On circular external difference families

Circular external difference families (CEDFs) are a recently-introduced variation of external difference families (EDFs) with applications to non-malleable threshold schemes: a $(v,m,\ell,1)$-CEDF is an $m$-sequence $(A_0, \ldots, A_{m-1})$ of $\ell$-subsets of an additive group $G$ of order $v$ such that $G\setminus\{0\}$ equals the multiset of all differences $a-a'$, with $(a,a')\in A_{i+1}\times A_{i}$ for some $i \in \mathbb{Z}_m$. When $G$ is the cyclic group, we speak of a cyclic CEDF. The existence of cyclic $(v,m,\ell,1)$-CEDFs is well understood when $m$ is even, while nonexistence is known when both $m$ and $\ell$ are odd. However, the case where $m$ is odd and $\ell$ is even has only been resolved in a few special cases. In this paper, we address this gap by constructing cyclic $(v,m,\ell,1)$-CEDFs for any odd $m>1$ when $\ell=2$, and for any even $\ell \ge 2$ when $m=3$. Notably, the latter result relies on the existence of a suitable tiling of the multiplicative semigroup of $\mathbb{Z}_v\setminus\{0\}$. Moreover, noting that every $(v,3,\ell,1)$-CEDF produces a $(v,3,\ell,2)$-EDF, we completely solve the existence problem for $(3\ell^2+1,3,\ell,2)$-EDFs over an abelian group. Our approach is based on representing the blocks as arithmetic progressions and analyzing their step patterns. We present two different ways to construct cyclic $(v,m,2,1)$-CEDFs for every odd $m>1$; their step patterns show that the resulting CEDFs are inequivalent. Many additional inequivalent CEDFs are obtained by translating suitable subsets within the CEDF.

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