Difference between revisions of "Proof of work"

From Bitcoin Wiki
Jump to: navigation, search
(List of algorithms: Remove altcoin promotion)
 
(16 intermediate revisions by 9 users not shown)
Line 1: Line 1:
A '''proof of work''' is a piece of data which was difficult (costly, time-consuming) to produce so as to satisfy certain requirements. It must be trivial to check whether data satisfies said requirements. Producing a proof of work can be a random process with low probability, so that a lot of trial and error is required ''on average'' before a valid proof of work is generated.  Bitcoin uses the [[Hashcash]] proof of work.
+
A '''proof of work''' is a piece of data which is difficult (costly, time-consuming) to produce but easy for others to verify and which satisfies certain requirements. Producing a proof of work can be a random process with low probability so that a lot of trial and error is required ''on average'' before a valid proof of work is generated.  Bitcoin uses the [[Hashcash]] proof of work system.
  
One application of this idea is using [http://en.wikipedia.org/wiki/Hashcash hashcash] as a method to preventing email spam, requiring a proof of work on the email's contents (including the To address), on every email. Legitimate emails will be able to do the work to generate the proof easily (not much work is required for a single email), but mass spam emailers will have difficulty generating the required proofs (which would require huge computational resources).
+
One application of this idea is using Hashcash as a method to preventing email spam, requiring a proof of work on the email's contents (including the To address), on every email. Legitimate emails will be able to do the work to generate the proof easily (not much work is required for a single email), but mass spam emailers will have difficulty generating the required proofs (which would require huge computational resources).
  
Hashcash proofs of work are used in Bitcoin for block generation. Proofs of work that are tied to the data of each block are required for the blocks to be accepted. The [[difficulty]] of this work is adjusted so as to limit the rate at which new blocks can be generated by the network to one every 10 minutes. Due to the very low probability of successful generation, this makes it unpredictable which worker computer in the network will be able to generate the next block.
+
Hashcash proofs of work are used in Bitcoin for block generation. In order for a block to be accepted by network participants, [[Mining|miners]] must complete a proof of work which covers all of the data in the block. The [[difficulty]] of this work is adjusted so as to limit the rate at which new blocks can be generated by the network to one every 10 minutes. Due to the very low probability of successful generation, this makes it unpredictable which worker computer in the network will be able to generate the next block.
  
 
For a block to be valid it must hash to a value less than the current [[target]]; this means that each block indicates that work has been done generating it. Each block contains the hash of the preceding block, thus each block has a [[block chain|chain]] of blocks that together contain a large amount of work. Changing a block (which can only be done by making a new block containing the same predecessor) requires regenerating all successors and redoing the work they contain. This protects the block chain from tampering.
 
For a block to be valid it must hash to a value less than the current [[target]]; this means that each block indicates that work has been done generating it. Each block contains the hash of the preceding block, thus each block has a [[block chain|chain]] of blocks that together contain a large amount of work. Changing a block (which can only be done by making a new block containing the same predecessor) requires regenerating all successors and redoing the work they contain. This protects the block chain from tampering.
 +
 +
The most widely used proof-of-work scheme is based on [[SHA-256]] and was introduced as a part of [[Bitcoin]]. Some other hashing algorithms that are used for proof-of-work include [https://en.wikipedia.org/wiki/Scrypt Scrypt], [https://en.wikipedia.org/wiki/BLAKE_(hash_function) Blake-256], [[CryptoNight]], [https://heavycoin.github.io/ HEFTY1], [https://131002.net/quark/quark_full.pdf Quark], [https://en.wikipedia.org/wiki/SHA-3 SHA-3], [https://github.com/floodyberry/scrypt-jane scrypt-jane], scrypt-n, and combinations thereof.
  
 
== Example ==
 
== Example ==
  
Let's say the base string that we are going to do work on is "Hello, world!". Our target is to find a variation of it that SHA-256 hashes to a value beginning with '000'. We vary the string by adding an integer value to the end called a [[nonce]] and incrementing it each time. Finding a match for "Hello, world!" takes us 4251 tries (but happens to have zeroes in the first four digits):
+
Let's say the base string that we are going to do work on is "Hello, world!". Our target is to find a variation of it that SHA-256 hashes to a value smaller than 2^240. We vary the string by adding an integer value to the end called a [[nonce]] and incrementing it each time, then interpreting the hash result as a long integer and checking whether it's smaller than the target 2^240. Finding a match for "Hello, world!" takes us 4251 tries.
  
  "Hello, world!0" => 1312af178c253f84028d480a6adc1e25e81caa44c749ec81976192e2ec934c64
+
  "Hello, world!0" => 1312af178c253f84028d480a6adc1e25e81caa44c749ec81976192e2ec934c64 = 2^252.253458683
  "Hello, world!1" => e9afc424b79e4f6ab42d99c81156d3a17228d6e1eef4139be78e948a9332a7d8
+
  "Hello, world!1" => e9afc424b79e4f6ab42d99c81156d3a17228d6e1eef4139be78e948a9332a7d8 = 2^255.868431117
  "Hello, world!2" => ae37343a357a8297591625e7134cbea22f5928be8ca2a32aa475cf05fd4266b7
+
  "Hello, world!2" => ae37343a357a8297591625e7134cbea22f5928be8ca2a32aa475cf05fd4266b7 = 2^255.444730341
 
  ...
 
  ...
  "Hello, world!4248" => 6e110d98b388e77e9c6f042ac6b497cec46660deef75a55ebc7cfdf65cc0b965
+
  "Hello, world!4248" => 6e110d98b388e77e9c6f042ac6b497cec46660deef75a55ebc7cfdf65cc0b965 = 2^254.782233115
  "Hello, world!4249" => c004190b822f1669cac8dc37e761cb73652e7832fb814565702245cf26ebb9e6
+
  "Hello, world!4249" => c004190b822f1669cac8dc37e761cb73652e7832fb814565702245cf26ebb9e6 = 2^255.585082774
  "Hello, world!4250" => 0000c3af42fc31103f1fdc0151fa747ff87349a4714df7cc52ea464e12dcd4e9
+
  "Hello, world!4250" => 0000c3af42fc31103f1fdc0151fa747ff87349a4714df7cc52ea464e12dcd4e9 = 2^239.61238653
  
4251 hashes on a modern computer is not very much work (most computers can achieve at least 4 million hashes per second). Bitcoin automatically varies the [[difficulty]] (and thus the amount of work required to generate a block) to keep a roughly constant rate of block generation. The probability of a single hash succeeding can be found [http://blockexplorer.com/q/probability here].
+
4251 hashes on a modern computer is not very much work (most computers can achieve at least 4 million hashes per second). Bitcoin automatically varies the [[target]] (and thus the amount of work required to generate a block) to keep a roughly constant rate of block generation.  
  
In Bitcoin things are a bit more complex, especially since the header contains the [http://en.wikipedia.org/wiki/Merkle_tree Merkle tree] which depends on the included [[transactions]]. This includes the generation transaction, a transaction "out of nowhere" to our own address, which in addition to providing the miner with incentive to do the work, also ensures that every miner hashes a unique data set.
+
In Bitcoin the hash value is also used as a reference to the block itself, so somebody might say that their [[transaction]] has been mined into [[block]] with hash <code>0000c3af42fc31103f1fdc0151fa747ff87349a4714df7cc52ea464e12dcd4e9</code>. The [[header]] of a [[block]] contains the [[Protocol_documentation#Merkle_Trees|Merkle tree]] which depends on the included [[transactions]]. This includes the generation transaction, a transaction "out of nowhere" to our own address, which in addition to providing the miner with incentive to do the work, also ensures that every miner hashes a unique data set.
  
 
== List of algorithms ==
 
== List of algorithms ==
Line 29: Line 31:
 
# hashcash with [[scrypt]] internal hash
 
# hashcash with [[scrypt]] internal hash
 
# [[Momentum]] birthday collision
 
# [[Momentum]] birthday collision
# Cuckoo cycle proof of work https://github.com/tromp/cuckoo  
+
# Cuckoo Cycle proof of work https://github.com/tromp/cuckoo  
# Various other proof of works functions (e.g. [[Ethereum]] had a few candidates)
+
# Various other proof of works functions
  
 
=== Proof of X ===
 
=== Proof of X ===
# [[Proof of Stake]] (Used in [[Peercoin]], [[Nxt]])
+
# [[Proof of Stake]]
# [[Proof of Burn]] (Used for the [[Counterparty]] IPO)
+
# [[Proof of Burn]]
 +
 
 +
=== Consensus ===
 +
# [https://www.stellar.org/papers/stellar-consensus-protocol.pdf Stellar Consensus Protocol]
  
 
[[Category:Vocabulary]]
 
[[Category:Vocabulary]]
 
[[Category:Proof-of-x]]
 
[[Category:Proof-of-x]]
 +
[[Category:Technical]]
  
 
[[fr:Preuve de travail]]
 
[[fr:Preuve de travail]]
 +
 +
==References==
 +
[[Distribution of nonces and hashes]]
 +
<references/>

Latest revision as of 00:41, 24 April 2019

A proof of work is a piece of data which is difficult (costly, time-consuming) to produce but easy for others to verify and which satisfies certain requirements. Producing a proof of work can be a random process with low probability so that a lot of trial and error is required on average before a valid proof of work is generated. Bitcoin uses the Hashcash proof of work system.

One application of this idea is using Hashcash as a method to preventing email spam, requiring a proof of work on the email's contents (including the To address), on every email. Legitimate emails will be able to do the work to generate the proof easily (not much work is required for a single email), but mass spam emailers will have difficulty generating the required proofs (which would require huge computational resources).

Hashcash proofs of work are used in Bitcoin for block generation. In order for a block to be accepted by network participants, miners must complete a proof of work which covers all of the data in the block. The difficulty of this work is adjusted so as to limit the rate at which new blocks can be generated by the network to one every 10 minutes. Due to the very low probability of successful generation, this makes it unpredictable which worker computer in the network will be able to generate the next block.

For a block to be valid it must hash to a value less than the current target; this means that each block indicates that work has been done generating it. Each block contains the hash of the preceding block, thus each block has a chain of blocks that together contain a large amount of work. Changing a block (which can only be done by making a new block containing the same predecessor) requires regenerating all successors and redoing the work they contain. This protects the block chain from tampering.

The most widely used proof-of-work scheme is based on SHA-256 and was introduced as a part of Bitcoin. Some other hashing algorithms that are used for proof-of-work include Scrypt, Blake-256, CryptoNight, HEFTY1, Quark, SHA-3, scrypt-jane, scrypt-n, and combinations thereof.

Example

Let's say the base string that we are going to do work on is "Hello, world!". Our target is to find a variation of it that SHA-256 hashes to a value smaller than 2^240. We vary the string by adding an integer value to the end called a nonce and incrementing it each time, then interpreting the hash result as a long integer and checking whether it's smaller than the target 2^240. Finding a match for "Hello, world!" takes us 4251 tries.

"Hello, world!0" => 1312af178c253f84028d480a6adc1e25e81caa44c749ec81976192e2ec934c64 = 2^252.253458683
"Hello, world!1" => e9afc424b79e4f6ab42d99c81156d3a17228d6e1eef4139be78e948a9332a7d8 = 2^255.868431117
"Hello, world!2" => ae37343a357a8297591625e7134cbea22f5928be8ca2a32aa475cf05fd4266b7 = 2^255.444730341
...
"Hello, world!4248" => 6e110d98b388e77e9c6f042ac6b497cec46660deef75a55ebc7cfdf65cc0b965 = 2^254.782233115
"Hello, world!4249" => c004190b822f1669cac8dc37e761cb73652e7832fb814565702245cf26ebb9e6 = 2^255.585082774
"Hello, world!4250" => 0000c3af42fc31103f1fdc0151fa747ff87349a4714df7cc52ea464e12dcd4e9 = 2^239.61238653

4251 hashes on a modern computer is not very much work (most computers can achieve at least 4 million hashes per second). Bitcoin automatically varies the target (and thus the amount of work required to generate a block) to keep a roughly constant rate of block generation.

In Bitcoin the hash value is also used as a reference to the block itself, so somebody might say that their transaction has been mined into block with hash 0000c3af42fc31103f1fdc0151fa747ff87349a4714df7cc52ea464e12dcd4e9. The header of a block contains the Merkle tree which depends on the included transactions. This includes the generation transaction, a transaction "out of nowhere" to our own address, which in addition to providing the miner with incentive to do the work, also ensures that every miner hashes a unique data set.

List of algorithms

Traditional proof of work

  1. hashcash with double iterated SHA256
  2. hashcash with scrypt internal hash
  3. Momentum birthday collision
  4. Cuckoo Cycle proof of work https://github.com/tromp/cuckoo
  5. Various other proof of works functions

Proof of X

  1. Proof of Stake
  2. Proof of Burn

Consensus

  1. Stellar Consensus Protocol

References

Distribution of nonces and hashes