Efficient Interactive Proofs for Non-Deterministic Bounded Space

Authors Joshua Cook , Ron D. Rothblum

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Author Details

Joshua Cook
  • Department of Computer Science, University of Texas Austin, TX, USA
Ron D. Rothblum
  • Faculty of Computer Science, Technion, Haifa, Israel


Thanks to Dana Moshkovitz for introducing the authors of this paper, and Anna Gál for finding the references relating low depth circuits to alternating algorithms.

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Joshua Cook and Ron D. Rothblum. Efficient Interactive Proofs for Non-Deterministic Bounded Space. In Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 275, pp. 47:1-47:22, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)


The celebrated IP = PSPACE Theorem gives an efficient interactive proof for any bounded-space algorithm. In this work we study interactive proofs for non-deterministic bounded space computations. While Savitch’s Theorem shows that nondeterministic bounded-space algorithms can be simulated by deterministic bounded-space algorithms, this simulation has a quadratic overhead. We give interactive protocols for nondeterministic algorithms directly to get faster verifiers. More specifically, for any non-deterministic space S algorithm, we construct an interactive proof in which the verifier runs in time Õ(n+S²). This improves on the best previous bound of Õ(n+S³) and matches the result for deterministic space bounded algorithms, up to polylog(S) factors. We further generalize to alternating bounded space algorithms. For any language L decided by a time T, space S algorithm that uses d alternations, we construct an interactive proof in which the verifier runs in time Õ(n + S log(T) + S d) and the prover runs in time 2^O(S). For d = O(log(T)), this matches the best known interactive proofs for deterministic algorithms, up to polylog(S) factors, and improves on the previous best verifier time for nondeterministic algorithms by a factor of log(T). We also improve the best prior verifier time for unbounded alternations by a factor of S. Using known connections of bounded alternation algorithms to bounded depth circuits, we also obtain faster verifiers for bounded depth circuits with unbounded fan-in.

Subject Classification

ACM Subject Classification
  • Theory of computation → Interactive proof systems
  • Interactive Proofs
  • Alternating Algorithms
  • AC0[2]
  • Doubly Efficient Proofs


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