Another Tradeoff Attack on Sprout-Like Stream Ciphers

Sprout is a new lightweight stream cipher with shorter internal state proposed at FSE 2015, using key-dependent state updating in the keystream generation phase. Some analyses have been available on eprint so far. In this paper, we extend the design paradigm in general and study the security of Sprout-like ciphers in a unified framework. Our new penetration is to investigate the k-normality of the augmented function, a vectorial Boolean function derived from the primitive. Based on it, a dedicated time/memory/data tradeoff attack is developed for such designs. It is shown that Sprout can be broken in 2(79-x-y) time, given [c . (2x + 2y -58) . 2(71-x-y)]-bit memory and 2(9+x+y)-bit keystream, where x/y is the number of forward/backward steps and c is a small constant. Our attack is highly flexible and compares favorably to all the previous results. With carefully chosen parameters, the new attack is at least 2(20) times faster than Lallemand/Naya-Plasencia attack at Crypto 2015, Maitra et al. attack and Banik attack, 2(10) times faster than Esgin/Kara attack with much less memory.