THE SHORTER BIT SEQUENCE INTERCEPT ATTACK ON LILI-128

Abstract

In this study, we investigated an attack on LILI- 128 keystream generator which required only 128 bits of the cipher text to be intercepted. We implemented the attack on a scaled down version of LILI- 128 due to hardware constraints. We modeled the scaled down LILI-41 keystream generator with same filter functions as in the original generator. The attacker need to intercept 52 bits (scaled down) of the keystream generator output from which it finds the initial state of both the LFSRs involved in the LILI-41 keystream generator. The results state that though the Attack is practically more feasible as it requires very few intercepted bits but its computational complexity is very high at the same time. The computational complexity achieved for the scaled down version is of the order and in terms of time it takes from few seconds to 5-6 hours, depending on the order in which the states are tried, to crack the initial state, but it definitely cracks. The major weakness of the generator in terms of this attack was analyzed to be the filter function for the clocking LFSR which is then changed and is attacked again, and this time the attack yields no results, hence the LILI generator is modified to resist this attack.