Direct Construction of Optimal Rotational-XOR Diffusion Primitives

Zhiyuan Guo, Renzhang Liu, Si Gao, Wenling Wu, Dongdai Lin
2017 IACR Transactions on Symmetric Cryptology  
As a core component of SPN block cipher and hash function, diffusion layer is mainly introduced by matrices built from maximum distance separable (MDS) codes. Up to now, most MDS constructions require to perform an equivalent or even exhaustive search. In this paper, we study the cyclic structure of rotational-XOR diffusion layer, a commonly used diffusion primitive over (𝔽b2)n, which consists of only rotation and XOR operations. First, by providing some novel properties on this class of
more » ... s, we prove the lower bound on the number of rotations for n≤4, and show the tightness of this bound for n=4. Next, through characterizing the relation among sub-matrices for each possible form, we eliminate all the other non-optimal cases. Finally, we present a direct construction of such MDS matrices, which allows to generate 4×4 perfect instances for arbitrary b≤4. Every example contains the fewest possible rotations, so under this strategy, our proposal costs the minimum gate equivalents (resp. cyclic shift instructions) in the hardware (resp. software) implementation. To the best of our knowledge, it is the first time that rotational-XOR MDS diffusion layers have been constructed without any auxiliary search.
doi:10.13154/tosc.v2017.i4.169-187 dblp:journals/tosc/GuoLGWL17 fatcat:yptvjuhubja2ne4yp3htpri4da