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.

fuzz/src/router.rs

@@ -216,6 +216,7 @@ pub fn do_test<Out: test_logger::Output>(data: &[u8], out: Out) {
 								},
 								funding_txo: Some(OutPoint { txid: bitcoin::Txid::from_slice(&[0; 32]).unwrap(), index: 0 }),
 								short_channel_id: Some(scid),
+								inbound_scid_alias: None,
 								channel_value_satoshis: slice_to_be64(get_slice!(8)),
 								user_channel_id: 0, inbound_capacity_msat: 0,
 								unspendable_punishment_reserve: None,

lightning-invoice/src/utils.rs

@@ -65,7 +65,7 @@ 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 {
+			let short_channel_id = match channel.get_inbound_payment_scid() {
 				Some(id) => id,
 				None => continue,
 			};
@@ -162,7 +162,7 @@ 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 {
+		let short_channel_id = match channel.get_inbound_payment_scid() {
 			Some(id) => id,
 			None => continue,
 		};
@@ -313,6 +313,13 @@ mod test {
 		assert_eq!(invoice.min_final_cltv_expiry(), MIN_FINAL_CLTV_EXPIRY as u64);
 		assert_eq!(invoice.description(), InvoiceDescription::Direct(&Description("test".to_string())));
 
+		// Invoice SCIDs should always use inbound SCID aliases over the real channel ID, if one is
+		// available.
+		assert_eq!(invoice.route_hints().len(), 1);
+		assert_eq!(invoice.route_hints()[0].0.len(), 1);
+		assert_eq!(invoice.route_hints()[0].0[0].short_channel_id,
+			nodes[1].node.list_usable_channels()[0].inbound_scid_alias.unwrap());
+
 		let payment_params = PaymentParameters::from_node_id(invoice.recover_payee_pub_key())
 			.with_features(invoice.features().unwrap().clone())
 			.with_route_hints(invoice.route_hints());

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 counterparty 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 avoid explicitly revealing
+	// the channel's funding UTXO.
+	// 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, fuzzing))]
@@ -947,6 +954,8 @@ impl<Signer: Sign> Channel<Signer> {
 
 			workaround_lnd_bug_4006: None,
 
+			latest_inbound_scid_alias: None,
+
 			#[cfg(any(test, fuzzing))]
 			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, fuzzing))]
 			historical_inbound_htlc_fulfills: HashSet::new(),
 
@@ -2151,6 +2162,15 @@ 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
+				// when 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 {
@@ -4198,6 +4218,11 @@ impl<Signer: Sign> Channel<Signer> {
 		self.short_channel_id
 	}
 
+	/// Allowed in any state (including after shutdown)
+	pub fn 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> {
@@ -5769,6 +5794,7 @@ 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(())
@@ -6024,6 +6050,7 @@ 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),
@@ -6039,6 +6066,7 @@ 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 {
@@ -6173,6 +6201,8 @@ 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, fuzzing))]
 			historical_inbound_htlc_fulfills,
 

lightning/src/ln/channelmanager.rs

@@ -1188,7 +1188,20 @@ pub struct ChannelDetails {
 	pub funding_txo: Option<OutPoint>,
 	/// The position of the funding transaction in the chain. None if the funding transaction has
 	/// not yet been confirmed and the channel fully opened.
+	///
+	/// Note that if [`inbound_scid_alias`] is set, it must be used for invoices and inbound
+	/// payments instead of this. See [`get_inbound_payment_scid`].
+	///
+	/// [`inbound_scid_alias`]: Self::inbound_scid_alias
+	/// [`get_inbound_payment_scid`]: Self::get_inbound_payment_scid
 	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.
+	///
+	/// [`short_channel_id`]: Self::short_channel_id
+	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
@@ -1274,6 +1287,15 @@ pub struct ChannelDetails {
 	pub is_public: bool,
 }
 
+impl ChannelDetails {
+	/// Gets the SCID which should be used to identify this channel for inbound payments. This
+	/// should be used for providing invoice hints or in any other context where our counterparty
+	/// will forward a payment to us.
+	pub fn get_inbound_payment_scid(&self) -> Option<u64> {
+		self.inbound_scid_alias.or(self.short_channel_id)
+	}
+}
+
 /// If a payment fails to send, it can be in one of several states. This enum is returned as the
 /// Err() type describing which state the payment is in, see the description of individual enum
 /// states for more.
@@ -1833,6 +1855,7 @@ 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.latest_inbound_scid_alias(),
 					channel_value_satoshis: channel.get_value_satoshis(),
 					unspendable_punishment_reserve: to_self_reserve_satoshis,
 					balance_msat,
@@ -5973,6 +5996,7 @@ 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

@@ -1578,6 +1578,7 @@ 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,
@@ -5101,6 +5102,7 @@ mod benches {
 				txid: bitcoin::Txid::from_slice(&[0; 32]).unwrap(), index: 0
 			}),
 			short_channel_id: Some(1),
+			inbound_scid_alias: None,
 			channel_value_satoshis: 10_000_000,
 			user_channel_id: 0,
 			balance_msat: 10_000_000,