[Resource Topic] 2018/702: Tight Proofs of Space and Replication

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Tight Proofs of Space and Replication

Authors: Ben Fisch


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 the construction can be tuned such that no adversary can pass verification using less than 1-\epsilon N space. Most notably, the degree of the graphs in our construction are independent of \epsilon, and the number of layers is only O(\log(1/\epsilon)). The proof size is O(d/\epsilon). The degree d depends on the depth robust graphs, which are only required to maintain \Omega(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 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.

ePrint: https://eprint.iacr.org/2018/702

Talk: https://www.youtube.com/watch?v=84kHVqvTVcw

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