Differential Cryptanalysis of the Lightweight Block Cipher Cypress-256

Abstract

This paper presents the results of differential cryptanalysis of the lightweight block cipher Cypress-256. The method for searching multi-round differential characteristic of the block cipher Cypress-256 is proposed. The searching assumes 1) building a big set of one-round differential characteristics and search for possible combinations of one-round characteristics into multi-round ones; 2) extending one-round differential characteristics with the probability up to certain threshold into multi-round characteristics. The following experiments show that the most probable one-round differential characteristics have input differences with 4-6 active bits which are distributed between different words. Besides that, high-probable one-round differential characteristics, which output differences have a small Hamming weight, cannot be extended to build high-probable multi-round differential characteristics. Due to application of the method assuming extension of one-round differential characteristics into multi-round ones, the differential characteristic up to 6 rounds was built, so 10-round block cipher Cypress-256 is resistant to differential cryptanalysis according to the requirements of practical criterion.