Cryptology ePrint Archive: Report 2016/035

Simple Proofs of Space-Time and Rational Proofs of Storage

Tal Moran and Ilan Orlov

Abstract: We introduce a new cryptographic primitive: Proofs of Space-Time (PoSTs) and construct an extremely simple, practical protocol for implementing these proofs. A PoST allows a prover to convince a verifier that she spent a ``space-time'' resource (storing data---space---over a period of time). Formally, we define the PoST resource as a trade-off between CPU work and space-time (under reasonable cost assumptions, a rational user will prefer to use the lower-cost space-time resource over CPU work).

Compared to a proof-of-work, a PoST requires less energy use, as the ``difficulty'' can be increased by extending the time period over which data is stored without increasing computation costs. Our definition is very similar to ``Proofs of Space'' [ePrint 2013/796, 2013/805] but, unlike the previous definitions, takes into account amortization attacks and storage duration. Moreover, our protocol uses a very different (and much simpler) technique, making use of the fact that we explicitly allow a space-time tradeoff, and doesn't require any non-standard assumptions (beyond random oracles). Unlike previous constructions, our protocol allows incremental difficulty adjustment, which can gracefully handle increases in the price of storage compared to CPU work. In addition, we show how, in a cryptocurrency context, the parameters of the scheme can be adjusted using a market-based mechanism, similar in spirit to the difficulty adjustment for PoW protocols.

Category / Keywords: cryptographic protocols / proofs of work, proofs of space, bitcoin, crypto-currency

Date: received 13 Jan 2016, last revised 28 Feb 2019

Contact author: talm at idc ac il

Available format(s): PDF | BibTeX Citation

Note: Updated version with simplified protocol and improved results.

Version: 20190228:222514 (All versions of this report)

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