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DevDocs: Block Chain: Proof of Work: Many corrections

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Luke Dashjr 2014-10-01 19:56:52 +00:00
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_includes/guide_block_chain.md
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@ -64,10 +64,10 @@ satoshi transaction fee.
{% autocrossref %} {% autocrossref %}
The block chain is collaboratively maintained on a peer-to-peer network, so The block chain is collaboratively maintained by anonymous peers on the network, so
Bitcoin requires each block prove a significant amount of work was invested in Bitcoin requires that each block prove a significant amount of work was invested in
its creation to ensure that untrustworthy peers who want to modify past blocks have its creation to ensure that untrustworthy peers who want to modify past blocks have
to work harder than trustworthy peers who only want to add new blocks to the to work harder than honest peers who only want to add new blocks to the
block chain. block chain.
Chaining blocks together makes it impossible to modify transactions included Chaining blocks together makes it impossible to modify transactions included
@ -87,18 +87,17 @@ hash of the block header which does not exceed a certain value. For
example, if the maximum possible hash value is <span example, if the maximum possible hash value is <span
class="math">2<sup>256</sup> − 1</span>, you can prove that you class="math">2<sup>256</sup> − 1</span>, you can prove that you
tried up to two combinations by producing a hash value less than <span tried up to two combinations by producing a hash value less than <span
class="math">2<sup>256</sup> − 1</span>. class="math">2<sup>255</sup></span>.
In the example given above, you will almost certainly produce a In the example given above, you will produce a successful hash on average every other try.
successful hash on your first try. You can even estimate the probability You can even estimate the probability
that a given hash attempt will generate a number below the [target][]{:#term-target}{:.term} that a given hash attempt will generate a number below the [target][]{:#term-target}{:.term}
threshold. Bitcoin itself does not track probabilities but instead threshold.
simply assumes that the lower it makes the target threshold, the more Bitcoin assumes a linear probability that the lower it makes the target threshold, the more hash attempts (on average) will need to be tried.
hash attempts, on average, will need to be tried.
New blocks will only be added to the block chain if their hash is at New blocks will only be added to the block chain if their hash is at
least as challenging as a [difficulty][]{:#term-difficulty}{:.term} value expected by the peer-to-peer least as challenging as a [difficulty][]{:#term-difficulty}{:.term} value expected by the consensus protocol.
network. Every 2,016 blocks, the network uses timestamps stored in each Every 2,016 blocks, the network uses timestamps stored in each
block header to calculate the number of seconds elapsed between generation block header to calculate the number of seconds elapsed between generation
of the first and last of those last 2,016 blocks. The ideal value is of the first and last of those last 2,016 blocks. The ideal value is
1,209,600 seconds (two weeks). 1,209,600 seconds (two weeks).
@ -123,7 +122,7 @@ propagate a modified block as the entire Bitcoin network expended
between the time the original block was created and the present time. between the time the original block was created and the present time.
Only if you acquired a majority of the network's hashing power Only if you acquired a majority of the network's hashing power
could you reliably execute such a [51 percent attack][]{:#term-51-attack}{:.term} against could you reliably execute such a [51 percent attack][]{:#term-51-attack}{:.term} against
transaction history. transaction history (although, it should be noted, that even less than 50% of the hashing power still has a good chance of performing such attacks).
The block header provides several easy-to-modify fields, such as a The block header provides several easy-to-modify fields, such as a
dedicated nonce field, so obtaining new hashes doesn't require waiting dedicated nonce field, so obtaining new hashes doesn't require waiting