Some results on lightweight stream ciphers Fountain v1 & Lizard

Abstract

In this paper, we propose cryptanalytic results on two lightweight stream ciphers: Fountain v1 and Lizard. The main results of this paper are the followings:$ - $ We propose a zero-sum distinguisher on reduced round Fountain v1. In this context, we study the non-randomness of the cipher with a careful selection of cube variables. Our obtained cube provides a zero-sum on Fountain v1 till $ 188 $ initialization rounds and significant non-randomness till $ 189 $ rounds. This results in a distinguishing attack on Fountain v1 with $ 189 $ initialization rounds. $ - $ Further, we find that the same cipher has a weakness against conditional Time-Memory-Data-Tradeoff (TMDTO). We show that TMDTO attack using sampling resistance has online complexity $ 2^{110} $ and offline complexity $ 2^{146} $. $ - $ Finally, we revisit the Time-Memory-Data-Tradeoff attack on Lizard by Maitra et al. (IEEE Transactions on Computers, 2018) and provide our observations on their work. We show that instead of choosing any random string, some particular strings would provide better results in their proposed attack technique.