Paper 2015/528

SpaceMint: A Cryptocurrency Based on Proofs of Space

Sunoo Park and Albert Kwon and Georg Fuchsbauer and Peter Gaži and Joël Alwen and Krzysztof Pietrzak

Abstract

Since its introduction in 2009, Bitcoin has become the most successful cryptocurrency ever deployed. However, the currency's dramatic expansion has also raised serious concerns about its long-term sustainability: (1) Bitcoin mining dynamics have shifted away from decentralization, as dedicated hardware and entry of governments and energy producers has gradually placed most mining power in the hands of a select few, and (2) the network's growth spurt has come with accompanying, vast amounts of energy constantly ``wasted'' solely for the purpose of sustaining the currency. In this work, we propose SpaceMint, a cryptocurrency based on proofs of space, instead of wasteful proofs of work. Mining in SpaceMint is designed to have low setup and overhead costs, yielding a fairer reward structure for small and large miners. Miners in SpaceMint dedicate disk space rather than computation. In our design, we adapt proof-of-space for the cryptocurrency setting, and propose a new block chain format and transaction types that prevent attacks that exploit the inexpensiveness of mining (from which alternative non-proof-of-work-based proposals have suffered). Our prototype shows that initializing 1TB for mining takes about a day (a one-off cost), and miners on average spend just a fraction of a second per block mined. We also provide a formal game-theoretic analysis modeling SpaceMint as an extensive game, and prove that following the protocol is an equilibrium, thereby arguing for the currency's stability and consensus.