Feedback With Carry Shift Registers Synthesis With the Euclidean Algorithm

Abstract

Feedback with carry shift registers (FCSR) were introduced by Klapper and Goresky (1994). They are very similar to classical linear feedback shift registers (LFSR) used in many pseudorandom generators. The main difference is the fact that the elementary additions are not additions modulo 2 but with propagation of carries. The mathematical models for LFSR are equivalently linear recurring sequences over GF(2) or rational series in the set GF(2)[[x]]. For FCSR, the good model is the one of rational 2-adic numbers. It is well known, that the series generated by a LFSR can be synthesized by either the Berlekamp-Massey algorithm for binary linear recurring sequences or the extended Euclidean algorithm in the set GF(2)[x] of binary polynomials. Klapper and Goresky (1997) give an algorithm for the FCSR synthesis. This algorithm is similar to those of Berlekamp-Massey and is based on De Weger and Mahler's rational approximation theory. In this correspondence, we prove that it is possible to synthesize the FCSR with the extended Euclidean algorithm in the ring /spl Zopf/ of integers. This algorithm is clearly equivalent to the previous one, however, it is simpler to understand, to implement, and to prove. Our algorithm is still valid in the case of g-adic integers where g is a positive integer. We also give a near-adaptative version of this algorithm.