Hashcash

Hashcash is a proof-of-work algorithm, which has been used as adenial-of-service counter measure technique in a number of systems.

A hashcash stamp constitutes a proof-of-work which takes aparameterizable amount of work to compute for the sender. The recipient(and indeed anyone as it is publicly auditable) can verify received hashcashstamps efficiently. Hashcash was invented byAdam Back in1997

At this point it is most widely used as thebitcoin mining function.

The emailanti-spam tool, like the proof-of-work algorithm, is also called hashcashand is used to create stamps to attach to mail to add a micro-cost tosending mail to deter spamming. The main use of the hashcash stamp is as awhite-listing hint to help hashcash users avoid losing email due to contentbased and blacklist based anti-spam systems.

Hashcash source code includes a library form, and also the algorithm isextremely simple to code from scratch with the availability of a hashlibrary. Verification can be done by a human eye (count leading 0s) evenwith availability of common preinstalled command line tools such as sha1sum. The algorithm works with a cryptographic hash, such as SHA1, SHA256 orcoming SHA3 that exhibits 2nd-preimage resistance. Note that 2nd-preimageresistance is a stronger hash property than the collision resistanceproperty.

• HashcashFAQ.
• Hashcashformat info
• Hashcash Paper Aug 02 - "Hashcash - A Denial of Service Counter-Measure" (5 years on), Tech Report,Adam Back
• The site is slowly moving over to this new domain, some of the content is still here: http://www.cypherspace.org/hashcash/