Introduce Padding for blinded payment paths

1. Introducing the padding increase the Tlvs size and hence reduced the
  max path length of payment paths.
2. This leads to reducing the max path length from 18 -> 17 in
   `forward_checks_failure` which is accounted for in this commit.

Author: shaavan

lightning/src/blinded_path/payment.rs

@@ -14,7 +14,7 @@
 use bitcoin::secp256k1::{self, PublicKey, Secp256k1, SecretKey};
 
 use crate::blinded_path::{BlindedHop, BlindedPath, IntroductionNode, NodeIdLookUp};
-use crate::blinded_path::utils;
+use crate::blinded_path::utils::{self, Padding};
 use crate::crypto::streams::ChaChaPolyReadAdapter;
 use crate::io;
 use crate::io::Cursor;
@@ -86,6 +86,7 @@ pub(crate) enum BlindedPaymentTlvs {
 }
 
 // Used to include forward and receive TLVs in the same iterator for encoding.
+#[derive(Clone)]
 enum BlindedPaymentTlvsRef<'a> {
 	Forward(&'a ForwardTlvs),
 	Receive(&'a ReceiveTlvs),
@@ -226,7 +227,6 @@ impl Writeable for ReceiveTlvs {
 
 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)?,
@@ -235,6 +235,19 @@ impl<'a> Writeable for BlindedPaymentTlvsRef<'a> {
 	}
 }
 
+impl<'a> Writeable for (usize, BlindedPaymentTlvsRef<'a>) {
+	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)
+	}
+}
+
 impl Readable for BlindedPaymentTlvs {
 	fn read<R: io::Read>(r: &mut R) -> Result<Self, DecodeError> {
 		_init_and_read_tlv_stream!(r, {
@@ -278,7 +291,15 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 		.chain(core::iter::once(&payee_node_id));
 	let tlvs = intermediate_nodes.iter().map(|node| BlindedPaymentTlvsRef::Forward(&node.tlvs))
 		.chain(core::iter::once(BlindedPaymentTlvsRef::Receive(&payee_tlvs)));
-	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 payment path by one hop, so make the second hop into the new

lightning/src/ln/blinded_payment_tests.rs

@@ -289,7 +289,7 @@ fn do_forward_checks_failure(check: ForwardCheckFail, intro_fails: bool) {
 	let mut route_params = get_blinded_route_parameters(amt_msat, payment_secret, 1, 1_0000_0000,
 		nodes.iter().skip(1).map(|n| n.node.get_our_node_id()).collect(),
 		&[&chan_upd_1_2, &chan_upd_2_3], &chanmon_cfgs[3].keys_manager);
-	route_params.payment_params.max_path_length = 18;
+	route_params.payment_params.max_path_length = 17;
 
 	let route = get_route(&nodes[0], &route_params).unwrap();
 	node_cfgs[0].router.expect_find_route(route_params.clone(), Ok(route.clone()));