Software Oriented Stream Ciphers Based upon FCSRs in Diversified Mode

Feedback with Carry Shift Registers (FCSRs) are a promising alternative to LFSRs for the design of stream ciphers. Most of the FCSR-based stream ciphers use a Galois representation. In this case, the control of a single bit leads to the control of the feedback values. This particular property was exploited to break most of the existing proposals. Recently, a new representation for FCSR automata was presented. This representation is a generalization of both Galois and Fibonacci representations. In this representation any cell can be used for a feedback for any other cell. With a good choice for the parameters, those new FCSR automatas are resistant to the previous attacks and the internal diffusion is significantly improved. Using this approach, a new hardware oriented version of F-FCSR has been recently proposed. In this paper, we propose a new design for FCSRs suitable for software applications. Using this approach, we present a new version of X-FCSR128 suitable for software applications which is really efficient in software.