$BTC
Background
Before bitcoin, several digital cash technologies were released, starting with David Chaum's ecash in the 1980s.The idea that solutions to computational puzzles could have some value was first proposed by cryptographers Cynthia Dwork and Moni Naor in 1992. The concept was independently rediscovered by Adam Back who developed Hashcash, a proof-of-work scheme for spam control in 1997. The first proposals for distributed digital scarcity-based cryptocurrencies came from cypherpunks Wei Dai (b-money) and Nick Szabo (bit gold) in 1998. In 2004, Hal Finney developed the first currency based on reusable proof of work.These various attempts were not successful: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.