Design of a Linear Layer Optimised for Bitsliced 32-bit Implementation


  • Gaëtan Leurent Inria, Paris, France
  • Clara Pernot Inria, Paris, France



Bitsliced cipher, Linear layer, Branch number


The linear layer of block ciphers plays an important role in their security In particular, ciphers designed following the wide-trail strategy use the branch number of the linear layer to derive bounds on the probability of linear and differential trails. At FSE 2014, the LS-design construction was introduced as a simple and regular structure to design bitsliced block ciphers. It considers the internal state as a bit matrix, and applies alternatively an identical S-Box on all the columns, and an identical L-Box on all the lines. Security bounds are derived from the branch number of the L-Box.
In this paper, we focus on bitsliced linear layers inspired by the LS-design construction and the Spook AEAD algorithm. We study the construction of bitsliced linear transformations with efficient implementations using XORs and rotations (optimized for bitsliced ciphers implemented on 32-bit processors), and a high branch number. In order to increase the density of the activity patterns, the linear layer is designed on the whole state, rather than using multiple parallel copies of an L-Box. Our main result is a linear layer for 128-bit ciphers with branch number 21, improving upon the best 32-bit transformation with branch number 12, and the one of Spook with branch number 16.




How to Cite

Leurent, G., & Pernot, C. (2024). Design of a Linear Layer Optimised for Bitsliced 32-bit Implementation. IACR Transactions on Symmetric Cryptology, 2024(1), 441–458.