Cryptanalysis of full sprout. (English) Zbl 1375.94142

Gennaro, Rosario (ed.) et al., Advances in cryptology – CRYPTO 2015. 35th annual cryptology conference, Santa Barbara, CA, USA, August 16–20, 2015. Proceedings. Part I. Berlin: Springer (ISBN 978-3-662-47988-9/pbk; 978-3-662-47989-6/ebook). Lecture Notes in Computer Science 9215, 663-682 (2015).
Summary: 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 shows that a more careful analysis should be done in order to instantiate the proposed design method.
For the entire collection see [Zbl 1319.94002].


94A60 Cryptography
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