MATHEMATICAL APPROACH FOR RECOVERING ENCRYPTION KEYOF STREAM CIPHER SYSTEM

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.