Proof-of-work system

proof-of-workproof of workreusable proof of workcryptographic puzzlesPoWproof of work systemProof-of-work algorithmsproofs of workreusable proof of work systemreusable 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.wikipedia
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Hashcash

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.
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.

Client Puzzle Protocol

client puzzlecomputation-based puzzle protocols
This idea is also known as a CPU cost function, client puzzle, computational puzzle or CPU pricing function.
It is an implementation of a proof-of-work system (POW).

Cryptocurrency

cryptocurrenciescrypto-currencyaltcoin
Proof-of-Stake, Proof of bandwidth, and other approaches have been discussed in the context of cryptocurrency.
Like bitcoin and other cryptocurrencies that would follow it, bit gold (not to be confused with the later gold-based exchange, BitGold) was described as an electronic currency system which required users to complete a proof of work function with solutions being cryptographically put together and published.

Proof-of-space

Proof of space (PoSpace) proposals apply the same principle by proving a dedicated amount of memory or disk space instead of CPU time.
Proofs of space are very similar to proofs of work, except that instead of computation, storage is used.

Cynthia Dwork

DworkC. Dwork
The concept was invented by Cynthia Dwork and Moni Naor as presented in a 1993 journal article.
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.

Proof-of-stake

proof of stakeDPOS (Delegated Proof of Stake Algorithm)generates
Proof-of-Stake, Proof of bandwidth, and other approaches have been discussed in the context of cryptocurrency.
In contrast, the algorithm of proof-of-work-based cryptocurrencies such as bitcoin uses mining; that is, the solving of computationally intensive puzzles to validate transactions and create new blocks.

Bitcoin

bitcoinsbitcoin walletBitcoin blockchain
Proof-of-work systems are being used as a primitive by other more complex cryptographic systems such as bitcoin which uses a system similar to Hashcash.
The receiver of the first bitcoin transaction was cypherpunk Hal Finney, who created the first reusable proof-of-work system (RPOW) in 2004.

CAPTCHA

capchacaptchasSimpleCaptcha
It is distinct from a CAPTCHA, which is intended for a human to solve quickly, rather than a computer.
Proof-of-work system

Guided tour puzzle protocol

Guided tour puzzle protocol
Guided tour puzzle protocol can be seen as a form of proof-of-work (POW) protocol.

Blockchain

Blockchainsblockchain technologyblock
The difficulty is periodically adjusted to keep the block time around a target time.
For example, bitcoin uses a proof-of-work system, where the chain with the most cumulative proof-of-work is considered the valid one by the network.

Hal Finney (computer scientist)

Hal Finney
Computer scientist Hal Finney built on the proof-of-work idea, yielding a system that exploited reusable proof of work ("RPOW").
In 2004, Finney created the first reusable proof of work system before bitcoin.

Mining pool

miners pool resourcesmining operationpool
Within the Bitcoin community there are groups working together in mining pools.
A "share" is awarded to members of the mining pool who present a valid partial proof-of-work.

Bitmessage

Bitmessage.ch
Bitmessage
Bitmessage uses a proof-of-work system to protect the network from flooding.

Nick Szabo

bit gold
* bit gold Bit gold.
In Szabo’s bit gold structure, a participant would dedicate computer power to solving cryptographic puzzles.

Denial-of-service attack

denial of servicedistributed denial of serviceDDoS
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.

Spamming

spamspammersspammer
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.

Moni Naor

M. NaorNaor
The concept was invented by Cynthia Dwork and Moni Naor as presented in a 1993 journal article.

Markus Jakobsson

The term "Proof of Work" or PoW was first coined and formalized in a 1999 paper by Markus Jakobsson and Ari Juels.

Shell money

cowriesnzimbushells
An early example of the proof-of-work system used to give value to a currency is the shell money of the Solomon Islands.

Solomon Islands

Solomons🇸🇧Solomon
An early example of the proof-of-work system used to give value to a currency is the shell money of the Solomon Islands.

Email

e-mailelectronic maile-mails
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.

Year 2038 problem

19 January 203832-bit computer clocks overflow2036 Problem
For instance the following header represents about 2 52 hash computations to send a message to on January 19, 2038:

SHA-1

sha1secure hashSHA
It is verified with a single computation by checking that the SHA-1 hash of the stamp (omit the header name including the colon and any amount of whitespace following it up to the digit '1') begins with 52 binary zeros, that is 13 hexadecimal zeros:

Uniform distribution (continuous)

uniform distributionuniformuniformly distributed
Known-solution protocols tend to have slightly lower variance than unbounded probabilistic protocols because the variance of a rectangular distribution is lower than the variance of a Poisson distribution (with the same mean).

Poisson distribution

PoissonPoisson-distributedPoissonian
Known-solution protocols tend to have slightly lower variance than unbounded probabilistic protocols because the variance of a rectangular distribution is lower than the variance of a Poisson distribution (with the same mean).