On the cycle decomposition of the WG-NLFSR

Recently, Kalikinkar Mandal and Guang Gong presented a family of nonlinear pseudorandom number generators using Welch-Gong Transformations in their paper [6]. They also performed the cycle decomposition of the WG-NLFSR recurrence relations over different finite fields by computer simulations where the nonlinear recurrence relation is composed of a characteristic polynomial and a WG permutation. In this paper, we mainly prove that the state transition transformation of the WGNLFSR is an even permutation. We also prove that the number of the cycles in the cycle decomposition of WG-NLFSR is even. And we apply our results to the filtering WG7-NLFSR to prove that the period of the sequences generated by WG7-NLFSR can not be maximum.