Update on F-FCSR Stream Cipher
François Arnault, Thierry Pierre Berger, Cédric Lauradoux · 2006
The F-FCSR family of algorithms have been presented about one year ago with [2] and [1]. While some flaws where found in the initial propositions (on the IV-setup procedure, and a TMD tradeoff attack), there are yet no known weaknesses of the core of these algorithms. We sum up here some of the properties of the automaton that are better understood now, and that have been presented in [2], [3], [4], and [6] and we propose two revised algorithms correcting all known weaknesses. 1 Recalls on F-FCSR 1.1 FCSR automaton Detailed descriptions can be found in [3, 1, 2]. A Feedback with Carry Shift Register (FCSR) is an automaton which computes the binary expansion of a 2-adic number p/q, where p and q are some integers, with q is odd. We will assume that q n/2. 1.1.1 Software description of the transition function The FCSR automaton contains two registers (sets of cells): the main register M and the carries register C. The main register M contains n cells. We denote mi (0 ≤ i ≤ n − 1) the binary digits contained in these cells and we call the integer m = ∑n−1 i=0 mi2 i the content (or state) of M.