Additive Differentials for ARX Mappings with Probability Exceeding 1/4

We consider the additive differential probabilities of functions \( x \oplus y \) and \( (x \oplus y) \lll r \), where \( x, y \in \mathbb {Z}_2^n \) and \( 1 \leq r < n \). The probabilities are used for the differential cryptanalysis of ARX ciphers that operate only with addition modulo \( 2^n \), bitwise XOR ( \( \oplus \)), and bit rotations ( \( \lll r \)). A complete characterization of differentials whose probability exceeds \( 1/4 \) is obtained. All possible values of their probabilities are \( 1/3 + 4^{2 - i} / 6 \) for \( i \in \{1, \dots , n\} \). We describe differentials with each of these probabilities and calculate the number of these values. We also calculate the number of all considered differentials. It is \( 48n - 68 \) for \( x \oplus y \) and \( 24n - 30 \) for \( (x \oplus y) \lll r \), where \( n \geq 2 \). We compare differentials of both mappings under the given constraint.