International Association
for Cryptologic Research

A new method for reducing the internal state size of stream cipher registers has been proposed in FSE 2015, allowing to reduce the area in hardware implementations. Along with it, an instantiated proposal of a cipher was also proposed: Sprout. In this paper, we analyze the security of Sprout, and we propose an attack that recovers the whole key more than $2^{10}$ times faster than exhaustive search and has very low data complexity. The attack can be seen as a divide-and-conquer evolved technique, that exploits the non-linear influence of the key bits on the update function. We have implemented the attack on a toy version of Sprout, that conserves the main properties exploited in the attack. The attack completely matches the expected complexities predicted by our theoretical cryptanalysis, which proves its validity. We believe that our attack raises legitimate questions regarding the security of the proposed design method.