# Feedback with Carry Shift Registers

In sequence design, a **Feedback with Carry Shift Register** (or FCSR) is the arithmetic or with carry analog of a Linear feedback shift register (LFSR). If is an integer, then an N-ary FCSR of length is a finite state device with a state consisting of a vector of elements in and an integer .

FCSRs are analyzed using number theory. Associated with the FCSR is a connection integer . Associated with the output sequence is the N-adic number The fundamental theorem of FCSRs says that there is an integer so that , a rational number. The output sequence is strictly periodic if and only if is between and . It is possible to express u as a simple quadratic polynomial involving the initial state and the q_{i}. including near uniform distribution of sub-blocks, ideal arithmetic autocorrelations, and the arithmetic shift and add property. They are the with-carry analog of m-sequences or maximum length sequences.

There are efficient algorithms for FCSR synthesis. This is the problem: given a prefix of a sequence, construct a minimal length FCSR that outputs the sequence. This can be solved with a variant of Mahler and De Weger's lattice based analysis of N-adic numbers when ; If L is the size of the smallest FCSR that outputs the sequence (called the N-adic complexity of the sequence), then all these algorithms require a prefix of length about to be successful and have quadratic time complexity. It follows that, as with LFSRs and linear complexity, any stream cipher whose N-adic complexity is low should not be used for cryptography.

FCSRs and LFSRs are special cases of a very general algebraic construction of sequence generators called Algebraic Feedback Shift Registers (AFSRs) in which the integers are replaced by an arbitrary ring R and N is replaced by an arbitrary non-unit in R. A general reference on the subject of LFSRs, FCSRs, and AFSRs is the book.

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