A Chosen IV Related Key Attack on Grain-128a

Due to the symmetric padding used in the stream cipher Grain v1 and Grain-128, it is possible to find Key-IV pairs that generate shifted keystreams efficiently. Based on this observation, Lee et al. presented a chosen IV related Key attack on Grain v1 and Grain-128 at ACISP 2008. Later, the... See full abstract

Due to the symmetric padding used in the stream cipher Grain v1 and Grain-128, it is possible to find Key-IV pairs that generate shifted keystreams efficiently. Based on this observation, Lee et al. presented a chosen IV related Key attack on Grain v1 and Grain-128 at ACISP 2008. Later, the designers introduced Grain-128a having an asymmetric padding. As a result, the existing idea of chosen IV related Key attack does not work on this new design. In this paper, we present a Key recovery attack on Grain-128a, in a chosen IV related Key setting. We show that using around ?·2^32 (? is a experimentally determined constant and it is sufficient to estimate it as 2^8) related Keys and ?·2^64 chosen IVs, it is possible to obtain 32·? simple nonlinear equations and solve them to recover the Secret Key in Grain-128a.
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