Abstract :
Stream cipher system plays an important role in many practical encryption systems. Moreover it can be an ideal source in random number generation. Shift registers are the backbone of such systems. This paper presents a method for attacking and recovering the basic key for the general stream cipher systems. Different criteria should be studied carefully when a key stream generated via these systems. Depending on the type of feedback connection of different stages, the output stream may be classified as linear and nonlinear. Different parameters specify complexity degree of the output of these systems. Proposed system achieved to recover the initial cipher key via two modules. The First module focuses on simulating some general stream cipher systems including shift registers with different lengths, while the second recovers the basic or the initial key which is generated from the first module. Recovering key attached by normalizing polynomial equations to set of linear equations. Nonlinearity output normalized to linear equation before recovering the key. The proposed technique overcomes the complexity parameter of linear and nonlinear stream sequence compared with other techniques in this field. C++ version 4.5 where used in implementing the proposed system.
