Introduce Padding for BlindedMessage Paths.

A note of Compact Blinded Paths:

Compact Blinded paths are intended to be as short as possible. So
to maintain there compactness, we don't apply padding to them.

Author: shaavan

lightning/src/blinded_path/message.rs

@@ -14,7 +14,7 @@ use bitcoin::secp256k1::{self, PublicKey, Secp256k1, SecretKey};
 #[allow(unused_imports)]
 use crate::prelude::*;
 
-use crate::blinded_path::utils;
+use crate::blinded_path::utils::{self, BlindedPathWithPadding};
 use crate::blinded_path::{BlindedHop, BlindedPath, Direction, IntroductionNode, NodeIdLookUp};
 use crate::crypto::streams::ChaChaPolyReadAdapter;
 use crate::io;
@@ -265,7 +265,6 @@ impl Writeable for ForwardTlvs {
 			NextMessageHop::NodeId(pubkey) => (Some(pubkey), None),
 			NextMessageHop::ShortChannelId(scid) => (None, Some(scid)),
 		};
-		// TODO: write padding
 		encode_tlv_stream!(writer, {
 			(2, short_channel_id, option),
 			(4, next_node_id, option),
@@ -277,7 +276,6 @@ impl Writeable for ForwardTlvs {
 
 impl Writeable for ReceiveTlvs {
 	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
-		// TODO: write padding
 		encode_tlv_stream!(writer, {
 			(65537, self.context, option),
 		});
@@ -490,6 +488,10 @@ impl_writeable_tlv_based!(DNSResolverContext, {
 	(0, nonce, required),
 });
 
+/// Represents the padding round off size (in bytes) that is used
+/// to pad message blinded path's [`BlindedHop`]
+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],
@@ -499,6 +501,8 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 		.iter()
 		.map(|node| node.node_id)
 		.chain(core::iter::once(recipient_node_id));
+	let is_compact = intermediate_nodes.iter().any(|node| node.short_channel_id.is_some());
+
 	let tlvs = pks
 		.clone()
 		.skip(1) // The first node's TLVs contains the next node's pubkey
@@ -512,7 +516,15 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 		})
 		.chain(core::iter::once(ControlTlvs::Receive(ReceiveTlvs { context: Some(context) })));
 
-	let path = pks.zip(tlvs);
-
-	utils::construct_blinded_hops(secp_ctx, path, session_priv)
+	if is_compact {
+		let path = pks.zip(tlvs);
+		utils::construct_blinded_hops(secp_ctx, path, session_priv)
+	} else {
+		let path =
+			pks.zip(tlvs.map(|tlv| BlindedPathWithPadding {
+				tlvs: tlv,
+				round_off: MESSAGE_PADDING_ROUND_OFF,
+			}));
+		utils::construct_blinded_hops(secp_ctx, path, session_priv)
+	}
 }

lightning/src/blinded_path/payment.rs

@@ -494,7 +494,6 @@ impl Writeable for UnauthenticatedReceiveTlvs {
 
 impl<'a> Writeable for BlindedPaymentTlvsRef<'a> {
 	fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
-		// TODO: write padding
 		match self {
 			Self::Forward(tlvs) => tlvs.write(w)?,
 			Self::Receive(tlvs) => tlvs.write(w)?,