Tight Proofs of Space and Replication

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