Add fake scid namespace for intercepted HTLCs

This is useful for LSPs who wish to create a just-in-time channel for end users
receiving a lightning payment. These fake scids will be encoded into route
hints in end user invoices, and signal to LDK to create an event triggering the
JIT channel, after which the payment will be received.

Co-authored-by: John Cantrell <johncantrell97@gmail.com>
Co-authored-by: Valentine Wallace <vwallace@protonmail.com>

Author: valentinewallace

lightning/src/util/scid_utils.rs

@@ -63,6 +63,8 @@ pub fn scid_from_parts(block: u64, tx_index: u64, vout_index: u64) -> Result<u64
 /// LDK has multiple reasons to generate fake short channel ids:
 /// 1) outbound SCID aliases we use for private channels
 /// 2) phantom node payments, to get an scid for the phantom node's phantom channel
+/// 3) payments intended to be intercepted will route using a fake scid (this is typically used so
+///    the forwarding node can open a JIT channel to the next hop)
 pub(crate) mod fake_scid {
 	use bitcoin::hash_types::BlockHash;
 	use bitcoin::hashes::hex::FromHex;
@@ -91,6 +93,7 @@ pub(crate) mod fake_scid {
 	pub(crate) enum Namespace {
 		Phantom,
 		OutboundAlias,
+		Intercept
 	}
 
 	impl Namespace {
@@ -150,7 +153,7 @@ pub(crate) mod fake_scid {
 		}
 	}
 
-	/// Returns whether the given fake scid falls into the given namespace.
+	/// Returns whether the given fake scid falls into the phantom namespace.
 	pub fn is_valid_phantom(fake_scid_rand_bytes: &[u8; 32], scid: u64, genesis_hash: &BlockHash) -> bool {
 		let block_height = scid_utils::block_from_scid(&scid);
 		let tx_index = scid_utils::tx_index_from_scid(&scid);
@@ -160,11 +163,21 @@ pub(crate) mod fake_scid {
 			&& valid_vout == scid_utils::vout_from_scid(&scid) as u8
 	}
 
+	/// Returns whether the given fake scid falls into the intercept namespace.
+	pub fn is_valid_intercept(fake_scid_rand_bytes: &[u8; 32], scid: u64, genesis_hash: &BlockHash) -> bool {
+		let block_height = scid_utils::block_from_scid(&scid);
+		let tx_index = scid_utils::tx_index_from_scid(&scid);
+		let namespace = Namespace::Intercept;
+		let valid_vout = namespace.get_encrypted_vout(block_height, tx_index, fake_scid_rand_bytes);
+		block_height >= segwit_activation_height(genesis_hash)
+			&& valid_vout == scid_utils::vout_from_scid(&scid) as u8
+	}
+
 	#[cfg(test)]
 	mod tests {
 		use bitcoin::blockdata::constants::genesis_block;
 		use bitcoin::network::constants::Network;
-		use crate::util::scid_utils::fake_scid::{is_valid_phantom, MAINNET_SEGWIT_ACTIVATION_HEIGHT, MAX_TX_INDEX, MAX_NAMESPACES, Namespace, NAMESPACE_ID_BITMASK, segwit_activation_height, TEST_SEGWIT_ACTIVATION_HEIGHT};
+		use crate::util::scid_utils::fake_scid::{is_valid_intercept, is_valid_phantom, MAINNET_SEGWIT_ACTIVATION_HEIGHT, MAX_TX_INDEX, MAX_NAMESPACES, Namespace, NAMESPACE_ID_BITMASK, segwit_activation_height, TEST_SEGWIT_ACTIVATION_HEIGHT};
 		use crate::util::scid_utils;
 		use crate::util::test_utils;
 		use crate::sync::Arc;
@@ -174,6 +187,10 @@ pub(crate) mod fake_scid {
 			let phantom_namespace = Namespace::Phantom;
 			assert!((phantom_namespace as u8) < MAX_NAMESPACES);
 			assert!((phantom_namespace as u8) <= NAMESPACE_ID_BITMASK);
+
+			let intercept_namespace = Namespace::Intercept;
+			assert!((intercept_namespace as u8) < MAX_NAMESPACES);
+			assert!((intercept_namespace as u8) <= NAMESPACE_ID_BITMASK);
 		}
 
 		#[test]
@@ -203,6 +220,18 @@ pub(crate) mod fake_scid {
 			assert!(!is_valid_phantom(&fake_scid_rand_bytes, invalid_fake_scid, &testnet_genesis));
 		}
 
+		#[test]
+		fn test_is_valid_intercept() {
+			let namespace = Namespace::Intercept;
+			let fake_scid_rand_bytes = [0; 32];
+			let testnet_genesis = genesis_block(Network::Testnet).header.block_hash();
+			let valid_encrypted_vout = namespace.get_encrypted_vout(0, 0, &fake_scid_rand_bytes);
+			let valid_fake_scid = scid_utils::scid_from_parts(1, 0, valid_encrypted_vout as u64).unwrap();
+			assert!(is_valid_intercept(&fake_scid_rand_bytes, valid_fake_scid, &testnet_genesis));
+			let invalid_fake_scid = scid_utils::scid_from_parts(1, 0, 12).unwrap();
+			assert!(!is_valid_intercept(&fake_scid_rand_bytes, invalid_fake_scid, &testnet_genesis));
+		}
+
 		#[test]
 		fn test_get_fake_scid() {
 			let mainnet_genesis = genesis_block(Network::Bitcoin).header.block_hash();