# “Subtractive Sets over Cyclotomic Rings: Limits of Schnorr-like Arguments over Lattices” Accepted to CRYPTO 2021

The paper “Subtractive Sets over Cyclotomic Rings: Limits of Schnorr-like Arguments over Lattices” has been accepted to CRYPTO 2021. This is a joint work of Russell W. F. Lai with Martin R. Albrecht.

Below you will find the abstract of the paper.

We study when (dual) Vandermonde systems of the form ${V}_T^{{(\intercal)}} \cdot \vec{z} = s\cdot \vec{w}$ admit a solution $\vec{z}$ over a ring $\mathcal{R}$, where ${V}_T$ is the Vandermonde matrix defined by a set $T$ and where the “slack” $s$ is a measure of the quality of solutions. To this end, we propose the notion of $(s,t)$-subtractive sets over a ring $\mathcal{R}$, with the property that if $S$ is $(s,t)$-subtractive then the above (dual) Vandermonde systems defined by any $t$-subset $T \subseteq S$ are solvable over $\mathcal{R}$. The challenge is then to find large sets $S$ while minimising (the norm of) $s$ when given a ring $\mathcal{R}$.

By constructing families of $(s,t)$-subtractive sets $S$ of size $n = poly(\lambda)$ over cyclotomic rings $\mathcal{R} = \mathbb{Z}[\zeta_{p^\ell}]$ for prime $p$, we construct Schnorr-like lattice-based proofs of knowledge for the SIS relation $A \cdot \vec{x} = s \cdot \vec{y} \bmod q$ with $O(1/n)$ knowledge error, and $s = 1$ in case $p = poly(\lambda)$. Our technique slots naturally into the lattice Bulletproof framework from Crypto’20, producing lattice-based succinct arguments for NP with better parameters.

We then give matching impossibility results constraining $n$ relative to $s$, which suggest that our Bulletproof-compatible protocols are optimal unless fundamentally new techniques are discovered. Noting that the knowledge error of lattice Bulletproofs is $\Omega(\log k/n)$ for witnesses in $\mathcal{R}^k$ and subtractive set size $n$, our result represents a barrier to practically efficient lattice-based succinct arguments in the Bulletproof framework.

Beyond these main results, the concept of $(s,t)$-subtractive sets bridges group-based threshold cryptography to lattice settings, which we demonstrate by relating it to distributed pseudorandom functions.