F-FCSR: Design of a New Class of Stream Ciphers

  • François Arnault
  • Thierry P. Berger
Part of the Lecture Notes in Computer Science book series (LNCS, volume 3557)

Abstract

In this paper we present a new class of stream ciphers based on a very simple mechanism. The heart of our method is a Feedback with Carry Shift Registers (FCSR) automaton. This automaton is very similar to the classical LFSR generators, except the fact that it performs operations with carries. Its properties are well mastered: proved period, non-degenerated states, good statistical properties, high non-linearity.

The only problem to use such an automaton directly is the fact that the mathematical structure (2-adic fraction) can be retrieved from few bits of its output using an analog of the Berlekamp-Massey algorithm.

To mask this structure, we propose to use a filter on the cells of the FCSR automaton. Due to the high non-linearity of this automaton, the best filter is simply a linear filter, that is a XOR on some internal states. We call such a generator a Filtered FCSR (F-FCSR) generator.

We propose four versions of our generator: the first uses a static filter with a single output at each iteration of the generator (F-FCSR-SF1). A second with an 8 bit output (F-FCSR-SF8). The third and the fourth are similar, but use a dynamic filter depending on the key (F-FCSR-DF1 and F-FCSR-DF8). We give limitations on the use of the static filter versions, in scope of the time/memory/data tradeoff attack.

These stream ciphers are very fast and efficient, especially for hardware implementations.

Keywords

stream cipher pseudorandom generator feedback with carry shift register 2-adic fractions 

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Copyright information

© Springer-Verlag Berlin Heidelberg 2005

Authors and Affiliations

  • François Arnault
    • 1
  • Thierry P. Berger
    • 1
  1. 1.LACOUniversité de LimogesLimoges CEDEXFrance

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