Maximums of the Additive Differential Probability of Exclusive-Or

At FSE 2004, Lipmaa et al. studied the additive differential probability adp⊕(α, β → γ) of exclusive-or where differences α, β, γ ∈ F2 are expressed using addition modulo 2. This probability is used in the analysis of symmetrickey primitives that combine XOR and modular addition, such as the increasingly popular Addition-Rotation-XOR (ARX) constructions. The focus of this paper is on maximal differentials, which are helpful when constructing differential trails. We provide the missing proof for Theorem 3 of the FSE 2004 paper, which states that maxα,β adp⊕(α, β → γ) = adp⊕(0, γ → γ) for all γ. Furthermore, we prove that there always exist either two or eight distinct pairs α, β such that adp⊕(α, β → γ) = adp⊕(0, γ → γ), and we obtain recurrence formulas for calculating adp⊕. To gain insight into the range of possible differential probabilities, we also study other properties such as the minimum value of adp⊕(0, γ → γ), and we find all γ that satisfy this minimum value.