Affiliation: COSIC/KU Leuven
Rotational-XOR Cryptanalysis of Reduced-round SPECK
In this paper we formulate a SAT/SMT model for Rotational-XOR (RX) cryptanalysis in ARX primitives for the first time. The model is successfully applied to the block cipher family Speck, and distinguishers covering more rounds than previously are found, as well as RX-characteristics requiring less data to detect. In particular, we present distinguishers for 10, 11 and 12 rounds for Speck32/64 which have better probabilities than the previously known 9-round differential characteristic, for a certain weak key class. For versions of Speck48, we present several distinguishers, among which the longest one covering 15 rounds, while the previously best differential characteristic only covered 11.
Rotational Cryptanalysis in the Presence of Constants
Rotational cryptanalysis is a statistical method for attacking ARX constructions. It was previously shown that ARX-C, i.e., ARX with the injection of constants can be used to implement any function. In this paper we investigate how rotational cryptanalysis is affected when constants are injected into the state. We introduce the notion of an RX-difference, generalizing the idea of a rotational difference. We show how RX-differences behave around modular addition, and give a formula to calculate their transition probability. We experimentally verify the formula using Speck32/64, and present a 7-round distinguisher based on RX-differences. We then discuss two types of constants: round constants, and constants which are the result of using a fixed key, and provide recommendations to designers for optimal choice of parameters.