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Hashcash
Hashcash is a proof-of-work system used to limit email spam and denial-of-service attacks, and more recently has become known for its use in bitcoin (and other cryptocurrencies) as part of the mining algorithm. Hashcash was proposed in May 1997 by Adam Back.
==How it works==
Hashcash is a proof-of-work algorithm that requires a selectable amount of work to compute, but the proof can be verified efficiently. For email uses, a textual encoding of a hashcash stamp is added to the header of an email to prove the sender has expended a modest amount of CPU time calculating the stamp prior to sending the email. In other words, as the sender has taken a certain amount of time to generate the stamp and send the email, it is unlikely that they are a spammer. The receiver can, at negligible computational cost, verify that the stamp is valid. However, the only known way to find a header with the necessary properties is brute force, trying random values until the answer is found; though testing an individual string is easy, if satisfactory answers are rare enough it will require a substantial number of tries to find the answer.
The hypothesis is that spammers, whose business model relies on their ability to send large numbers of emails with very little cost per message, will cease to be profitable if there is even a small cost for each spam they send. Receivers can verify whether a sender made such an investment and use the results to help filter email.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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