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114 lines
4.3 KiB
114 lines
4.3 KiB
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BIP: 30
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Title: Duplicate transactions
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Author: Pieter Wuille <pieter.wuille@gmail.com>
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Status: Final
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Type: Standards Track
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Created: 2012-02-22
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-------------------------------------------------
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[[abstract]]
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Abstract
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~~~~~~~~
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This document gives a specification for dealing with duplicate
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transactions in the block chain, in an attempt to solve certain problems
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the reference implementations has with them.
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[[motivation]]
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Motivation
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~~~~~~~~~~
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So far, the Bitcoin reference implementation always assumed duplicate
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transactions (transactions with the same identifier) didn't exist. This
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is not true; in particular coinbases are easy to duplicate, and by
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building on duplicate coinbases, duplicate normal transactions are
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possible as well. Recently, an attack that exploits the reference
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implementation's dealing with duplicate transactions was described and
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demonstrated. It allows reverting fully-confirmed transactions to a
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single confirmation, making them vulnerable to become unspendable
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entirely. Another attack is possible that allows forking the block chain
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for a subset of the network.
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[[specification]]
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Specification
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~~~~~~~~~~~~~
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To counter this problem, the following network rule is introduced:
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* Blocks are not allowed to contain a transaction whose identifier
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matches that of an earlier, not-fully-spent transaction in the same
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chain.
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This rule initially applied to all blocks whose timestamp is after March
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15, 2012, 00:00 UTC (testnet: February 20, 2012 00:00 UTC). It was later
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extended by Commit
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https://github.com/bitcoin/bitcoin/commit/ab91bf39b7c11e9c86bb2043c24f0f377f1cf514[Apply
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BIP30 checks to all blocks except the two historic violations.] to apply
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to all blocks except the two historic blocks at heights 91842 and 91880
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on the main chain that had to be grandfathered in.
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[[rationale]]
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Rationale
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~~~~~~~~~
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Whatever solution is used, the following law must be obeyed to guarantee
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sane behaviour: the set of usable transactions outputs must not be
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modified by adding blocks to the chain and removing them again. This
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happens during a reorganisation, and the current Bitcoin reference
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implementation does not obey this law in case the temporarily added
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blocks contain a duplicate transaction.
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There are several potential solutions to this problem:
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1. Guarantee that all coinbases are unique, making duplicate
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transactions very hard to create.
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2. Remember previous remaining outputs of a given transaction
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identifier, in case a new transaction with the same identifier is added.
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3. Only allow duplicate transactions in case the previous instance of
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the transaction had no spendable outputs left. Removing a block from the
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chain can then safely reset the removed transaction's outputs to
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nothing.
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The first option is probably the most complete one, as it also
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guarantees transaction identifiers are unique. However, implementing it
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requires several changes that need to be accepted throughout the
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network. Furthermore, it does not prevent duplicate transactions based
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on earlier duplicate coinbases. The second option is impossible to
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implement in a forward-compatible way, as it potentially renders
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currently-invalid blocks valid. In this document we choose for the third
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option, because it only requires a trivial change.
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Fully-spent transactions are allowed to be duplicated in order not to
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hinder pruning at some point in the future. Not allowing any transaction
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to be duplicated would require evidence to be kept for each transaction
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ever made.
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[[backward-compatibility]]
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Backward compatibility
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~~~~~~~~~~~~~~~~~~~~~~
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The addition of this rule only makes some previously-valid blocks
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invalid. This implies that if the rule is implemented by a supermajority
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of miners, it is not possible to fork the block chain in a permanent way
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between nodes with and without the new rule.
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[[implementation]]
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Implementation
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~~~~~~~~~~~~~~
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A patch for the reference client can be found on
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https://github.com/sipa/bitcoin/tree/nooverwritetx
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This BIP was implemented in Commit
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https://github.com/bitcoin/bitcoin/commit/a206b0ea12eb4606b93323268fc81a4f1f952531[Do
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not allow overwriting unspent transactions (BIP 30)] There have been
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additional commits to refine the implementation of this BIP.
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[[acknowledgements]]
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Acknowledgements
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~~~~~~~~~~~~~~~~
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Thanks to Russell O'Connor for finding and demonstrating this problem,
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and helping test the patch.
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