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Properties of the additive differential probability $\mathrm{adp}^{\mathrm{XR}}$ of the composition of bitwise XOR and a bit rotation are investigated, where the differences are expressed using addition modulo $2^n$. This composition is widely used in ARX constructions consisting of additions modulo $2^n$, bit rotations and bitwise XORs. Differential cryptanalysis of such primitives may involve maximums of $\mathrm{adp}^{\mathrm{XR}}$, where some of its input or output differences are fixed. Although there is an efficient way to calculate this probability, many its properties are still unknown. In this work we find maximums of $\mathrm{adp}^{\mathrm{XR}}$, where the rotation is one bit left/right and one of its input differences is fixed. Some symmetries of $\mathrm{adp}^{\mathrm{XR}}$ are obtained as well. Also, we provide all its impossible differentials in terms of regular expression patterns. The number of them is estimated. It turned out to be maximal for the one bit left rotation and noticeably less than the number of impossible differentials of bitwise XOR.

We consider the function $x^{-1}$ that inverses a finite field element $x \in \mathbb{F}_{p^n}$ ($p$ is prime, $0^{-1} = 0$) and affine $\mathbb{F}_{p}$-subspaces of $\mathbb{F}_{p^n}$ such that their images are affine subspaces as well. It is proven that the image of an affine subspace $L$, $|L| > 2$, is an affine subspace if and only if $L = q \mathbb{F}_{p^k}$, where $q \in \mathbb{F}_{p^n}^{*}$ and $k \mid n$. In other words, it is either a subfield of $\mathbb{F}_{p^n}$ or a subspace consisting of all elements of a subfield multiplied by $q$. This generalizes the results that were obtained for linear invariant subspaces in 2006. As a consequence, we propose a sufficient condition providing that a function $A(x^{-1}) + b$ has no invariant affine subspaces $U$ of cardinality $2 < |U| < p^n$ for an invertible linear transformation $A: \mathbb{F}_{p^n} \to \mathbb{F}_{p^n}$ and $b \in \mathbb{F}_{p^n}^{*}$. As an example, it is shown that the condition works for S-box of AES. Also, we demonstrate that some functions of the form $\alpha x^{-1} + b$ have no invariant affine subspaces except for $\mathbb{F}_{p^n}$, where $\alpha, b \in \mathbb{F}_{p^n}^{*}$ and $n$ is arbitrary.