The Method of Searching for Differential Trails of ARX-based Block Cipher Cypress

The method of searching for multi-round differential trails of ARX-based block cipher Cypress is proposed. Cypress is a lightweight block cipher which was designed for using in post quantum period and it operates 256/512-bit blocks and keys. Cypress is based on Feistel network with ARX-transformation as a round function. The proposed method of searching for differential trails is based on several assumptions made within the presented mathematical model (Cypress is a Markov cipher, the input differences of one-round differential trails with high probability have a small Hamming weight etc.). The searching for multi-round differential trails consists in extending probable one-round differential trails to several rounds. Using the method proposed, the efficient differential trail was found for 6-round Cypress-256, proving that 10-round Cypress-256 is resistant against differential cryptanalysis according to the requirements of practical criterion.