Track SCID aliases from our counterparty and use them in invoices

New `funding_locked` messages can include SCID aliases which our
counterparty will recognize as "ours" for the purposes of relaying
transactions to us. This avoids telling the world about our
on-chain transactions every time we want to receive a payment, and
will allow for receiving payments before the funding transaction
appears on-chain.

Here we store the new SCID aliases and use them in invoices instead
of he "standard" SCIDs.

Author: TheBlueMatt

lightning-invoice/src/utils.rs

@@ -65,9 +65,11 @@ pub fn create_phantom_invoice<Signer: Sign, K: Deref>(
 
 	for hint in phantom_route_hints {
 		for channel in &hint.channels {
-			let short_channel_id = match channel.short_channel_id {
-				Some(id) => id,
-				None => continue,
+			let short_channel_id = if let Some(scid) = channel.inbound_scid_alias { scid } else {
+				match channel.short_channel_id {
+					Some(id) => id,
+					None => continue,
+				}
 			};
 			let forwarding_info = match &channel.counterparty.forwarding_info {
 				Some(info) => info.clone(),
@@ -162,9 +164,11 @@ where
 	let our_channels = channelmanager.list_usable_channels();
 	let mut route_hints = vec![];
 	for channel in our_channels {
-		let short_channel_id = match channel.short_channel_id {
-			Some(id) => id,
-			None => continue,
+		let short_channel_id = if let Some(scid) = channel.inbound_scid_alias { scid } else {
+			match channel.short_channel_id {
+				Some(id) => id,
+				None => continue,
+			}
 		};
 		let forwarding_info = match channel.counterparty.forwarding_info {
 			Some(info) => info,

lightning/src/ln/channel.rs

@@ -695,6 +695,13 @@ pub(super) struct Channel<Signer: Sign> {
 
 	/// This channel's type, as negotiated during channel open
 	channel_type: ChannelTypeFeatures,
+
+	// Our counerparty can offer us SCID aliases which they will map to this channel when routing
+	// outbound payments. These can be used in invoice route hints to provide privacy of which
+	// on-chain transaction is ours.
+	// We only bother storing the most recent SCID alias at any time, though our counterparty has
+	// to store all of them.
+	latest_inbound_scid_alias: Option<u64>,
 }
 
 #[cfg(any(test, feature = "fuzztarget"))]
@@ -947,6 +954,8 @@ impl<Signer: Sign> Channel<Signer> {
 
 			workaround_lnd_bug_4006: None,
 
+			latest_inbound_scid_alias: None,
+
 			#[cfg(any(test, feature = "fuzztarget"))]
 			historical_inbound_htlc_fulfills: HashSet::new(),
 
@@ -1252,6 +1261,8 @@ impl<Signer: Sign> Channel<Signer> {
 
 			workaround_lnd_bug_4006: None,
 
+			latest_inbound_scid_alias: None,
+
 			#[cfg(any(test, feature = "fuzztarget"))]
 			historical_inbound_htlc_fulfills: HashSet::new(),
 
@@ -2141,6 +2152,15 @@ impl<Signer: Sign> Channel<Signer> {
 			return Err(ChannelError::Ignore("Peer sent funding_locked when we needed a channel_reestablish. The peer is likely lnd, see https://github.com/lightningnetwork/lnd/issues/4006".to_owned()));
 		}
 
+		if let Some(scid_alias) = msg.short_channel_id_alias {
+			if Some(scid_alias) != self.short_channel_id {
+				// The scid alias provided can be used to route payments *from* our counterparty,
+				// i.e. can be used for inbound payments and provided in invoices, but is not used
+				// whrn routing outbound payments.
+				self.latest_inbound_scid_alias = Some(scid_alias);
+			}
+		}
+
 		let non_shutdown_state = self.channel_state & (!MULTI_STATE_FLAGS);
 
 		if non_shutdown_state == ChannelState::FundingSent as u32 {
@@ -4188,6 +4208,11 @@ impl<Signer: Sign> Channel<Signer> {
 		self.short_channel_id
 	}
 
+	/// Allowed in any state (including after shutdown)
+	pub fn get_latest_inbound_scid_alias(&self) -> Option<u64> {
+		self.latest_inbound_scid_alias
+	}
+
 	/// Returns the funding_txo we either got from our peer, or were given by
 	/// get_outbound_funding_created.
 	pub fn get_funding_txo(&self) -> Option<OutPoint> {
@@ -5758,6 +5783,7 @@ impl<Signer: Sign> Writeable for Channel<Signer> {
 			(13, self.channel_creation_height, required),
 			(15, preimages, vec_type),
 			(17, self.announcement_sigs_state, required),
+			(19, self.latest_inbound_scid_alias, option),
 		});
 
 		Ok(())
@@ -6013,6 +6039,7 @@ impl<'a, Signer: Sign, K: Deref> ReadableArgs<(&'a K, u32)> for Channel<Signer>
 		// If we read an old Channel, for simplicity we just treat it as "we never sent an
 		// AnnouncementSignatures" which implies we'll re-send it on reconnect, but that's fine.
 		let mut announcement_sigs_state = Some(AnnouncementSigsState::NotSent);
+		let mut latest_inbound_scid_alias = None;
 
 		read_tlv_fields!(reader, {
 			(0, announcement_sigs, option),
@@ -6028,6 +6055,7 @@ impl<'a, Signer: Sign, K: Deref> ReadableArgs<(&'a K, u32)> for Channel<Signer>
 			(13, channel_creation_height, option),
 			(15, preimages_opt, vec_type),
 			(17, announcement_sigs_state, option),
+			(19, latest_inbound_scid_alias, option),
 		});
 
 		if let Some(preimages) = preimages_opt {
@@ -6162,6 +6190,8 @@ impl<'a, Signer: Sign, K: Deref> ReadableArgs<(&'a K, u32)> for Channel<Signer>
 
 			workaround_lnd_bug_4006: None,
 
+			latest_inbound_scid_alias,
+
 			#[cfg(any(test, feature = "fuzztarget"))]
 			historical_inbound_htlc_fulfills,
 

lightning/src/ln/channelmanager.rs

@@ -1187,6 +1187,11 @@ pub struct ChannelDetails {
 	/// The position of the funding transaction in the chain. None if the funding transaction has
 	/// not yet been confirmed and the channel fully opened.
 	pub short_channel_id: Option<u64>,
+	/// An optional [`short_channel_id`] alias for this channel, randomly generated by our
+	/// counterparty and usable in place of [`short_channel_id`] in invoice route hints. Our
+	/// counterparty will recognize the alias provided here in place of the [`short_channel_id`]
+	/// when they see a payment to be routed to us.
+	pub inbound_scid_alias: Option<u64>,
 	/// The value, in satoshis, of this channel as appears in the funding output
 	pub channel_value_satoshis: u64,
 	/// The value, in satoshis, that must always be held in the channel for us. This value ensures
@@ -1830,6 +1835,7 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 					},
 					funding_txo: channel.get_funding_txo(),
 					short_channel_id: channel.get_short_channel_id(),
+					inbound_scid_alias: channel.get_latest_inbound_scid_alias(),
 					channel_value_satoshis: channel.get_value_satoshis(),
 					unspendable_punishment_reserve: to_self_reserve_satoshis,
 					balance_msat,
@@ -5955,6 +5961,7 @@ impl_writeable_tlv_based!(ChannelCounterparty, {
 });
 
 impl_writeable_tlv_based!(ChannelDetails, {
+	(1, inbound_scid_alias, option),
 	(2, channel_id, required),
 	(4, counterparty, required),
 	(6, funding_txo, option),

lightning/src/routing/router.rs

@@ -1563,6 +1563,7 @@ mod tests {
 			},
 			funding_txo: Some(OutPoint { txid: bitcoin::Txid::from_slice(&[0; 32]).unwrap(), index: 0 }),
 			short_channel_id,
+			inbound_scid_alias: None,
 			channel_value_satoshis: 0,
 			user_channel_id: 0,
 			balance_msat: 0,