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107 lines
5.1 KiB
Plaintext
107 lines
5.1 KiB
Plaintext
<pre>
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BIP: 64
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Layer: Peer Services
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Title: getutxo message
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Author: Mike Hearn <hearn@vinumeris.com>
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Comments-Summary: No comments yet.
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Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0064
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Status: Draft
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Type: Standards Track
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Created: 2014-06-10
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</pre>
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==Abstract==
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This document describes a small P2P protocol extension that performs UTXO lookups given a set of outpoints.
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==Motivation==
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All full Bitcoin nodes maintain a database called the unspent transaction output set. This set is
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how double spending is checked for: to be valid a transaction must identify unspent outputs in this
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set using an identifier called an "outpoint", which is merely the hash of the output's containing
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transaction plus an index.
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The ability to query this can sometimes be useful for a lightweight/SPV client which does not have
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the full UTXO set at hand. For example, it can be useful in applications implementing assurance
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contracts to do a quick check when a new pledge becomes visible to test whether that pledge was
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already revoked via a double spend. Although this message is not strictly necessary because e.g.
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such an app could be implemented by fully downloading and storing the block chain, it is useful for
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obtaining acceptable performance and resolving various UI cases.
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Another example of when this data can be useful is for performing floating fee calculations in an
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SPV wallet. This use case requires some other changes to the Bitcoin protocol however, so we will
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not dwell on it here.
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==Specification==
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Two new messages are defined. The "getutxos" message has the following structure:
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{|class="wikitable"
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! Field Size !! Description !! Data type !! Comments
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|-
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| 1 || check mempool || bool || Whether to apply mempool transactions during the calculation, thus exposing their UTXOs and removing outputs that they spend.
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| ? || outpoints || vector<COutPoint> || The list of outpoints to be queried. Each outpoint is serialized in the same way it is in a tx message.
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|}
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The response message "utxos" has the following structure:
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{|class="wikitable"
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! Field Size !! Description !! Data type !! Comments
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| 4 || chain height || uint32 || The height of the chain at the moment the result was calculated.
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| 32 || chain tip hash || uint256 || Block hash of the top of the chain at the moment the result was calculated.
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| ? || hit bitmap || byte[] || 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.
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|-
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| ? || result utxos || result[] || A list of result objects (defined below), one for each outpoint that is unspent (i.e. has a bit set in the bitmap).
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|}
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The result object is defined as:
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{|class="wikitable"
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! Field Size !! Description !! Data type !! Comments
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| 4 || tx version || uint32 || The version number of the transaction the UTXO was found in.
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|-
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| 4 || height || uint32 || The height of the block containing the defining transaction, or 0x7FFFFFFF if the tx is in the mempool.
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|-
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| ? || output || CTxOut || The output itself, serialized in the same way as in a tx message.
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|}
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==Backward compatibility==
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Nodes indicate support by advertising a protocol version above 70003 and by setting a new
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NODE_GETUTXO flag in their nServices field, which has a value of 2 (the second bit of the field).
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==Authentication==
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The UTXO set is not currently authenticated by anything. There are proposals to resolve this by
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introducing a new consensus rule that commits to a root hash of the UTXO set in blocks, however this
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feature is not presently available in the Bitcoin protocol. Once it is, the utxos message could be
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upgraded to include Merkle branches showing inclusion of the UTXOs in the committed sets.
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If the requesting client is looking up outputs for a signed transaction that they have locally, the
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client can partly verify the returned output by running the input scripts with it. Currently this
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verifies only that the script is correct. A future version of the Bitcoin protocol is likely to also
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allow the value to be checked in this way. It does not show that the output is really unspent or was
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ever actually created in the block chain however. Additionally, the form of the provided scriptPubKey
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should be checked before execution to ensure the remote peer doesn't just set the script to OP_TRUE.
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If the requesting client has a mapping of chain heights to block hashes in the best chain e.g.
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obtained via getheaders, then they can obtain a proof that the output did at one point exist by
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requesting the block and searching for the output within it. When combined with Bloom filtering this
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can be reasonably efficient.
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Note that even when the outputs are being checked against something this protocol has the same
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security model as Bloom filtering: a remote node can lie through omission by claiming the requested
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UTXO does not exist / was already spent (they are the same, from the perspective of a full node).
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Querying multiple nodes and combining their answers can be a partial solution to this, although as
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nothing authenticates the Bitcoin P2P network a man in the middle could still yield incorrect
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results.
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==Implementation==
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https://github.com/bitcoin/bitcoin/pull/4351/files
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