Biased differential distinguisher – Cryptanalysis of reduced-round SKINNY

SKINNY is a lightweight tweakable block cipher which received a great deal of cryptanalytic attention due to its elegant structure and efficiency. Despite the cryptanalytic efforts the security margins are remaining high. This has led to SKINNY being used as a component of multiple submissions in the NIST Lightweight Competition, an effort to standardize a lightweight AEAD scheme. Inspired by the SKINNY competitions, multiple attacks on it were reported in different settings (e.g. single vs. related-tweakey) using different techniques (impossible differentials, zero-correlation, meet-in-the-middle, etc.). In this paper we revisit some of these attacks, identify issues with several of them, and offer a series of improved attacks which were experimentally verified. Our best attack can attack up to 18 rounds of SKINNY-64 using 2⁶⁰ chosen plaintexts data, 2¹¹⁶ time, and 2¹¹² memory.