Paper 2026/326

Special Soundness and Binding Properties: A Framework for Tightly Secure zk-SNARKs

Abstract

Interactive arguments often combine polynomial IOPs with polynomials commitment schemes (PCSs). Frequently, the interactive argument is proven to be knowledge sound, but this incurs a high security loss when applying the Fiat-Shamir transformation to obtain a non-interactive argument in the random oracle model (ROM). We introduce the notion of special soundness for polynomial IOPs, which surprisingly has not been considered before. We study relations between various binding properties of univariate PCSs. In the case of the KZG PCS, these properties can be based on falsifiable assumptions. We prove that a special-sound polynomial IOP plus a PCS under suitable binding notions gives a computationally special-sound interactive argument. By Attema, Fehr, and Klooss (TCC 2022), applying Fiat-Shamir to this argument yields a tightly knowledge-sound argument (or zk-SNARK) in the ROM under the same assumptions. In the case of the KZG PCS, we add various batching optimizations to our compiler and prove that they preserve computational special soundness. This yields a generic approach for achieving efficient zk-SNARKs with constant proof size and tight knowledge soundness in the ROM under falsifiable assumptions.