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@ -4,7 +4,48 @@
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[[mtp]]
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=== Median Time Past (MTP)
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FIXME
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((("scripting", "timelocks",
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"Median-Tme-Past")))((("Median-Tme-Past")))((("timelocks",
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"Median-Tme-Past")))As part of the activation of relative timelocks,
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there was also a change in the way "time" is calculated for timelocks
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(both absolute and relative). In bitcoin there is a subtle, but very
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significant, difference between wall time and consensus time. Bitcoin is
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a decentralized network, which means that each participant has his or
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her own perspective of time. Events on the network do not occur
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instantaneously everywhere. Network latency must be factored into the
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perspective of each node. Eventually everything is synchronized to
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create a common ledger. Bitcoin reaches consensus every 10 minutes about
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the state of the ledger as it existed in the _past_.
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The timestamps set in block headers are set by the miners. There is a
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certain degree of latitude allowed by the consensus rules to account for
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differences in clock accuracy between decentralized nodes. However, this
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creates an unfortunate incentive for miners to lie about the time in a
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block so as to earn extra fees by including timelocked transactions that
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are not yet mature. See the following section for more information.
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To remove the incentive to lie and strengthen the security of timelocks,
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a BIP was proposed and activated at the same time as the BIPs for
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relative timelocks. This is BIP-113, which defines a new consensus
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measurement of time called _Median-Time-Past_.
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Median-Time-Past is calculated by taking the timestamps of the last 11
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blocks and finding the median. That median time then becomes consensus
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time and is used for all timelock calculations. By taking the midpoint
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from approximately two hours in the past, the influence of any one
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block's timestamp is reduced. By incorporating 11 blocks, no single
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miner can influence the timestamps in order to gain fees from
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transactions with a timelock that hasn't yet matured.
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Median-Time-Past changes the implementation of time calculations for
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+nLocktime+, +CLTV+, +nSequence+, and +CSV+. The consensus time
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calculated by Median-Time-Past is always approximately one hour behind
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wall clock time. If you create timelock transactions, you should account
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for it when estimating the desired value to encode in +nLocktime+,
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+nSequence+, +CLTV+, and +CSV+.
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Median-Time-Past is specified in
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https://github.com/bitcoin/bips/blob/master/bip-0113.mediawiki[BIP-113].
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[[duplicate_transactions]]
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=== Preventing Duplicate Transactions
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David A. Harding
1 year ago