#ArbitrageTradingStrategy Before bitcoin, several digital cash technologies were released, starting with David Chaum's ecash in the 1980s.[10] The idea that solutions to computational puzzles could have some value was first proposed by cryptographers Cynthia Dwork and Moni Naor in 1992.[11][10] The concept was independently rediscovered by Adam Back who developed Hashcash, a proof-of-work scheme for spam control in 1997.[10] The first proposals for distributed digital scarcity-based cryptocurrencies came from cypherpunks Wei Dai (b-money) and Nick Szabo (bit gold) in 1998.[12] In 2004, Hal Finney developed the first currency based on reusable proof of work.[13] These various attempts were not successful:[10] Chaum's concept required centralized control and no banks wanted to sign on, Hashcash had no protection against double-spending, while b-money and bit gold were not resistant to Sybil attacks.[10]