present some new results about the Welch-Gong (WG) transformations, followed by a description of the WG stream cipher family which is built upon an LFSR and a WG transformation over an extension field. The randomness properties of keystreams produced by a decimated WG cipher are derived based on the new results. We also discuss the selection criteria for choosing the optimal parameters for a WG cipher in order to achieve the maximum level of security. Finally, we present the optimal parameters for the WG transformations over F_2m, 7 ≤ m ≤ 16 based on the proposed criteria.