International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: Tight Proofs of Space and Replication

Ben Fisch
DOI: 10.1007/978-3-030-17656-3_12 (login may be required)
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Abstract: We construct a concretely practical proof-of-space (PoS) with arbitrarily tight security based on stacked depth robust graphs and constant-degree expander graphs. A proof-of-space (PoS) is an interactive proof system where a prover demonstrates that it is persistently using space to store information. A PoS is arbitrarily tight if the honest prover uses exactly N space and for any $$\epsilon > 0$$ϵ>0 the construction can be tuned such that no adversary can pass verification using less than $$(1-\epsilon ) N$$(1-ϵ)N space. Most notably, the degree of the graphs in our construction are independent of $$\epsilon $$ϵ, and the number of layers is only $$O(\log (1/\epsilon ))$$O(log(1/ϵ)). The proof size is $$O(d/\epsilon )$$O(d/ϵ). The degree d depends on the depth robust graphs, which are only required to maintain $$\varOmega (N)$$Ω(N) depth in subgraphs on 80% of the nodes. Our tight PoS is also secure against parallel attacks.Tight proofs of space are necessary for proof-of-replication (PoRep), which is a publicly verifiable proof that the prover is dedicating unique resources to storing one or more retrievable replicas of a specified file. Our main PoS construction can be used as a PoRep, but data extraction is as inefficient as replica generation. We present a second variant of our construction called ZigZag PoRep that has fast/parallelizable data extraction compared to replica generation and maintains the same space tightness while only increasing the number of levels by roughly a factor two.
Video from EUROCRYPT 2019
  title={Tight Proofs of Space and Replication},
  booktitle={Advances in Cryptology – EUROCRYPT 2019},
  series={Advances in Cryptology – EUROCRYPT 2019},
  author={Ben Fisch},