¡¥uer based e-cash protocols 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. Hal Finney developed reusable proof of work(RPOW) using hashcash as its proof of work algorithm. ¡¦