Proof-of-work

• Proof-of-work

A proof-of-work (POW) system (or protocol, or function) is an economic measure to deter denial of service attacks and other service abuses such as spam on a network by requiring some work from the service requester, usually meaning processing time by a computer

A key feature of these schemes is their asymmetry: the work must be moderately hard (but feasible) on the requester side but easy to check for the service provider. This idea is also known as a CPU cost function, client puzzle, computational puzzle or CPU pricing function. It is distinct from a CAPTCHA, which is intended for a human to solve quickly, rather than a computer

http://en.wikipedia.org/wiki/Proof-of-work

• Hashcash

Hashcash is a proof-of-work system designed to limit email spam and denial-of-service attacks.
Hashcash is a method of adding a textual stamp 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

The theory is that spammers, whose business model relies on their ability to send large numbers of emails with very little cost per message, cannot afford this investment into each individual piece of spam they send. Receivers can verify whether a sender made such an investment and use the results to help filter email.

http://en.wikipedia.org/wiki/Hashcash