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

@@ -65,7 +65,6 @@ 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),
@@ -77,7 +76,6 @@ 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),
 		});
@@ -138,17 +136,25 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 ) -> Result<Vec<BlindedHop>, secp256k1::Error> {
 	let pks = intermediate_nodes.iter().map(|node| &node.node_id)
 		.chain(core::iter::once(&recipient_node_id));
-	let tlvs = pks.clone()
+	let tlvs: Vec<ControlTlvs> = pks.clone()
 		.skip(1) // The first node's TLVs contains the next node's pubkey
 		.zip(intermediate_nodes.iter().map(|node| node.short_channel_id))
 		.map(|(pubkey, scid)| match scid {
 			Some(scid) => NextMessageHop::ShortChannelId(scid),
 			None => NextMessageHop::NodeId(*pubkey),
 		})
 		.map(|next_hop| ControlTlvs::Forward(ForwardTlvs { next_hop, next_blinding_override: None }))
-		.chain(core::iter::once(ControlTlvs::Receive(ReceiveTlvs{ context: Some(context) })));
+		.chain(core::iter::once(ControlTlvs::Receive(ReceiveTlvs{ context: Some(context) })))
+		.collect();
 
-	utils::construct_blinded_hops(secp_ctx, pks, tlvs, session_priv)
+	let max_length = tlvs.iter()
+		.max_by_key(|c| c.serialized_length())
+		.map(|c| c.serialized_length())
+		.unwrap_or(0);
+
+	let length_tlvs = tlvs.into_iter().map(move |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

@@ -359,3 +359,16 @@ 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)
+	}
+}