The most famous cryptocurrency, Bitcoin, was created in 2009. However, cryptographers have been experimenting with cryptocurrencies over the past four decades. In 1983, an American inventor named David Chaum designed an electronic cash system called DigiCash that was based on cryptographic algorithms. In 1997, a British cryptographer named Adam Back created HashCash that used the proof-of-work scheme to prevent email spam. In 1998, b-money and bit gold were released by Wei Dai and Nick Szabo, respectively.