A different but relevant concept called hashcash was introduced by Adam Back in 1997 as a PoW system to control e-mail spam. The idea is quite simple: if legitimate users want to send e-mails then they are required to compute a hash as a proof that they have spent a reasonable amount of computing resources before sending the e-mail. Generating hashcash is a compute intensive process but does not inhibit a legitimate user from sending the e-mail because the usual number of e-mails required to be sent by a legitimate user is presumably quite low. On the other hand, if a spammer wants to send e-mails, usually thousands in number, then it becomes infeasible to compute hashcash for all e-mails, thus making the spamming effort expensive; as a result this mechanism can be used to thwart e-mail spamming. Hashcash takes a considerable amount of computing resources to compute but is easy and quick to verify. Verification is performed by the user who receives the e-mail. Hashcash is popularized by its use in the bitcoin mining process. This idea of using computational puzzles or pricing functions to prevent e-mail spam was introduced originally in 1992 by Cynthia Dwork and Moni Naor. Pricing function was the name given to the hard functions that are required to be computed before access to a resource can be granted. Later, Adam Back invented hashcash independently in 1997, which introduced the usage of computing hash functions as PoW.
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