Cryptology ePrint Archive: Report 2016/333

Proof of Space from Stacked Expanders

Ling Ren and Srinivas Devadas

Abstract: Recently, proof of space (PoS) has been suggested as a more egalitarian alternative to the traditional hash-based proof of work. In PoS, a prover proves to a verifier that it has dedicated some specified amount of space. A closely related notion is memory-hard functions (MHF), functions that require a lot of memory/space to compute. While making promising progress, existing PoS and MHF have several problems. First, there are large gaps between the desired space-hardness and what can be proven. Second, it has been pointed out that PoS and MHF should require a lot of space not just at some point, but throughout the entire computation/protocol; few proposals considered this issue. Third, the two existing PoS constructions are both based on a class of graphs called superconcentrators, which are either hard to construct or add a logarithmic factor overhead to efficiency. In this paper, we construct PoS from stacked expander graphs. Our constructions are simpler, more efficient and have tighter provable space-hardness than prior works. Our results also apply to a recent MHF called Balloon hash. We show Balloon hash has tighter space-hardness than previously believed and consistent space-hardness throughout its computation.

Category / Keywords: cryptographic protocols / Proof of space, graph pebbling

Original Publication (in the same form): IACR-TCC-2016

Date: received 25 Mar 2016, last revised 24 Aug 2016

Contact author: renling at mit edu

Available format(s): PDF | BibTeX Citation

Version: 20160825:041842 (All versions of this report)

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