Introduce Dummy Hop support in Blinded Path Constructor

Adds a new constructor for blinded paths that allows specifying
the number of dummy hops.
This enables users to insert arbitrary hops before the real destination,
enhancing privacy by making it harder to infer the sender–receiver
distance or identify the final destination.

Lays the groundwork for future use of dummy hops in blinded path construction.

Co-authored-by: valentinewallace <[email protected]>

Author: shaavan

lightning/src/blinded_path/message.rs

@@ -69,7 +69,6 @@ impl BlindedMessagePath {
 	/// pubkey in `node_pks` will be the destination node.
 	///
 	/// Errors if no hops are provided or if `node_pk`(s) are invalid.
-	//  TODO: make all payloads the same size with padding + add dummy hops
 	pub fn new<ES: Deref, T: secp256k1::Signing + secp256k1::Verification>(
 		intermediate_nodes: &[MessageForwardNode], recipient_node_id: PublicKey,
 		context: MessageContext, entropy_source: ES, secp_ctx: &Secp256k1<T>,
@@ -89,9 +88,53 @@ impl BlindedMessagePath {
 			blinding_point: PublicKey::from_secret_key(secp_ctx, &blinding_secret),
 			blinded_hops: blinded_hops(
 				secp_ctx,
+				entropy_source,
+				None,
 				intermediate_nodes,
 				recipient_node_id,
 				context,
+				0,
+				&blinding_secret,
+			)
+			.map_err(|_| ())?,
+		}))
+	}
+
+	/// Create a path for an onion message, to be forwarded along `node_pks`.
+	///
+	/// Additionally allows appending a number of dummy hops before the final hop,
+	/// increasing the total path length and enhancing privacy by obscuring the true
+	/// distance between sender and recipient.
+	///
+	/// The last node pubkey in `node_pks` will be the destination node.
+	///
+	/// Errors if no hops are provided or if `node_pk`(s) are invalid.
+	pub fn new_with_dummy_hops<ES: Deref, T: secp256k1::Signing + secp256k1::Verification>(
+		intermediate_nodes: &[MessageForwardNode], dummy_hops_count: u8,
+		recipient_node_id: PublicKey, context: MessageContext, entropy_source: ES,
+		expanded_key: &inbound_payment::ExpandedKey, secp_ctx: &Secp256k1<T>,
+	) -> Result<Self, ()>
+	where
+		ES::Target: EntropySource,
+	{
+		let introduction_node = IntroductionNode::NodeId(
+			intermediate_nodes.first().map_or(recipient_node_id, |n| n.node_id),
+		);
+		let blinding_secret_bytes = entropy_source.get_secure_random_bytes();
+		let blinding_secret =
+			SecretKey::from_slice(&blinding_secret_bytes[..]).expect("RNG is busted");
+
+		Ok(Self(BlindedPath {
+			introduction_node,
+			blinding_point: PublicKey::from_secret_key(secp_ctx, &blinding_secret),
+			blinded_hops: blinded_hops(
+				secp_ctx,
+				entropy_source,
+				Some(expanded_key),
+				intermediate_nodes,
+				recipient_node_id,
+				context,
+				dummy_hops_count,
 				&blinding_secret,
 			)
 			.map_err(|_| ())?,
@@ -563,13 +606,23 @@ impl_writeable_tlv_based!(DNSResolverContext, {
 pub(crate) const MESSAGE_PADDING_ROUND_OFF: usize = 100;
 
 /// Construct blinded onion message hops for the given `intermediate_nodes` and `recipient_node_id`.
-pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
-	secp_ctx: &Secp256k1<T>, intermediate_nodes: &[MessageForwardNode],
-	recipient_node_id: PublicKey, context: MessageContext, session_priv: &SecretKey,
-) -> Result<Vec<BlindedHop>, secp256k1::Error> {
+pub(super) fn blinded_hops<ES: Deref, T: secp256k1::Signing + secp256k1::Verification>(
+	secp_ctx: &Secp256k1<T>, entropy_source: ES,
+	expanded_key: Option<&inbound_payment::ExpandedKey>, intermediate_nodes: &[MessageForwardNode],
+	recipient_node_id: PublicKey, context: MessageContext, dummy_hops_count: u8,
+	session_priv: &SecretKey,
+) -> Result<Vec<BlindedHop>, secp256k1::Error>
+where
+	ES::Target: EntropySource,
+{
+	if expanded_key.is_none() && dummy_hops_count > 0 {
+		debug_assert!(false, "Dummy hops are not supported without expanded keys");
+	}
+
 	let pks = intermediate_nodes
 		.iter()
 		.map(|node| node.node_id)
+		.chain((0..dummy_hops_count).map(|_| recipient_node_id))
 		.chain(core::iter::once(recipient_node_id));
 	let is_compact = intermediate_nodes.iter().any(|node| node.short_channel_id.is_some());
 
@@ -584,6 +637,12 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 		.map(|next_hop| {
 			ControlTlvs::Forward(ForwardTlvs { next_hop, next_blinding_override: None })
 		})
+		.chain((0..dummy_hops_count).map(|_| {
+			let dummy_tlv = UnauthenticatedDummyTlv {};
+			let nonce = Nonce::from_entropy_source(&*entropy_source);
+			let hmac = dummy_tlv.hmac_data(nonce, expanded_key.unwrap());
+			ControlTlvs::Dummy(DummyTlv { dummy_tlv, authentication: (hmac, nonce) })
+		}))
 		.chain(core::iter::once(ControlTlvs::Receive(ReceiveTlvs { context: Some(context) })));
 
 	if is_compact {