Prior to the release of David Chaum and Stefan Brands. Adam Back developed hashcash, a proof-of-work scheme for spam control. The first proposals for distributed digital scarcity based cryptocurrencies were Wei Dai's b-money and Nick Szabo's bit gold. Finney developed a reusable proof of work (RPOW) using hashcash as its proof of work algorithm. In a bit gold proposal, Nick Szabo also investigated some additional enabling aspects including a Byzantine fault-tolerant asset to store and transfer chained proof-of-work solutions. Satoshi Nakamoto has suspects including Dai, Szabo, and Finney - and accompanying denials. The possibility that Satoshi Nakamoto was a computer collective in the European financial sector has also been discussed.