2 research outputs found
Algebraic Attack on the Alternating Step(r,s)Generator
The Alternating Step(r,s) Generator, ASG(r,s), is a clock-controlled sequence
generator which is recently proposed by A. Kanso. It consists of three
registers of length l, m and n bits. The first register controls the clocking
of the two others. The two other registers are clocked r times (or not clocked)
(resp. s times or not clocked) depending on the clock-control bit in the first
register. The special case r=s=1 is the original and well known Alternating
Step Generator. Kanso claims there is no efficient attack against the ASG(r,s)
since r and s are kept secret. In this paper, we present an Alternating Step
Generator, ASG, model for the ASG(r,s) and also we present a new and efficient
algebraic attack on ASG(r,s) using 3(m+n) bits of the output sequence to find
the secret key with O((m^2+n^2)*2^{l+1}+ (2^{m-1})*m^3 + (2^{n-1})*n^3)
computational complexity. We show that this system is no more secure than the
original ASG, in contrast to the claim of the ASG(r,s)'s constructor.Comment: 5 pages, 2 figures, 2 tables, 2010 IEEE International Symposium on
Information Theory (ISIT2010),June 13-18, 2010, Austin, Texa
Modified Alternating Step Generators
Irregular clocking of feedback shift registers is a popular technique to improve parameters of keystream generators in stream ciphers. Another technique is to implement nonlinear functions. We join these techniques and propose Modified Alternating Step Generators built with linear and nonlinear feedback shift registers. Adequate nonlinear Boolean functions are used as feedbacks or as filtering functions of shift registers in order to increase complexity of sequences produced by individual registers and the whole generator. We investigate basic parameters of proposed keystream generators, such as period, linear complexity and randomness