On APN Exponents and the Differential and Boomerang Properties of Binomials in Characteristic 3
Pith reviewed 2026-05-25 03:16 UTC · model grok-4.3
The pith
Two classes of binomials over finite fields of characteristic 3 achieve boomerang uniformity zero and local perfect nonlinearity.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central claim is that two explicit classes of binomials built from APN exponents produced by the Zha-Wang parametrization are locally perfect nonlinear and possess boomerang uniformity zero, while the binomial with exponent r = 3^n - 3 is locally almost perfect nonlinear with boomerang uniformity one for every odd n at least five and has its full boomerang spectrum determined by character-sum evaluation.
What carries the argument
The binomial functions F_r(x) = x^r (1 + χ(x)) over F_{3^n} constructed from APN exponents of the Zha-Wang form, together with the evaluation of character sums that determine the boomerang spectrum.
If this is right
- The two proved classes give infinite families of functions attaining the absolute minimum boomerang uniformity in characteristic 3.
- The binomial with exponent 3^n - 3 supplies an explicit example whose boomerang spectrum is completely known for all odd n at least five.
- Numerical verification up to n = 13 indicates that the Zha-Wang list already captures every previously recorded APN power function in this setting.
- The proofs extend earlier binomial constructions with low boomerang uniformity from other characteristics to the case of characteristic 3.
Where Pith is reading between the lines
- If a complete classification of APN exponents in characteristic 3 ever appears, the same binomial construction would immediately produce additional families with boomerang uniformity zero.
- The pattern observed for odd n suggests that direct computation of the boomerang uniformity for the exponent 3^n - 3 at the next few odd values of n could serve as a quick consistency check.
- Analogous binomial constructions might be attempted in characteristic 2 or 5 once comparable parametrizations of APN exponents become available.
Load-bearing premise
The Zha-Wang parametrization supplies every APN exponent that produces the claimed binomial families.
What would settle it
An explicit APN exponent for some n greater than 13 that lies outside the Zha-Wang list and yields a binomial with boomerang uniformity strictly larger than zero would falsify the claimed generality of the two zero-uniformity classes.
read the original abstract
Recent studies on binomials of the form $F_r(x) = x^r(1 + \chi(x))$ over $\mathbb{F}_{p^n}$ have shown that these functions can exhibit very low boomerang uniformity. In this paper, we focus on the specific behavior of such binomials in characteristic $3$, where instances of extremely low boomerang uniformity-namely $0$ or $1$-seem to arise more frequently than in other characteristics. First, we provide a systematic analysis of Almost Perfect Nonlinear (APN) power functions in characteristic $3$. We present an explicit parametrization of APN exponents arising from the construction of Zha and Wang and demonstrate through numerical results for $n \le 13$ that this generalized framework accounts for several previously known and sporadic APN instances. Building on this classification, we identify and rigorously prove two classes of binomials $F_r$ that are locally-PN and possess the minimum possible boomerang uniformity of $0$. These classes involve exponents derived from the aforementioned APN construction and the differentially 4-uniform exponent $r = 2 \cdot 3^{\frac{n-1}{2}} + 1$. Furthermore, we analyze the binomial $F_r$ with $r = 3^n - 3$, proving that it is locally-APN with boomerang uniformity $1$ when $n\ge 5$ is odd, and completely determine its boomerang spectrum through the evaluation of character sums. Our results clarify and extend existing studies on the cryptographic properties of binomials, providing a systematic characterization of several classes of binomials with very low boomerang uniformity in characteristic $3$.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper provides a systematic analysis of APN power functions in characteristic 3 by giving an explicit parametrization of APN exponents from the Zha-Wang construction, supported by numerical checks for n ≤ 13. It proves that two classes of binomials F_r are locally-PN with boomerang uniformity 0, using exponents from the APN construction and r = 2 · 3^{(n-1)/2} + 1. For the binomial with r = 3^n - 3 when n is odd and ≥5, it shows local-APN property with boomerang uniformity 1 and fully determines the boomerang spectrum using character sum evaluations.
Significance. If the results hold, the paper makes a significant contribution to the study of cryptographic properties of functions over finite fields of characteristic 3. The explicit parametrization and rigorous proofs of minimal boomerang uniformity for specific binomials, along with the complete boomerang spectrum determination, provide valuable insights for S-box design. The use of character-sum techniques for the spectrum result is a strength, offering a parameter-free derivation in that part.
minor comments (1)
- [Abstract] The symbol χ(x) is introduced without prior definition or reference; a brief clarification would improve readability for readers unfamiliar with the context.
Simulated Author's Rebuttal
We thank the referee for the positive evaluation, the recognition of the paper's contributions to APN exponents and boomerang uniformity in characteristic 3, and the recommendation for minor revision. No specific major comments were raised in the report.
Circularity Check
No significant circularity identified
full rationale
The paper derives its main results on boomerang uniformity and spectra via explicit parametrization of APN exponents taken from the external Zha-Wang construction together with direct character-sum evaluations. Numerical checks up to n=13 serve only to confirm coverage of known cases and are not premises of the uniformity proofs. No self-definitional reductions, fitted inputs renamed as predictions, load-bearing self-citations, or ansatzes smuggled via citation appear in the derivation chain. The central theorems remain independent of the paper's own inputs.
Axiom & Free-Parameter Ledger
axioms (2)
- standard math Finite fields of characteristic 3 satisfy the usual field axioms and Frobenius automorphism properties
- domain assumption Character sums over finite fields can be evaluated to obtain exact boomerang spectra
Reference graph
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discussion (0)
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