Establishing Equations: The Complexity of Algebraic and Fast Algebraic Attacks Revisited
Algebraic and fast algebraic attacks have posed serious threats to some deployed LFSR-based stream ciphers. Previous works on this topic focused on reducing the time complexity by lowering the degree of the equations, speeding up the substitution step by Fast Fourier Transform and analysis of Boolean functions exhibiting the optimal algebraic immunity. All of these works shared and overlooked a common base, i.e., establishing an adequate equation system first, which actually in some cases dominates the time or memory complexity if the direct methods are used, especially in fast algebraic attacks. In this paper, we present a complete analysis of the establishing equation procedure and show how the Frobenius form of the monomial state rewriting matrix can be applied to considerably reduce the complexity of this step.
KeywordsAlgebraic attack Stream cipher Establishing equations Coefficient sequence
We would like to thank anonymous referees for their helpful comments and suggestions, especially a reviewer of Asiacrypt 2013. This work was supported by the National Grand Fundamental Research 973 Program of China (Grant No. 2013CB338002, No. 2013CB834203), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA06010701), IIE’s Research Project on Cryptography (Grant No. Y3Z0016102) and the programs of the National Natural Science Foundation of China (Grant Nos. 61379142, 60833008, 60603018, 61173134, 91118006, 61272476). Supported by the National Natural Science Foundation of China under Grant No. 91118006, the National Grand Fundamental Research 973 Program of China under Grant No. 2013CB338003.
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