On Attacks on Filtering Generators Using Linear Subspace Structures

The filter generator consists of a linear feedback shift register (LFSR) and a Boolean filtering function that combines some bits from the shift register to create a key stream. A new attack on the filter generator has recently been described by Rønjom and Helleseth [6]. This paper gives an alternative and extended attack to reconstruct the initial state of the LFSR using the underlying subspace structure of the filter sequence. This improved attack provides further insight and more flexibility in performing the attack by Rønjom and Helleseth. The main improvements are that it does not use the coefficient sequences that were fundamental in the previous attack and works better in the unlikely cases when the original attack needed some modifications.