Provide our peers with SCID aliases and forward payments with them

This creates an SCID alias for all of our outbound channels, which
we send to our counterparties as a part of the `funding_locked`
message and then recognize in any HTLC forwarding instructions.

Note that we generate an SCID alias for all channels, including
already open ones, even though we currently have no way of
communicating to our peers the SCID alias for already-open
channels.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -702,6 +702,12 @@ pub(super) struct Channel<Signer: Sign> {
 	// 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>,
+
+	// We always offer our counterparty a static SCID alias, which we recognize as for this channel
+	// if we see it in HTLC forwarding instructions. We don't bother rotating the alias given we
+	// don't currently support node id aliases and eventually privacy should be provided with
+	// blinded paths instead of simple scid+node_id aliases.
+	outbound_scid_alias: u64,
 }
 
 #[cfg(any(test, fuzzing))]
@@ -807,7 +813,8 @@ impl<Signer: Sign> Channel<Signer> {
 	// Constructors:
 	pub fn new_outbound<K: Deref, F: Deref>(
 		fee_estimator: &F, keys_provider: &K, counterparty_node_id: PublicKey, their_features: &InitFeatures,
-		channel_value_satoshis: u64, push_msat: u64, user_id: u64, config: &UserConfig, current_chain_height: u32
+		channel_value_satoshis: u64, push_msat: u64, user_id: u64, config: &UserConfig, current_chain_height: u32,
+		outbound_scid_alias: u64
 	) -> Result<Channel<Signer>, APIError>
 	where K::Target: KeysInterface<Signer = Signer>,
 	      F::Target: FeeEstimator,
@@ -955,6 +962,7 @@ impl<Signer: Sign> Channel<Signer> {
 			workaround_lnd_bug_4006: None,
 
 			latest_inbound_scid_alias: None,
+			outbound_scid_alias,
 
 			#[cfg(any(test, fuzzing))]
 			historical_inbound_htlc_fulfills: HashSet::new(),
@@ -993,7 +1001,8 @@ impl<Signer: Sign> Channel<Signer> {
 	/// Assumes chain_hash has already been checked and corresponds with what we expect!
 	pub fn new_from_req<K: Deref, F: Deref, L: Deref>(
 		fee_estimator: &F, keys_provider: &K, counterparty_node_id: PublicKey, their_features: &InitFeatures,
-		msg: &msgs::OpenChannel, user_id: u64, config: &UserConfig, current_chain_height: u32, logger: &L
+		msg: &msgs::OpenChannel, user_id: u64, config: &UserConfig, current_chain_height: u32, logger: &L,
+		outbound_scid_alias: u64
 	) -> Result<Channel<Signer>, ChannelError>
 		where K::Target: KeysInterface<Signer = Signer>,
 		      F::Target: FeeEstimator,
@@ -1262,6 +1271,7 @@ impl<Signer: Sign> Channel<Signer> {
 			workaround_lnd_bug_4006: None,
 
 			latest_inbound_scid_alias: None,
+			outbound_scid_alias,
 
 			#[cfg(any(test, fuzzing))]
 			historical_inbound_htlc_fulfills: HashSet::new(),
@@ -3477,7 +3487,7 @@ impl<Signer: Sign> Channel<Signer> {
 			Some(msgs::FundingLocked {
 				channel_id: self.channel_id(),
 				next_per_commitment_point,
-				short_channel_id_alias: None,
+				short_channel_id_alias: Some(self.outbound_scid_alias),
 			})
 		} else { None };
 
@@ -3699,7 +3709,7 @@ impl<Signer: Sign> Channel<Signer> {
 				funding_locked: Some(msgs::FundingLocked {
 					channel_id: self.channel_id(),
 					next_per_commitment_point,
-					short_channel_id_alias: None,
+					short_channel_id_alias: Some(self.outbound_scid_alias),
 				}),
 				raa: None, commitment_update: None, mon_update: None,
 				order: RAACommitmentOrder::CommitmentFirst,
@@ -3735,7 +3745,7 @@ impl<Signer: Sign> Channel<Signer> {
 			Some(msgs::FundingLocked {
 				channel_id: self.channel_id(),
 				next_per_commitment_point,
-				short_channel_id_alias: None,
+				short_channel_id_alias: Some(self.outbound_scid_alias),
 			})
 		} else { None };
 
@@ -4223,6 +4233,17 @@ impl<Signer: Sign> Channel<Signer> {
 		self.latest_inbound_scid_alias
 	}
 
+	/// Allowed in any state (including after shutdown)
+	pub fn outbound_scid_alias(&self) -> u64 {
+		self.outbound_scid_alias
+	}
+	/// Only allowed immediately after deserialization if get_outbound_scid_alias returns 0,
+	/// indicating we were written by an old LDK which did not set outbound SCID aliases.
+	pub fn set_outbound_scid_alias(&mut self, outbound_scid_alias: u64) {
+		assert_eq!(self.outbound_scid_alias, 0);
+		self.outbound_scid_alias = outbound_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> {
@@ -4475,7 +4496,7 @@ impl<Signer: Sign> Channel<Signer> {
 					return Some(msgs::FundingLocked {
 						channel_id: self.channel_id,
 						next_per_commitment_point,
-						short_channel_id_alias: None,
+						short_channel_id_alias: Some(self.outbound_scid_alias),
 					});
 				}
 			} else {
@@ -5795,6 +5816,7 @@ impl<Signer: Sign> Writeable for Channel<Signer> {
 			(15, preimages, vec_type),
 			(17, self.announcement_sigs_state, required),
 			(19, self.latest_inbound_scid_alias, option),
+			(21, self.outbound_scid_alias, required),
 		});
 
 		Ok(())
@@ -6051,6 +6073,7 @@ impl<'a, Signer: Sign, K: Deref> ReadableArgs<(&'a K, u32)> for Channel<Signer>
 		// 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;
+		let mut outbound_scid_alias = None;
 
 		read_tlv_fields!(reader, {
 			(0, announcement_sigs, option),
@@ -6067,6 +6090,7 @@ impl<'a, Signer: Sign, K: Deref> ReadableArgs<(&'a K, u32)> for Channel<Signer>
 			(15, preimages_opt, vec_type),
 			(17, announcement_sigs_state, option),
 			(19, latest_inbound_scid_alias, option),
+			(21, outbound_scid_alias, option),
 		});
 
 		if let Some(preimages) = preimages_opt {
@@ -6202,6 +6226,8 @@ impl<'a, Signer: Sign, K: Deref> ReadableArgs<(&'a K, u32)> for Channel<Signer>
 			workaround_lnd_bug_4006: None,
 
 			latest_inbound_scid_alias,
+			// Later in the ChannelManager deserialization phase we scan for channels and assign scid aliases if its missing
+			outbound_scid_alias: outbound_scid_alias.unwrap_or(0),
 
 			#[cfg(any(test, fuzzing))]
 			historical_inbound_htlc_fulfills,
@@ -6325,7 +6351,7 @@ mod tests {
 		let secp_ctx = Secp256k1::new();
 		let node_id = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[42; 32]).unwrap());
 		let config = UserConfig::default();
-		match Channel::<EnforcingSigner>::new_outbound(&&fee_estimator, &&keys_provider, node_id, &features, 10000000, 100000, 42, &config, 0) {
+		match Channel::<EnforcingSigner>::new_outbound(&&fee_estimator, &&keys_provider, node_id, &features, 10000000, 100000, 42, &config, 0, 42) {
 			Err(APIError::IncompatibleShutdownScript { script }) => {
 				assert_eq!(script.into_inner(), non_v0_segwit_shutdown_script.into_inner());
 			},
@@ -6347,7 +6373,7 @@ mod tests {
 
 		let node_a_node_id = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[42; 32]).unwrap());
 		let config = UserConfig::default();
-		let node_a_chan = Channel::<EnforcingSigner>::new_outbound(&&fee_est, &&keys_provider, node_a_node_id, &InitFeatures::known(), 10000000, 100000, 42, &config, 0).unwrap();
+		let node_a_chan = Channel::<EnforcingSigner>::new_outbound(&&fee_est, &&keys_provider, node_a_node_id, &InitFeatures::known(), 10000000, 100000, 42, &config, 0, 42).unwrap();
 
 		// Now change the fee so we can check that the fee in the open_channel message is the
 		// same as the old fee.
@@ -6373,13 +6399,13 @@ mod tests {
 		// Create Node A's channel pointing to Node B's pubkey
 		let node_b_node_id = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[42; 32]).unwrap());
 		let config = UserConfig::default();
-		let mut node_a_chan = Channel::<EnforcingSigner>::new_outbound(&&feeest, &&keys_provider, node_b_node_id, &InitFeatures::known(), 10000000, 100000, 42, &config, 0).unwrap();
+		let mut node_a_chan = Channel::<EnforcingSigner>::new_outbound(&&feeest, &&keys_provider, node_b_node_id, &InitFeatures::known(), 10000000, 100000, 42, &config, 0, 42).unwrap();
 
 		// Create Node B's channel by receiving Node A's open_channel message
 		// Make sure A's dust limit is as we expect.
 		let open_channel_msg = node_a_chan.get_open_channel(genesis_block(network).header.block_hash());
 		let node_b_node_id = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[7; 32]).unwrap());
-		let mut node_b_chan = Channel::<EnforcingSigner>::new_from_req(&&feeest, &&keys_provider, node_b_node_id, &InitFeatures::known(), &open_channel_msg, 7, &config, 0, &&logger).unwrap();
+		let mut node_b_chan = Channel::<EnforcingSigner>::new_from_req(&&feeest, &&keys_provider, node_b_node_id, &InitFeatures::known(), &open_channel_msg, 7, &config, 0, &&logger, 42).unwrap();
 
 		// Node B --> Node A: accept channel, explicitly setting B's dust limit.
 		let mut accept_channel_msg = node_b_chan.accept_inbound_channel();
@@ -6443,7 +6469,7 @@ mod tests {
 
 		let node_id = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[42; 32]).unwrap());
 		let config = UserConfig::default();
-		let mut chan = Channel::<EnforcingSigner>::new_outbound(&&fee_est, &&keys_provider, node_id, &InitFeatures::known(), 10000000, 100000, 42, &config, 0).unwrap();
+		let mut chan = Channel::<EnforcingSigner>::new_outbound(&&fee_est, &&keys_provider, node_id, &InitFeatures::known(), 10000000, 100000, 42, &config, 0, 42).unwrap();
 
 		let commitment_tx_fee_0_htlcs = Channel::<EnforcingSigner>::commit_tx_fee_msat(chan.feerate_per_kw, 0, chan.opt_anchors());
 		let commitment_tx_fee_1_htlc = Channel::<EnforcingSigner>::commit_tx_fee_msat(chan.feerate_per_kw, 1, chan.opt_anchors());
@@ -6492,12 +6518,12 @@ mod tests {
 		// Create Node A's channel pointing to Node B's pubkey
 		let node_b_node_id = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[42; 32]).unwrap());
 		let config = UserConfig::default();
-		let mut node_a_chan = Channel::<EnforcingSigner>::new_outbound(&&feeest, &&keys_provider, node_b_node_id, &InitFeatures::known(), 10000000, 100000, 42, &config, 0).unwrap();
+		let mut node_a_chan = Channel::<EnforcingSigner>::new_outbound(&&feeest, &&keys_provider, node_b_node_id, &InitFeatures::known(), 10000000, 100000, 42, &config, 0, 42).unwrap();
 
 		// Create Node B's channel by receiving Node A's open_channel message
 		let open_channel_msg = node_a_chan.get_open_channel(chain_hash);
 		let node_b_node_id = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[7; 32]).unwrap());
-		let mut node_b_chan = Channel::<EnforcingSigner>::new_from_req(&&feeest, &&keys_provider, node_b_node_id, &InitFeatures::known(), &open_channel_msg, 7, &config, 0, &&logger).unwrap();
+		let mut node_b_chan = Channel::<EnforcingSigner>::new_from_req(&&feeest, &&keys_provider, node_b_node_id, &InitFeatures::known(), &open_channel_msg, 7, &config, 0, &&logger, 42).unwrap();
 
 		// Node B --> Node A: accept channel
 		let accept_channel_msg = node_b_chan.accept_inbound_channel();
@@ -6554,7 +6580,7 @@ mod tests {
 		// Create a channel.
 		let node_b_node_id = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[42; 32]).unwrap());
 		let config = UserConfig::default();
-		let mut node_a_chan = Channel::<EnforcingSigner>::new_outbound(&&feeest, &&keys_provider, node_b_node_id, &InitFeatures::known(), 10000000, 100000, 42, &config, 0).unwrap();
+		let mut node_a_chan = Channel::<EnforcingSigner>::new_outbound(&&feeest, &&keys_provider, node_b_node_id, &InitFeatures::known(), 10000000, 100000, 42, &config, 0, 42).unwrap();
 		assert!(node_a_chan.counterparty_forwarding_info.is_none());
 		assert_eq!(node_a_chan.holder_htlc_minimum_msat, 1); // the default
 		assert!(node_a_chan.counterparty_forwarding_info().is_none());
@@ -6619,7 +6645,7 @@ mod tests {
 		let counterparty_node_id = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[42; 32]).unwrap());
 		let mut config = UserConfig::default();
 		config.channel_options.announced_channel = false;
-		let mut chan = Channel::<InMemorySigner>::new_outbound(&&feeest, &&keys_provider, counterparty_node_id, &InitFeatures::known(), 10_000_000, 100000, 42, &config, 0).unwrap(); // Nothing uses their network key in this test
+		let mut chan = Channel::<InMemorySigner>::new_outbound(&&feeest, &&keys_provider, counterparty_node_id, &InitFeatures::known(), 10_000_000, 100000, 42, &config, 0, 42).unwrap(); // Nothing uses their network key in this test
 		chan.holder_dust_limit_satoshis = 546;
 		chan.counterparty_selected_channel_reserve_satoshis = Some(0); // Filled in in accept_channel