Prior to the release of bitcoin, there were a number of digital cash technologies, starting with the issuer-based ecash protocols of David Chaum and Stefan Brands.[3][4][5] The idea that solutions to computational puzzles could have some value was first proposed by cryptographers Cynthia Dwork and Moni Naor in 1992. The idea was independently rediscovered by Adam Back who developed hashcash, a proof-of-work scheme for spam control in 1997.[6] The first proposals for distributed digital scarcity-based cryptocurrencies were Wei Dai's b-money[7] and Nick Szabo's bit gold.[8][9] Hal Finney developed reusable proof of work (RPOW) using hashcash as its proof of work algorithm.[10]
In the bit gold proposal which proposed a collectible market-based mechanism for inflation control, Nick Szabo also investigated some additional aspects including a Byzantine fault-tolerant agreement protocol based on quorum addresses to store and transfer the chained proof-of-work solutions, which was vulnerable to Sybil attacks,
