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.wikipedia
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Adam Back

Back, Adam
Hashcash was proposed in 1997 by Adam Back and described more formally in Back's paper "Hashcash - A Denial of Service Counter-Measure"
He is the inventor of hashcash, the proof-of-work system used by several anti-spam systems.

Bitcoin

bitcoinsbitcoin walletBitcoin blockchain
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.
The system used is based on Adam Back's 1997 anti-spam scheme, Hashcash.

Cynthia Dwork

DworkC. Dwork
A similar idea was first proposed by Cynthia Dwork and Moni Naor and Eli Ponyatovski in their 1992 paper "Pricing via Processing or Combatting Junk Mail", but using weakened signature schemes rather than the simpler and more efficient hash-based approach introduced in hashcash.
With Naor she also first presented the idea of, and a technique for, combating e-mail spam by requiring a proof of computational effort, also known as proof-of-work - a key technology underlying hashcash and bitcoin.

Cryptographic hash function

cryptographic hashhashhashing
It then computes the 160-bit SHA-1 hash of the header.
One popular system – used in Bitcoin mining and Hashcash – uses partial hash inversions to prove that work was done, to unlock a mining reward in Bitcoin and as a good-will token to send an e-mail in Hashcash.

Blockchain

Blockchainsblockchain technologyblock
Blocks accepted from miners form the bitcoin blockchain that is a growing ledger of every bitcoin transaction since the coin's first creation.
Nakamoto improved the design in an important way using a Hashcash-like method to add blocks to the chain without requiring them to be signed by a trusted party.

Proof-of-work system

proof-of-workproof of workreusable proof of work
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.
One popular system, used in Hashcash, uses partial hash inversions to prove that work was done, as a good-will token to send an e-mail.

Apache SpamAssassin

SpamAssassinmessage scoring systemspam protection
SpamAssassin has been able to check for Hashcash stamps since version 2.70, assigning a negative score (i.e. less likely to be spam) for valid, unspent Hashcash stamps.
Hashcash email stamps based on proof-of-work

SHA-2

SHA-256sha256Secure Hash Algorithm 2
Although most cryptocurrencies use the SHA-256 hash function, the same ASIC technology could be used to create hashcash solvers that are three orders of magnitude faster than a consumer CPU, reducing the computational hurdle for spammers.
Hashcash

Email spam

spame-mail spamspam email
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.

Denial-of-service attack

denial of servicedistributed denial of serviceDDoS
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.

Cryptocurrency

cryptocurrenciescrypto-currencyaltcoin
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.

Moni Naor

M. NaorNaor
A similar idea was first proposed by Cynthia Dwork and Moni Naor and Eli Ponyatovski in their 1992 paper "Pricing via Processing or Combatting Junk Mail", but using weakened signature schemes rather than the simpler and more efficient hash-based approach introduced in hashcash.

Header (computing)

headerheaderspacket header
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.

Brute-force attack

brute force attackbrute forcebrute-force
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, satisfactory answers are rare enough it will require a substantial number of tries to find the answer.

Base64

Radix-64base-64Base64 encoded
rand: String of random characters, encoded in base-64 format.

SHA-1

sha1secure hashSHA
It then computes the 160-bit SHA-1 hash of the header.

Geometric distribution

geometricgeometrically distributedgeometric random variable
The number of times that the sender needs to try to get a valid hash value is modeled by geometric distribution.

Exponential growth

exponentiallyexponentialgrow exponentially
The time needed to compute such a hash collision is exponential with the number of zero bits.

Micropayment

micropaymentsmicrotransactionsmicro-transactions
The Hashcash system has the advantage over micropayment proposals applying to legitimate e-mail that no real money is involved.

Embedded system

embedded systemsembeddedembedded device
This can mean sacrificing accessibility from low-end embedded systems or else running the risk of hostile hosts not being challenged enough to provide an effective filter from spam.

Mailing list

mailing listsmailinglistlistwashing
Examples of each include, respectively, a centralized e-mail topology (like a mailing list), in which some server is to send an enormous amount of legitimate e-mails, and botnets or cluster farms with which spammers can increase their processing power enormously.

Botnet

botnetscommand and controlcommand and control server
Examples of each include, respectively, a centralized e-mail topology (like a mailing list), in which some server is to send an enormous amount of legitimate e-mails, and botnets or cluster farms with which spammers can increase their processing power enormously.

Moore's law

computational powerincreasing performanceMoore’s Law
Another projected problem is that computers continue to get faster according to Moore's law.

Application-specific integrated circuit

ASICASICsapplication specific integrated circuit
The rise of cryptocurrency has created a demand for ASIC-based mining machines.