extension-bolt: taproot gossip (features 32/33)

[ellemouton]

Overview

The initial version of the Lightning Network gossip protocol as defined in BOLT 7 was designed around P2WSH funding transactions. For these channels, the channel_announcement message is used to advertise the channel to the rest of the network. Nodes in the network use the content of this message to prove that the channel is sufficiently bound to the Lightning Network context and that it is owned by the nodes advertising the channel. This proof and verification protocol is, however, not compatible with SegWit V1 (P2TR) outputs and so cannot be used to advertise the channels defined in the Simple Taproot Channel proposal. This document thus aims to define an updated gossip protocol that will allow nodes to both advertise and verify taproot channels. This part of the update affects the announcement_signatures and channel_announcement messages.

The opportunity is also taken to rework the node_announcement and channel_update messages to take advantage of BIP-340 signatures and TLV fields. Timestamp fields are also updated to be block heights instead of Unix timestamps.