Add option to use scid alias from intercept namespace

Author: aylagreystone

lightning/src/ln/channelmanager.rs

@@ -3776,14 +3776,24 @@ where
 
 	#[rustfmt::skip]
 	fn create_and_insert_outbound_scid_alias(&self) -> u64 {
+		self.create_and_insert_outbound_scid_alias_with_intercept(false)
+	}
+
+	#[rustfmt::skip]
+	fn create_and_insert_outbound_scid_alias_with_intercept(&self, use_intercept_namespace: bool) -> u64 {
 		let height = self.best_block.read().unwrap().height;
 		let mut outbound_scid_alias = 0;
 		let mut i = 0;
 		loop {
 			if cfg!(fuzzing) { // fuzzing chacha20 doesn't use the key at all so we always get the same alias
 				outbound_scid_alias += 1;
 			} else {
-				outbound_scid_alias = fake_scid::Namespace::OutboundAlias.get_fake_scid(height, &self.chain_hash, &self.fake_scid_rand_bytes, &self.entropy_source);
+				let namespace = if use_intercept_namespace {
+					fake_scid::Namespace::Intercept
+				} else {
+					fake_scid::Namespace::OutboundAlias
+				};
+				outbound_scid_alias = namespace.get_fake_scid(height, &self.chain_hash, &self.fake_scid_rand_bytes, &self.entropy_source);
 			}
 			if outbound_scid_alias != 0 && self.outbound_scid_aliases.lock().unwrap().insert(outbound_scid_alias) {
 				break;
@@ -3850,9 +3860,10 @@ where
 		}
 
 		let mut channel = {
-			let outbound_scid_alias = self.create_and_insert_outbound_scid_alias();
-			let their_features = &peer_state.latest_features;
 			let config = if override_config.is_some() { override_config.as_ref().unwrap() } else { &self.default_configuration };
+			let use_intercept_scid = config.channel_handshake_config.intercept_htlcs_on_channel;
+			let outbound_scid_alias = self.create_and_insert_outbound_scid_alias_with_intercept(use_intercept_scid);
+			let their_features = &peer_state.latest_features;
 			match OutboundV1Channel::new(&self.fee_estimator, &self.entropy_source, &self.signer_provider, their_network_key,
 				their_features, channel_value_satoshis, push_msat, user_channel_id, config,
 				self.best_block.read().unwrap().height, outbound_scid_alias, temporary_channel_id, &*self.logger)
@@ -8191,7 +8202,8 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 		}
 
 		// Now that we know we have a channel, assign an outbound SCID alias.
-		let outbound_scid_alias = self.create_and_insert_outbound_scid_alias();
+		let use_intercept_scid = config.channel_handshake_config.intercept_htlcs_on_channel;
+		let outbound_scid_alias = self.create_and_insert_outbound_scid_alias_with_intercept(use_intercept_scid);
 		channel.context_mut().set_outbound_scid_alias(outbound_scid_alias);
 
 		if let Some(message_send_event) = message_send_event {
@@ -8407,7 +8419,8 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 			},
 		};
 
-		let outbound_scid_alias = self.create_and_insert_outbound_scid_alias();
+		let use_intercept_scid = self.default_configuration.channel_handshake_config.intercept_htlcs_on_channel;
+		let outbound_scid_alias = self.create_and_insert_outbound_scid_alias_with_intercept(use_intercept_scid);
 		channel.context_mut().set_outbound_scid_alias(outbound_scid_alias);
 
 		if let Some(message_send_event) = message_send_event {
@@ -16303,6 +16316,7 @@ mod tests {
 				to_self_delay: Some(200),
 				max_accepted_htlcs: Some(5),
 				channel_reserve_proportional_millionths: Some(20000),
+				intercept_htlcs_on_channel: None,
 			}),
 			update_overrides: None,
 		};

lightning/src/ln/functional_tests.rs

@@ -7474,6 +7474,7 @@ pub fn test_manually_accept_inbound_channel_request() {
 			to_self_delay: None,
 			max_accepted_htlcs: Some(3),
 			channel_reserve_proportional_millionths: None,
+			intercept_htlcs_on_channel: None,
 		}),
 		update_overrides: Some(ChannelConfigUpdate {
 			forwarding_fee_proportional_millionths: None,

lightning/src/util/config.rs

@@ -233,6 +233,18 @@ pub struct ChannelHandshakeConfig {
 	///
 	/// [`max_htlcs`]: crate::ln::chan_utils::max_htlcs
 	pub our_max_accepted_htlcs: u16,
+	/// If set, this channel's SCID alias will be generated in the intercept namespace, causing
+	/// all HTLCs sent to this channel to trigger [`Event::HTLCIntercepted`] events instead of
+	/// being automatically forwarded. This enables manual control over all payments to the channel.
+	///
+	/// Requires [`UserConfig::accept_intercept_htlcs`] to be `true`. Useful for scenarios like
+	/// fee-taking or conditional forwarding.
+	///
+	/// Default value: `false`
+	///
+	/// [`Event::HTLCIntercepted`]: crate::events::Event::HTLCIntercepted
+	/// [`UserConfig::accept_intercept_htlcs`]: crate::util::config::UserConfig::accept_intercept_htlcs
+	pub intercept_htlcs_on_channel: bool,
 }
 
 impl Default for ChannelHandshakeConfig {
@@ -250,6 +262,7 @@ impl Default for ChannelHandshakeConfig {
 			#[cfg(test)]
 			negotiate_anchor_zero_fee_commitments: false,
 			our_max_accepted_htlcs: 50,
+			intercept_htlcs_on_channel: false,
 		}
 	}
 }
@@ -273,6 +286,7 @@ impl Readable for ChannelHandshakeConfig {
 			#[cfg(test)]
 			negotiate_anchor_zero_fee_commitments: Readable::read(reader)?,
 			our_max_accepted_htlcs: Readable::read(reader)?,
+			intercept_htlcs_on_channel: Readable::read(reader)?,
 		})
 	}
 }
@@ -1009,6 +1023,9 @@ pub struct ChannelHandshakeConfigUpdate {
 	/// The Proportion of the channel value to configure as counterparty's channel reserve. See
 	/// [`ChannelHandshakeConfig::their_channel_reserve_proportional_millionths`].
 	pub channel_reserve_proportional_millionths: Option<u32>,
+	/// If set, allows this channel's SCID alias to be generated in the intercept namespace. See
+	/// [`ChannelHandshakeConfig::intercept_htlcs_on_channel`].
+	pub intercept_htlcs_on_channel: Option<bool>,
 }
 
 impl ChannelHandshakeConfig {
@@ -1039,5 +1056,9 @@ impl ChannelHandshakeConfig {
 		if let Some(channel_reserve) = config.channel_reserve_proportional_millionths {
 			self.their_channel_reserve_proportional_millionths = channel_reserve;
 		}
+
+		if let Some(intercept_htlcs) = config.intercept_htlcs_on_channel {
+			self.intercept_htlcs_on_channel = intercept_htlcs;
+		}
 	}
 }