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  LIP: 006
  Layer: Peer Services
  Title: MWEB P2P Messages
  Author: David Burkett <davidburkett38@gmail.com>
  Comments-Summary: No comments yet.
  Comments-URI: https://github.com/litecoin/lips/wiki/Comments:LIP-0007
  Status: Draft
  Type: Standards Track
  Created: 2022-06-23
  License: PD

Table of Contents

Abstract

In this LIP, we define new p2p protocol messages for supporting more efficient syncing of MWEB components.

Motivation

The most popular wallets for LTC users are light clients, which don't download and validate the blockchain. Instead, they rely on full nodes to help find transactions belonging to the wallet, significantly reducing the amount of data that must be downloaded, albeit with a weaker security model.

Unfortunately, the existing methods for finding a wallet's transactions cannot be applied to MWEB transactions, so a new approach is necessary.

The p2p protocol messages being added in this LIP will allow light clients to add MWEB support without reducing security or forfeiting privacy gains, while only downloading near-minimal amount of data.

Specification

New messages

MWEBUTXOsRequest

A getmwebutxos message is introduced which can be used to request a batch of MWEB UTXOs.

Field Name Type Size Encoding Purpose
block_hash Binary blob 32 bytes The output from a double-SHA256 of the block header, as used elsewhere The blockhash of the snapshot in time that UTXOs are requested for
start_index VarInt 1-8 bytes As used to encode unsigned values elsewhere The leaf index of the first UTXO being requested
num_requested uint16_t 2 bytes Little Endian The maximum number of UTXOs the peer would like in this batch
output_format uint8_t 1 byte MSB first The requested MWEB output serialization format

The supported MWEB output serialization format values are:

  1. FULL_UTXO(0x00) - The fully serialized UTXO containing the commitment, public keys, output message, rangeproof, and signature.
  2. HASH_ONLY(0x01) - The blake3 hash of the UTXO.
  3. COMPACT_UTXO(0x02) - The UTXO without the rangeproof. The blake3 hash of the rangeproof is provided, which is enough to calculate and verify the hash of the UTXO.

MWEBUTXOs

A peer can respond to getmwebutxos requests with a mwebutxos message containing:

Field Name Type Size Encoding Purpose
output_format uint8_t 1 byte MSB first The serialization format of the UTXOs that follow (FULL_UTXO, HASH_ONLY, or COMPACT_UTXO)
utxos_length CompactSize 1 or 3 bytes As used to encode array lengths elsewhere The number of MWEB UTXOs that follow
utxos List of MWEB UTXOs variable size * utxos_length leaf_index + output The serialized MWEB UTXOs
- leaf_index VarInt 1-8 bytes As used to encode unsigned values elsewhere Leaf index of the UTXO
- output MWEBOutput, Hash, or CompactMWEBOutput Variable size Encoded using the format specified in the output_format field The serialized output
parent_hashes_length CompactSize 1 or 3 bytes As used to encode array lengths elsewhere The number of parent hashes that follow
parent_hashes List of blake3 hashes 32 bytes * parent_hashes_length The parent hashes necessary to verify the outputs belong to the output PMMR

HogExAndMWEBHeader

When a getdata request with a type field of MSG_MWEB_HEADER (0x20000008) is made using a recent block hash, peers may respond with a mwebheader message containing:

Field Name Type Size Encoding Purpose
merkle merkleblock variable Encoded as defined in BIP-0037 Proves the hash of the final transaction in the block (the HogEx)
hogex tx variable Encoded as defined in BIP-0144 Commits to the hash of the MWEB header
mweb_header mwebheader 180 bytes Encoded as defined in LIP-0003 Contains the leafset_root and output_root values necessary for validating UTXOs

MWEBLeafset

When a getdata request with a type field of MSG_MWEB_LEAFSET (0x20000009) is made using a recent block hash, peers may respond with a mwebleafset message containing:

Field Name Type Size Encoding Purpose
block_hash Binary blob 32 bytes The output from a double-SHA256 of the block header, as used elsewhere The blockhash of the block the leafset is provided for
size CompactSize 1, 3, 5, or 9 bytes As used to encode array lengths elsewhere The number of bytes in the serialized bitset that follows
leafset Binary blob variable A zero-padded bitset serialized in big-endian form Indicates the leaf indices of all unspent UTXOs

Copyright

This document is placed in the public domain.