Paper 2026/347

Relaxed Modular PCS from Arbitrary PCS and Applications to SNARKs for Integers

Abstract

\emph{Modular Polynomial Commitment Schemes (Mod-PCS)} extend standard PCSs by enabling provable evaluation of integer polynomials modulo a random modulus, providing a natural foundation for SNARKs that operate directly over large integers without emulating arithmetic in finite fields. Only two Mod-PCS constructions are known. The first (Campanelli and Hall-Andersen, IACR ePrint 2024) serves primarily as a feasibility result and is impractical and not post-quantum secure due to its reliance on groups of unknown order. The second (Garetta et al., CRYPTO 2025) introduces the weaker notion of \emph{relaxed} Mod-PCS, but is not fully succinct: committing to a multilinear polynomial with $N$ terms and $B$-bit coefficients requires $O(\sqrt{N}B)$ proof size and verification time. We present a black-box transformation that builds relaxed Mod-PCS from any standard PCS, enabling new constructions. Instantiating our transformation with a tensor-code PCS yields the first relaxed Mod-PCS with $O(\log (N+B))$ proof size and verifier time, which is transparent and plausibly post-quantum secure. Using this scheme within the framework of Garetta et al., we obtain the first fully succinct SNARK for the Customizable Constraint System over $\mathbb{Z}_B$, achieving $O(B\log N + N\log N \log B)$ prover time and $O(\log (N+B))$ verifier time and proof size. Our approach relies on a commitment-switching technique for integer polynomials and a new batched integer commitment scheme from any PCS. We further introduce improved arguments for integer addition and multiplication, correctness of the number-theoretic transform, and general Diophantine relations over committed integers.

Note: Bibliography Problem Fixed Author affiliation changed Carry propagation gap fixed in multiplication protocol Rotation check added