Algebraic analysis of LEX

Authors:
Muhammad Reza Z'aba;Håvard Raddum;Leonie Simpson;Ed Dawson;Matt Henricksen;Kenneth Wong
Affiliations:
Queensland University of Technology, Brisbane, Queensland, Australia;University of Bergen, Norway;Queensland University of Technology, Brisbane, Queensland, Australia;Queensland University of Technology, Brisbane, Queensland, Australia;Institute for Infocomm Research, South Tower, Singapore;Queensland University of Technology, Brisbane, Queensland, Australia
Venue:
AISC '09 Proceedings of the Seventh Australasian Conference on Information Security - Volume 98
Year:
2009

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Abstract

LEX is a stream cipher that progressed to Phase 3 of the eSTREAM stream cipher project. In this paper, we show that the security of LEX against algebraic attacks relies on a small equation system not being solvable faster than exhaustive search. We use the byte leakage in LEX to construct a system of 21 equations in 17 variables. This is very close to the requirement for an efficient attack, i.e. a system containing 16 variables. The system requires only 36 bytes of keystream, which is very low.