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107 lines
4.9 KiB
Plaintext
<pre>
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BIP: 131
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Layer: Consensus (hard fork)
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Title: "Coalescing Transaction" Specification (wildcard inputs)
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Author: Chris Priest <cp368202@ohiou.edu>
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Comments-Summary: No comments yet.
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Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0131
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Status: Rejected
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Type: Standards Track
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Created: 2015-11-30
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License: PD
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</pre>
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==Abstract==
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This specification defines a new type of transaction that supplements (not replaces)
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normal "non coalescing" bitcoin transactions.
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==Motivation==
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Normal "non-coalescing" Bitcoin Transactions have one large inefficiency: When you want to spend
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from multiple inputs with the exact same scriptPubKey, you have to list each
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input separately, along with the same signature multiple times, even though the signature expresses the same information.
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This bloats the transaction size and makes it expensive to spend from small value inputs.
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Because small value inputs are expensive to send, they remain in the UTXO pool
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which full nodes have to keep around. It is believed that long term increase of the UTXO
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set can have negative scaling consequences on the network.
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If maximum blocksize is made to increase *slower* than the actual number of transactions bitcoin users are sending
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to the network, this problem is projected to get worse. In other words, as transaction
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fees increase, the minimum economical value of a spending a UTXO will increase.
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==Specification==
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=== Redefinition of Transaction version ===
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First, this BIP redefines the version field on transactions. The first four bytes
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are defined as the version number, while the last four bytes are defined as the
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transaction type. Type "0000" denotes normal transactions, and "0001" defines
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coalescing transaction.
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Examples:
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version 1 transaction with normal inputs:
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version: 10000000
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version 2 transaction with normal inputs:
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version: 20000000
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version 2 transaction with coalescing inputs:
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version: 20000001
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Essentially the last bit in the version field is set to 1 to enable wildcard inputs for all
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inputs present in the transaction.
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=== Wildcard inputs ===
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A coalescing transaction is formulated the exact same way as a version 1 transaction
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with one exception: each input is treated as a "wildcard input".
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A wildcard input beings the value of all inputs with the exact same scriptPubKey
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in a block lower or equal to the block the wildcard input is confirmed into.
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== Changes needed to implement ==
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The bitcoin code needs to be modified in three places in order to handle Coalescing Transactions.
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1. <b>Full Node Coalescing validation</b> - When a full node receives a coalescing transaction, it has to
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aggregate the value of all the UTXOs in the blockchain older than the input
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with the same scriptPubKey. If this value is greater than or equal to the
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amount of all outputs, then that coalescing transaction is valid and can be propagated.
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2. <b>Full Node Non-Coalescing validation</b> - When a non-coalescing transaction comes in, the code needs to be modified
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to check if each input has not been spent by a coalescing transaction. If there exist any
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coalescing transaction in the blockchain with the same scriptPubKey found in a block *after* that input,
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then the UTXO has been spent and the transaction is invalid.
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3. <b>Wallet</b> - The user facing wallet portion of the reference client should notify
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the user when their wallet contains many UTXOs that qualify it to benefit from
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a coalescing transaction. Wallets should not simply replace non-coalescing transactions
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with coalescing transactions in all instances.
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== Isn't this BIP bad because it encourage address re-use? ==
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Address re-use comes in two forms: re-use by the ''sender'', and re-use by the ''receiver''.
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Re-use by the sender is basically using the same address for the change output. This is generally considered bad
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since people looking through your transaction history can determine who you do business with. When
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you generate a new address for every change, your privacy is conserved as it is impossible to know which
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output is a recipient, and which output is the change output. This BIP has '''no effect''' on re-use
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by the sender.
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On the other hand, address re-use by the ''receiver'' occurs under completely different circumstances.
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When you publish an address and have multiple people send to that address, you are engaging in address re-use
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from the receiver. This activity has historically been considered bad because it leads to re-using a private key.
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When you re-use a private key too many times, you run the risk of an attacker performing statistical analysis
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on the multiple signatures, which can lead to an attacker finding out your private key.
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This BIP introduces a way to spend multiple inputs ''without'' re-using the private key. In a sense, this BIP
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fixes the problem that makes address re-use bad for the receiver. After this BIP becomes implemented
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and deployed, address re-use by the receiver will no longer be considered bad form.
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==Copyright==
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This document is placed in the public domain.
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