Hashcash is the proof of work function at the heart of bitcoin mining. It was initially developed in 1997 by Adam Back, a British cryptographer and researcher. He initially proposed hashcash as a tool to protect against denial of service attacks. Requiring the sender of an email, for example, to execute a bit of computation is no problem for a legitimate sender of several dozen or even several hundreds of email a day. For a spammer attempting to send billions of emails, however, the work required would be computationally onerous, thus slowing the flood of illegitimate email.