Introduce Padding for blinded message paths.

This commit make the size of each onion packet in a blinded path of
the same size, that is equal to the size of that largest onion
packet.

Author: shaavan

lightning/src/blinded_path/message.rs

@@ -44,6 +44,7 @@ pub struct ForwardNode {
 
 /// TLVs to encode in an intermediate onion message packet's hop data. When provided in a blinded
 /// route, they are encoded into [`BlindedHop::encrypted_payload`].
+#[derive(Clone)]
 pub(crate) struct ForwardTlvs {
 	/// The next hop in the onion message's path.
 	pub(crate) next_hop: NextMessageHop,
@@ -53,6 +54,7 @@ pub(crate) struct ForwardTlvs {
 }
 
 /// Similar to [`ForwardTlvs`], but these TLVs are for the final node.
+#[derive(Clone)]
 pub(crate) struct ReceiveTlvs {
 	/// If `context` is `Some`, it is used to identify the blinded path that this onion message is
 	/// sending to. This is useful for receivers to check that said blinded path is being used in
@@ -66,7 +68,6 @@ 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),
@@ -78,7 +79,6 @@ 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),
 		});
@@ -187,7 +187,14 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 		.map(|next_hop| ControlTlvs::Forward(ForwardTlvs { next_hop, next_blinding_override: None }))
 		.chain(core::iter::once(ControlTlvs::Receive(ReceiveTlvs{ context: Some(context) })));
 
-	utils::construct_blinded_hops(secp_ctx, pks, tlvs, session_priv)
+	let max_length = tlvs.clone()
+		.max_by_key(|c| c.serialized_length())
+		.map(|c| c.serialized_length())
+		.unwrap_or(0);
+
+	let length_tlvs = tlvs.map(|tlv| (max_length, tlv));
+
+	utils::construct_blinded_hops(secp_ctx, pks, length_tlvs, session_priv)
 }
 
 // Advance the blinded onion message path by one hop, so make the second hop into the new

lightning/src/blinded_path/utils.rs

@@ -151,6 +151,7 @@ impl Padding {
 }
 
 impl Readable for Padding {
+	/// Reads and discards the padding data.
 	#[inline]
 	fn read<R: io::Read>(reader: &mut R) -> Result<Self, DecodeError> {
 		loop {

lightning/src/onion_message/packet.rs

@@ -306,6 +306,7 @@ for Payload<ParsedOnionMessageContents<<H as CustomOnionMessageHandler>::CustomM
 /// or received. Thus we read a `ControlTlvs` rather than reading a [`ForwardTlvs`] or
 /// [`ReceiveTlvs`] directly. Also useful on the encoding side to keep forward and receive TLVs in
 /// the same iterator.
+#[derive(Clone)]
 pub(crate) enum ControlTlvs {
 	/// This onion message is intended to be forwarded.
 	Forward(ForwardTlvs),
@@ -359,3 +360,16 @@ impl Writeable for ControlTlvs {
 		}
 	}
 }
+
+impl Writeable for (usize, ControlTlvs) {
+	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
+		let length = self.0 - self.1.serialized_length();
+		let padding = Some(Padding::new(length));
+
+		encode_tlv_stream!(writer, {
+			(1, padding, option)
+		});
+
+		self.1.write(writer)
+	}
+}