Notes in Computer Science 4851

The filter generator consists of a linear feedback shift register (LFSR) and a Boolean filtering function that combines bits from the shift register to create a key stream. The nonlinear combiner generator employs several (LFSRs) and a Boolean function that combines bit from all the registers to generate the key stream. A new attack on the filter generator has recently been described by Rønjom and Helleseth who also extended the attack to linear feedback shift registers over an extension field GF (2). Some extensions and improvements of the attacks to the filter generator have been given by Rønjom, Gong and Helleseth. The purpose of this paper is to give a short overview of these attacks and to discuss how to extend these attacks to the nonlinear combiner generator.