View source for Hashcash

[[Hashcash]] is a "proof of work" system originally designed as an anti-spam measure.  [[Hashcash]] tokens are "minted" by finding a [[hash collision]] for a unique input string.  The result is included in the header of an email message as "proof" that the sender has done a certain amount of work to "pay for" the delivery of the message.  A 20-bit collision may take only a fraction of a second to compute, depending on the computer system.  25 bits on the same system would take 11 seconds, and 30 bits would take over 6 minutes.  The recipient determines how much work is required to pay for delivery.

The idea behind [[hashcash]] is that a legitimate email sender can easily afford a few seconds to mint a stamp for an outgoing email, but spammers who want to send millions of emails would be bogged down.

The [[hashcash]] project may be found [http://www.hashcash.org here].