mirror of
https://github.com/bitcoinbook/bitcoinbook
synced 2024-12-27 00:48:09 +00:00
114 lines
4.3 KiB
Plaintext
114 lines
4.3 KiB
Plaintext
-------------------------------------------------
|
|
BIP: 30
|
|
Title: Duplicate transactions
|
|
Author: Pieter Wuille <pieter.wuille@gmail.com>
|
|
Status: Final
|
|
Type: Standards Track
|
|
Created: 2012-02-22
|
|
-------------------------------------------------
|
|
|
|
[[abstract]]
|
|
Abstract
|
|
~~~~~~~~
|
|
|
|
This document gives a specification for dealing with duplicate
|
|
transactions in the block chain, in an attempt to solve certain problems
|
|
the reference implementations has with them.
|
|
|
|
[[motivation]]
|
|
Motivation
|
|
~~~~~~~~~~
|
|
|
|
So far, the Bitcoin reference implementation always assumed duplicate
|
|
transactions (transactions with the same identifier) didn't exist. This
|
|
is not true; in particular coinbases are easy to duplicate, and by
|
|
building on duplicate coinbases, duplicate normal transactions are
|
|
possible as well. Recently, an attack that exploits the reference
|
|
implementation's dealing with duplicate transactions was described and
|
|
demonstrated. It allows reverting fully-confirmed transactions to a
|
|
single confirmation, making them vulnerable to become unspendable
|
|
entirely. Another attack is possible that allows forking the block chain
|
|
for a subset of the network.
|
|
|
|
[[specification]]
|
|
Specification
|
|
~~~~~~~~~~~~~
|
|
|
|
To counter this problem, the following network rule is introduced:
|
|
|
|
* Blocks are not allowed to contain a transaction whose identifier
|
|
matches that of an earlier, not-fully-spent transaction in the same
|
|
chain.
|
|
|
|
This rule initially applied to all blocks whose timestamp is after March
|
|
15, 2012, 00:00 UTC (testnet: February 20, 2012 00:00 UTC). It was later
|
|
extended by Commit
|
|
https://github.com/bitcoin/bitcoin/commit/ab91bf39b7c11e9c86bb2043c24f0f377f1cf514[Apply
|
|
BIP30 checks to all blocks except the two historic violations.] to apply
|
|
to all blocks except the two historic blocks at heights 91842 and 91880
|
|
on the main chain that had to be grandfathered in.
|
|
|
|
[[rationale]]
|
|
Rationale
|
|
~~~~~~~~~
|
|
|
|
Whatever solution is used, the following law must be obeyed to guarantee
|
|
sane behaviour: the set of usable transactions outputs must not be
|
|
modified by adding blocks to the chain and removing them again. This
|
|
happens during a reorganisation, and the current Bitcoin reference
|
|
implementation does not obey this law in case the temporarily added
|
|
blocks contain a duplicate transaction.
|
|
|
|
There are several potential solutions to this problem:
|
|
|
|
1. Guarantee that all coinbases are unique, making duplicate
|
|
transactions very hard to create.
|
|
2. Remember previous remaining outputs of a given transaction
|
|
identifier, in case a new transaction with the same identifier is added.
|
|
3. Only allow duplicate transactions in case the previous instance of
|
|
the transaction had no spendable outputs left. Removing a block from the
|
|
chain can then safely reset the removed transaction's outputs to
|
|
nothing.
|
|
|
|
The first option is probably the most complete one, as it also
|
|
guarantees transaction identifiers are unique. However, implementing it
|
|
requires several changes that need to be accepted throughout the
|
|
network. Furthermore, it does not prevent duplicate transactions based
|
|
on earlier duplicate coinbases. The second option is impossible to
|
|
implement in a forward-compatible way, as it potentially renders
|
|
currently-invalid blocks valid. In this document we choose for the third
|
|
option, because it only requires a trivial change.
|
|
|
|
Fully-spent transactions are allowed to be duplicated in order not to
|
|
hinder pruning at some point in the future. Not allowing any transaction
|
|
to be duplicated would require evidence to be kept for each transaction
|
|
ever made.
|
|
|
|
[[backward-compatibility]]
|
|
Backward compatibility
|
|
~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The addition of this rule only makes some previously-valid blocks
|
|
invalid. This implies that if the rule is implemented by a supermajority
|
|
of miners, it is not possible to fork the block chain in a permanent way
|
|
between nodes with and without the new rule.
|
|
|
|
[[implementation]]
|
|
Implementation
|
|
~~~~~~~~~~~~~~
|
|
|
|
A patch for the reference client can be found on
|
|
https://github.com/sipa/bitcoin/tree/nooverwritetx
|
|
|
|
This BIP was implemented in Commit
|
|
https://github.com/bitcoin/bitcoin/commit/a206b0ea12eb4606b93323268fc81a4f1f952531[Do
|
|
not allow overwriting unspent transactions (BIP 30)] There have been
|
|
additional commits to refine the implementation of this BIP.
|
|
|
|
[[acknowledgements]]
|
|
Acknowledgements
|
|
~~~~~~~~~~~~~~~~
|
|
|
|
Thanks to Russell O'Connor for finding and demonstrating this problem,
|
|
and helping test the patch.
|