International Association
for Cryptologic Research

By the impossibility result of Gentry and Wichs, non-falsifiable assumptions are needed to construct (even non-zero-knowledge) adaptively sound succinct non-interactive arguments (SNARGs) for hard languages. It is important to understand whether this impossibility result is tight. While it is known how to construct adaptively sound non-succinct non-interactive arguments for $\mathsf{NP}$ from falsifiable assumptions, adaptively sound SNARGs for $\mathsf{NP}$ from non-falsifiable assumptions, and adaptively sound SNARGs for $\mathsf{P}$ from falsifiable assumptions, there are no known non-adaptively sound SNARGs for $\mathsf{NP}$ from falsifiable assumptions. We show that Gentry-Wichs is tight by constructing the latter. In addition, we prove it is non-adaptively knowledge-sound in the algebraic group model and Sub-ZK (i.e., zero-knowledge even if the CRS is subverted) under a non-falsifiable assumption.

A succinct functional commitment (SFC) scheme for a circuit class $\mathbf{CC}$ enables, for any circuit $\mathcal{C}\in \mathbf{CC}$, the committer to first succinctly commit to a vector $\vec{\alpha}$, and later succinctly open the commitment to $\mathcal{C} (\vec{\alpha}, \vec{\beta})$, where the verifier chooses $\vec{\beta}$ at the time of opening. Unfortunately, SFC commitment schemes are known only for severely limited function classes like the class of inner products. By making non-black-box use of SNARK-construction techniques, we propose a SFC scheme for the large class of semi-sparse polynomials. The new SFC scheme can be used to, say, efficiently (1) implement sparse polynomials, and (2) aggregate various interesting SFC (e.g., vector commitment and polynomial commitment) schemes. The new scheme is evaluation-binding under a new instantiation of the computational uber-assumption. We provide a thorough analysis of the new assumption.