Paper 2026/364 Original: eprint.iacr.org/2026/364 ↗

SPRINT: New Isogeny Proofs of Knowledge and Isogeny-Based Signatures

Abstract

Zero-knowledge proofs of knowledge are a fundamental building block in many isogeny-based cryptographic protocols, such as signature schemes based on identification-to-signature transformations, or multi-party ceremonies that avoid a trusted setup, in particular for generating supersingular elliptic curves with unknown endomorphism rings. In this paper, we construct SPRINT, an efficient polynomial IOP-based proof system that encodes the radical $2$-isogeny formulas into a system of multivariate polynomials. When combined with the recent polynomial commitment scheme (PCS) DeepFold, our construction yields substantial improvements over state-of-the-art isogeny proofs of knowledge. For the SQIsign NIST-I prime $p=5 \cdot 2^{248}-1$, our implementation takes only a few milliseconds for proving and verification, with proof sizes around 80 kB. Compared to the previous state-of-the-art proof system by den Hollander, Mula, Slamanig & Spindler (PQCrypto'26), we achieve speedups ranging from $1.1\times$ to $22.7\times$ for the prover and from $4.4\times$ to $109\times$ for the verifier, while achieving proof sizes that are $1.4\times$ to $17.5\times$ smaller, across different instantiations and parameter sets. Moreover, we study the weak simulation extractability of our proof system, which we can use as a starting point for a modular construction of signatures. We show that any Fiat–Shamir compiled interactive proof with a so-called canonical simulator is weakly simulation-extractable. We expect this general result to be applicable to other proof systems and thus of independent interest. Building on SPRINT and our wSE result, we introduce a new family of signature schemes whose security solely relies on the $\ell$-isogeny path problem, a foundational problem in isogeny-based cryptography. As a concrete instantiation, we construct a signature scheme using DeepFold as the PCS. Across the different parameter sets, a prototype implementation of our scheme sees significantly faster signing times than the highly optimized NIST specification for SQIsign, at the cost of much larger signature sizes and excluding a one-time precomputation phase. When this phase is instead included with the signing time, we still get almost on-par timings. Meanwhile our signature scheme relies on weaker assumptions and is constant-time when instantiated with a suitable PCS. Even though our signatures are large, the framework offers flexibility for tradeoffs and optimizations – both within a given PCS and by switching to alternative PCS constructions. In particular, it will naturally inherit efficiency gains from future advances in plausibly post-quantum secure PCS constructions.