Security Evaluation of Nonlinear Filterization Stream Cipher Based on Information Theory
Sattar B. Sadkhan, Dhilal M. Reda · 2014
One of the techniques for destroying the linearity inherent in LFSRs is to generate the keystream as a result of some nonlinear function of the stages of a single LFSR. Such keystream generators are called nonlinear filter generators, and the used nonlinear function is called the filtering function. We have proposed a new method to evaluate the nonlinear filtering function which is based on five parameters (length of register, seed, connection polynomial, filter structure, chosen entrances). To built the nonlinear function , the designer needs to built it in the program and change part of the chosen nonlinear function every time and see the results. We shorten this operation for the designer to write only one equation and the new method will generate all the possible forms for the input equation, and then analyzes the output sequence resulted from each state using frequency and serial tests to determine the best filter function and consequently we knew the probability distribution of the resulted key Based on the Information Theory, security evaluation of nonlinear filtering stream cipher was computed. Key word : Security Evaluation , Stream Cipher , nonlinear Filterization