Coefficient choice in Galois-ring lifts of APN permutations: sharp unit-direction spectra and one-term optimal lifts
Hiroki Fukui
Permutation polynomials over the Galois ring $\mathrm{GR}(2^k,m)$ are nonlinear components for symmetric primitives that add keys in the ring, as in secure computation modulo $2^k$. Rønjom and Sandrib showed that their differential uniformity is at least $2q^{k-1}$, where $q=2^m$. For the class studied here it is attained on top-layer differences, while almost all differences are units. We ask how small the difference table can be on unit differences when the residue permutation is fixed and only higher coefficient digits of its lift are chosen. Let $m\ge5$ be odd, $k\ge3$, and let $p$ be a reduced quadratic almost perfect nonlinear (APN) permutation of $\mathbb{F}_q$ whose derivative is nowhere zero and two-to-one. Over all lifts of $p$, the least possible maximum on unit differences is $q$ or $2q$, decided by two invariants of $p$ and the level $k$. We describe every lift attaining it and show that the coefficients modulo $8$ decide optimality at every level. For every odd $m\ge5$, residue permutations with at most $16$ monomials attain $q$ after one $2$-adic digit of one coefficient of the natural lift is changed. For an explicit infinite family, one added monomial lowers the maximum on unit differences from $4q$, or from at least $q^2$, to $2q$ at every level without raising the degree. Under a uniform starting state and independent uniform round keys, valuation preservation yields layerwise bounds for characteristics and two-round differentials that distinguish lifts with the same global uniformity. Core results are formalized in Lean 4.
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