Hashcash: Proof-of-work system, Adam Back, Cryptographic hash function, Embedded system, Brute force attack, Mozilla Thunderbird, Spam in blogs - Tapa blanda

Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Hashcash is a proof-of-work system designed to limit email spam and denial of service attacks. It was proposed in March 1997 by Adam Back. 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. 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.