How to Prove Post-quantum Security for Succinct Non-interactive Reductions

Hash-based succinct non-interactive arguments (SNARGs) are widely used in practice, owing to their ease of deployment, notable efficiency, and post-quantum properties. They are constructed via the BCS transformation, which combines an interactive oracle proof (IOP) and a hash-based vector commitment. This success has motivated the study of hash-based succinct non-interactive reductions (SNRDXs), used for recursively ensuring the correctness of distributed computations, by extending the BCS transformation to work with an interactive oracle reduction (IOR) rather than an IOP.

We prove that hash-based SNRDXs constructed from IORs are secure in the quantum random oracle model (QROM), provided the IOR satisfies a natural post-quantum analogue of state-restoration security; moreover, we show that (classical) round-by-round security implies post-quantum state-restoration security. Our results thus achieve a post-quantum analogue of the classical security of SNRDXs in the ROM, and generalize a prior result about SNARGs in the QROM to cover recent SNRDXs constructions.

Moreover, for SNRDXs we propose and achieve an adaptively-secure straightline quantum extraction property in the QROM, while prior work obtains non-adaptive security for SNARGs in the QROM. Along the way, we develop a modular framework for proving the security of the (extended) BCS transformation based on a new quantum extraction property for vector commitments (which we prove is achieved by Merkle commitments), mirroring classical security analyses and departing from prior "monolithic" post-quantum analyses. This demands a new commutator bound that shows the almost-commutativity between quantum extraction and quantum oracle queries, by bounding a natural classical extraction property.