[Bitcoin-development] Draft BIP for geutxos message

Mike Hearn mike at plan99.net
Thu Jul 10 14:29:12 UTC 2014


I opened up a pull req for a draft BIP for getutxo.

   https://github.com/bitcoin/bips/pull/88

I include a rendering below for your reading convenience. If you'd like to
comment on design/security/etc then please first familiarise yourself with
the long discussions that were already had here:

   https://github.com/bitcoin/bitcoin/pull/4351


  BIP: 45
  Title: getutxo message
  Author: Mike Hearn <hearn at vinumeris.com>
  Status: Draft
  Type: Standards Track
  Created: 2014-06-10

Table of Contents

   - Abstract
      <https://github.com/mikehearn/bips/commit/6058b92f5d9804ee4104649f53afc2fa53248c81?short_path=35c7795#Abstract>
      - Motivation
      <https://github.com/mikehearn/bips/commit/6058b92f5d9804ee4104649f53afc2fa53248c81?short_path=35c7795#Motivation>
      - Specification
      <https://github.com/mikehearn/bips/commit/6058b92f5d9804ee4104649f53afc2fa53248c81?short_path=35c7795#Specification>
      - Backward compatibility
      <https://github.com/mikehearn/bips/commit/6058b92f5d9804ee4104649f53afc2fa53248c81?short_path=35c7795#Backward_compatibility>
      - Authentication
      <https://github.com/mikehearn/bips/commit/6058b92f5d9804ee4104649f53afc2fa53248c81?short_path=35c7795#Authentication>
      - Implementation
      <https://github.com/mikehearn/bips/commit/6058b92f5d9804ee4104649f53afc2fa53248c81?short_path=35c7795#Implementation>

<https://github.com/mikehearn/bips/commit/6058b92f5d9804ee4104649f53afc2fa53248c81?short_path=35c7795#abstract>
Abstract

This document describes a small P2P protocol extension that performs UTXO
lookups given a set of outpoints.
<https://github.com/mikehearn/bips/commit/6058b92f5d9804ee4104649f53afc2fa53248c81?short_path=35c7795#motivation>
Motivation

All full Bitcoin nodes maintain a database called the unspent transaction
output set. This set is how double spending is checked for: to be valid a
transaction must identify unspent outputs in this set using an identifier
called an "outpoint", which is merely the hash of the output's containing
transaction plus an index.

The ability to query this can sometimes be useful for a lightweight/SPV
client which does not have the full UTXO set at hand. For example, it can
be useful in applications implementing assurance contracts to do a quick
check when a new pledge becomes visible to test whether that pledge was
already revoked via a double spend. Although this message is not strictly
necessary because e.g. such an app could be implemented by fully
downloading and storing the block chain, it is useful for obtaining
acceptable performance and resolving various UI cases.

Another example of when this data can be useful is for performing floating
fee calculations in an SPV wallet. This use case requires some other
changes to the Bitcoin protocol however, so we will not dwell on it here.
<https://github.com/mikehearn/bips/commit/6058b92f5d9804ee4104649f53afc2fa53248c81?short_path=35c7795#specification>
Specification

Two new messages are defined. The "getutxos" message has the following
structure:

Field SizeDescriptionData typeComments1check mempoolboolWhether to apply
mempool transactions during the calculation, thus exposing their UTXOs and
removing outputs that they spend.?outpointsvectorThe list of outpoints to
be queried. Each outpoint is serialized in the same way it is in a tx
message.

The response message "utxos" has the following structure:

Field SizeDescriptionData typeComments4chain heightuint32The height of the
chain at the moment the result was calculated.32chain tip hashuint256Block
hash of the top of the chain at the moment the result was calculated.?hit
bitmapbyte[]An array of bytes encoding one bit for each outpoint queried.
Each bit indicates whether the queried outpoint was found in the UTXO set
or not.?result utxosresult[]A list of result objects (defined below), one
for each outpoint that is unspent (i.e. has a bit set in the bitmap).

The result object is defined as:

Field SizeDescriptionData typeComments4tx versionuint32The version number
of the transaction the UTXO was found in.4heightuint256The height of the
block containing the defining transaction, or 0x7FFFFFFF if the tx is in
the mempool.?outputCTxOutThe output itself, serialized in the same way as
in a tx message.
<https://github.com/mikehearn/bips/commit/6058b92f5d9804ee4104649f53afc2fa53248c81?short_path=35c7795#backward-compatibility>Backward
compatibility

Nodes indicate support by advertising a protocol version above 70003 and by
setting a new NODE_GETUTXO flag in their nServices field, which has a value
of 2 (1
<https://github.com/mikehearn/bips/commit/6058b92f5d9804ee4104649f53afc2fa53248c81?short_path=35c7795#authentication>
Authentication

The UTXO set is not currently authenticated by anything. There are
proposals to resolve this by introducing a new consensus rule that commits
to a root hash of the UTXO set in blocks, however this feature is not
presently available in the Bitcoin protocol. Once it is, the utxos message
could be upgraded to include Merkle branches showing inclusion of the UTXOs
in the committed sets.

If the requesting client is looking up outputs for a signed transaction
that they have locally, the client can partly verify the returned output by
running the input scripts with it. Currently this verifies only that the
script is correct. A future version of the Bitcoin protocol is likely to
also allow the value to be checked in this way. It does not show that the
output is really unspent or was ever actually created in the block chain
however.

If the requesting client has a mapping of chain heights to block hashes in
the best chain e.g. obtained via getheaders, then they can obtain a proof
that the output did at one point exist by requesting the block and
searching for the output within it. When combined with Bloom filtering this
can be reasonably efficient.

Note that even when the outputs are being checked against something this
protocol has the same security model as Bloom filtering: a remote node can
lie through omission by claiming the requested UTXO does not exist / was
already spent (they are the same, from the perspective of a full node).
Querying multiple nodes and combining their answers can be a partial
solution to this, although as nothing authenticates the Bitcoin P2P network
a man in the middle could still yield incorrect results.
<https://github.com/mikehearn/bips/commit/6058b92f5d9804ee4104649f53afc2fa53248c81?short_path=35c7795#implementation>
Implementation

https://github.com/bitcoin/bitcoin/pull/4351/files
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