Introduce Padding for Payment and Message Blinded Tlvs

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),
 		});
@@ -495,6 +493,10 @@ 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],
@@ -504,6 +506,8 @@ 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
@@ -517,7 +521,15 @@ 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

@@ -14,7 +14,7 @@ use bitcoin::hashes::sha256::Hash as Sha256;
 use bitcoin::secp256k1::ecdh::SharedSecret;
 use bitcoin::secp256k1::{self, PublicKey, Secp256k1, SecretKey};
 
-use crate::blinded_path::utils;
+use crate::blinded_path::utils::{self, BlindedPathWithPadding};
 use crate::blinded_path::{BlindedHop, BlindedPath, IntroductionNode, NodeIdLookUp};
 use crate::crypto::streams::ChaChaPolyReadAdapter;
 use crate::io;
@@ -508,7 +508,6 @@ 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)?,
@@ -520,7 +519,10 @@ impl<'a> Writeable for BlindedPaymentTlvsRef<'a> {
 impl Readable for BlindedPaymentTlvs {
 	fn read<R: io::Read>(r: &mut R) -> Result<Self, DecodeError> {
 		_init_and_read_tlv_stream!(r, {
-			(1, _padding, option),
+			// Reasoning: Padding refers to filler data added to a packet to increase
+			// its size and obscure its actual length. Since padding contains no meaningful
+			// information, we can safely omit reading it here.
+			// (1, _padding, option),
 			(2, scid, option),
 			(8, next_blinding_override, option),
 			(10, payment_relay, option),
@@ -530,7 +532,6 @@ impl Readable for BlindedPaymentTlvs {
 			(65537, payment_context, option),
 			(65539, authentication, option),
 		});
-		let _padding: Option<utils::Padding> = _padding;
 
 		if let Some(short_channel_id) = scid {
 			if payment_secret.is_some() {
@@ -559,6 +560,10 @@ impl Readable for BlindedPaymentTlvs {
 	}
 }
 
+/// Represents the padding round off size (in bytes) that
+/// is used to pad payment bilnded path's [`BlindedHop`]
+pub(crate) const PAYMENT_PADDING_ROUND_OFF: usize = 30;
+
 /// Construct blinded payment hops for the given `intermediate_nodes` and payee info.
 pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 	secp_ctx: &Secp256k1<T>, intermediate_nodes: &[PaymentForwardNode], payee_node_id: PublicKey,
@@ -571,7 +576,9 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 		.map(|node| BlindedPaymentTlvsRef::Forward(&node.tlvs))
 		.chain(core::iter::once(BlindedPaymentTlvsRef::Receive(&payee_tlvs)));
 
-	let path = pks.zip(tlvs);
+	let path = pks.zip(
+		tlvs.map(|tlv| BlindedPathWithPadding { tlvs: tlv, round_off: PAYMENT_PADDING_ROUND_OFF }),
+	);
 
 	utils::construct_blinded_hops(secp_ctx, path, session_priv)
 }

lightning/src/blinded_path/utils.rs

@@ -18,12 +18,10 @@ use bitcoin::secp256k1::{self, PublicKey, Scalar, Secp256k1, SecretKey};
 use super::message::BlindedMessagePath;
 use super::{BlindedHop, BlindedPath};
 use crate::crypto::streams::ChaChaPolyWriteAdapter;
-use crate::ln::msgs::DecodeError;
+use crate::io;
 use crate::ln::onion_utils;
 use crate::onion_message::messenger::Destination;
-use crate::util::ser::{Readable, Writeable};
-
-use crate::io;
+use crate::util::ser::{Writeable, Writer};
 
 use core::borrow::Borrow;
 
@@ -196,19 +194,82 @@ fn encrypt_payload<P: Writeable>(payload: P, encrypted_tlvs_rho: [u8; 32]) -> Ve
 	write_adapter.encode()
 }
 
-/// Blinded path encrypted payloads may be padded to ensure they are equal length.
+/// A data structure used exclusively to pad blinded path payloads, ensuring they are of
+/// equal length. Padding is written at Type 1 for compatibility with the lightning specification.
 ///
-/// Reads padding to the end, ignoring what's read.
-pub(crate) struct Padding {}
-impl Readable for Padding {
-	#[inline]
-	fn read<R: io::Read>(reader: &mut R) -> Result<Self, DecodeError> {
+/// For more details, see the [BOLTs Specification - Encrypted Recipient Data](https://github.com/lightning/bolts/blob/8707471dbc23245fb4d84c5f5babac1197f1583e/04-onion-routing.md#inside-encrypted_recipient_data-encrypted_data_tlv).
+pub(crate) struct BlindedPathPadding {
+	length: usize,
+}
+
+impl BlindedPathPadding {
+	/// Creates a new [`BlindedPathPadding`] instance with a specified size.
+	/// Use this method when defining the padding size before writing
+	/// an encrypted payload.
+	pub fn new(length: usize) -> Self {
+		Self { length }
+	}
+}
+
+impl Writeable for BlindedPathPadding {
+	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
+		const BUFFER_SIZE: usize = 1024;
+		let buffer = [0u8; BUFFER_SIZE];
+
+		let mut remaining = self.length;
 		loop {
-			let mut buf = [0; 8192];
-			if reader.read(&mut buf[..])? == 0 {
+			let to_write = core::cmp::min(remaining, BUFFER_SIZE);
+			writer.write_all(&buffer[..to_write])?;
+			remaining -= to_write;
+			if remaining == 0 {
 				break;
 			}
 		}
-		Ok(Self {})
+		Ok(())
+	}
+}
+
+/// Padding storage requires two extra bytes:
+/// - One byte for the type.
+/// - One byte for the padding length.
+/// This constant accounts for that overhead.
+const TLV_OVERHEAD: usize = 2;
+
+/// A generic struct that applies padding to blinded path TLVs, rounding their size off to `round_off`
+pub(crate) struct BlindedPathWithPadding<T: Writeable> {
+	pub(crate) tlvs: T,
+	pub(crate) round_off: usize,
+}
+
+impl<T: Writeable> Writeable for BlindedPathWithPadding<T> {
+	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
+		let tlv_length = self.tlvs.serialized_length();
+		let total_length = tlv_length + TLV_OVERHEAD;
+
+		let padding_length =
+			(total_length + self.round_off - 1) / self.round_off * self.round_off - total_length;
+
+		let padding = Some(BlindedPathPadding::new(padding_length));
+
+		encode_tlv_stream!(writer, {
+			(1, padding, option),
+		});
+
+		self.tlvs.write(writer)
 	}
 }
+
+#[cfg(test)]
+/// Checks if all the packets in the blinded path are properly padded.
+pub fn is_padded(hops: &[BlindedHop], padding_round_off: usize) -> bool {
+	let first_hop = hops.first().expect("BlindedPath must have at least one hop");
+	let first_payload_size = first_hop.encrypted_payload.len();
+
+	// The unencrypted payload data is padded before getting encrypted.
+	// Assuming the first payload is padded properly, get the extra data length.
+	let extra_length = first_payload_size % padding_round_off;
+	hops.iter().all(|hop| {
+		// Check that every packet is padded to the round off length subtracting the extra length.
+		(hop.encrypted_payload.len() - extra_length) % padding_round_off == 0
+	})
+}

lightning/src/ln/blinded_payment_tests.rs

@@ -13,7 +13,8 @@ use bitcoin::secp256k1::{PublicKey, Scalar, Secp256k1, SecretKey, schnorr};
 use bitcoin::secp256k1::ecdh::SharedSecret;
 use bitcoin::secp256k1::ecdsa::{RecoverableSignature, Signature};
 use crate::blinded_path;
-use crate::blinded_path::payment::{BlindedPaymentPath, Bolt12RefundContext, PaymentForwardNode, ForwardTlvs, PaymentConstraints, PaymentContext, PaymentRelay, UnauthenticatedReceiveTlvs};
+use crate::blinded_path::payment::{BlindedPaymentPath, Bolt12RefundContext, ForwardTlvs, PaymentConstraints, PaymentContext, PaymentForwardNode, PaymentRelay, UnauthenticatedReceiveTlvs, PAYMENT_PADDING_ROUND_OFF};
+use crate::blinded_path::utils::is_padded;
 use crate::events::{Event, HTLCDestination, PaymentFailureReason};
 use crate::ln::types::ChannelId;
 use crate::types::payment::{PaymentHash, PaymentSecret};
@@ -353,7 +354,7 @@ 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 = 17;
+	route_params.payment_params.max_path_length = 16;
 
 	let route = get_route(&nodes[0], &route_params).unwrap();
 	node_cfgs[0].router.expect_find_route(route_params.clone(), Ok(route.clone()));
@@ -878,6 +879,8 @@ fn do_multi_hop_receiver_fail(check: ReceiveCheckFail) {
 		nodes.iter().skip(1).map(|n| n.node.get_our_node_id()).collect(), &[&chan_upd_1_2],
 		&chanmon_cfgs[2].keys_manager);
 
+	route_params.payment_params.max_path_length = 17;
+
 	let route = if check == ReceiveCheckFail::ProcessPendingHTLCsCheck {
 		let mut route = get_route(&nodes[0], &route_params).unwrap();
 		// Set the final CLTV expiry too low to trigger the failure in process_pending_htlc_forwards.
@@ -1426,6 +1429,42 @@ fn fails_receive_tlvs_authentication() {
 	);
 }
 
+#[test]
+fn blinded_payment_path_padding() {
+	// Make sure that for a blinded payment path, all encrypted payloads are padded to equal lengths.
+	let chanmon_cfgs = create_chanmon_cfgs(5);
+	let node_cfgs = create_node_cfgs(5, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(5, &node_cfgs, &[None, None, None, None, None]);
+	let mut nodes = create_network(5, &node_cfgs, &node_chanmgrs);
+	create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 1_000_000, 0);
+	create_announced_chan_between_nodes_with_value(&nodes, 1, 2, 1_000_000, 0);
+	let chan_upd_2_3 = create_announced_chan_between_nodes_with_value(&nodes, 2, 3, 1_000_000, 0).0.contents;
+	let chan_upd_3_4 = create_announced_chan_between_nodes_with_value(&nodes, 3, 4, 1_000_000, 0).0.contents;
+
+	// Get all our nodes onto the same height so payments don't fail for CLTV violations.
+	connect_blocks(&nodes[0], nodes[4].best_block_info().1 - nodes[0].best_block_info().1);
+	connect_blocks(&nodes[1], nodes[4].best_block_info().1 - nodes[1].best_block_info().1);
+	connect_blocks(&nodes[2], nodes[4].best_block_info().1 - nodes[2].best_block_info().1);
+	assert_eq!(nodes[4].best_block_info().1, nodes[3].best_block_info().1);
+
+	let amt_msat = 5000;
+	let (payment_preimage, payment_hash, payment_secret) = get_payment_preimage_hash(&nodes[4], Some(amt_msat), None);
+
+	let blinded_path = blinded_payment_path(payment_secret, 1, 1_0000_0000,
+		nodes.iter().skip(2).map(|n| n.node.get_our_node_id()).collect(), &[&chan_upd_2_3, &chan_upd_3_4],
+		&chanmon_cfgs[4].keys_manager
+	);
+
+	assert!(is_padded(&blinded_path.blinded_hops(), PAYMENT_PADDING_ROUND_OFF));
+
+	let route_params = RouteParameters::from_payment_params_and_value(PaymentParameters::blinded(vec![blinded_path]), amt_msat);
+
+	nodes[0].node.send_payment(payment_hash, RecipientOnionFields::spontaneous_empty(), PaymentId(payment_hash.0), route_params, Retry::Attempts(0)).unwrap();
+	check_added_monitors(&nodes[0], 1);
+	pass_along_route(&nodes[0], &[&[&nodes[1], &nodes[2], &nodes[3], &nodes[4]]], amt_msat, payment_hash, payment_secret);
+	claim_payment(&nodes[0], &[&nodes[1], &nodes[2], &nodes[3], &nodes[4]], payment_preimage);
+}
+
 fn secret_from_hex(hex: &str) -> SecretKey {
 	SecretKey::from_slice(&<Vec<u8>>::from_hex(hex).unwrap()).unwrap()
 }

lightning/src/onion_message/functional_tests.rs

@@ -21,8 +21,9 @@ use super::offers::{OffersMessage, OffersMessageHandler};
 use super::packet::{OnionMessageContents, Packet};
 use crate::blinded_path::message::{
 	AsyncPaymentsContext, BlindedMessagePath, DNSResolverContext, MessageContext,
-	MessageForwardNode, OffersContext,
+	MessageForwardNode, OffersContext, MESSAGE_PADDING_ROUND_OFF,
 };
+use crate::blinded_path::utils::is_padded;
 use crate::blinded_path::EmptyNodeIdLookUp;
 use crate::events::{Event, EventsProvider};
 use crate::ln::msgs::{self, BaseMessageHandler, DecodeError, OnionMessageHandler};
@@ -596,6 +597,65 @@ fn too_big_packet_error() {
 	assert_eq!(err, SendError::TooBigPacket);
 }
 
+#[test]
+fn test_blinded_path_padding_for_full_length_path() {
+	// Check that for a full blinded path, all encrypted payload are padded to rounded-off length.
+	let nodes = create_nodes(4);
+	let test_msg = TestCustomMessage::Pong;
+
+	let secp_ctx = Secp256k1::new();
+	let intermediate_nodes = [
+		MessageForwardNode { node_id: nodes[1].node_id, short_channel_id: None },
+		MessageForwardNode { node_id: nodes[2].node_id, short_channel_id: None },
+	];
+	// Update the context to create a larger final receive TLVs, ensuring that
+	// the hop sizes vary before padding.
+	let context = MessageContext::Custom(vec![0u8; 42]);
+	let blinded_path = BlindedMessagePath::new(
+		&intermediate_nodes,
+		nodes[3].node_id,
+		context,
+		&*nodes[3].entropy_source,
+		&secp_ctx,
+	)
+	.unwrap();
+
+	assert!(is_padded(&blinded_path.blinded_hops(), MESSAGE_PADDING_ROUND_OFF));
+
+	let destination = Destination::BlindedPath(blinded_path);
+	let instructions = MessageSendInstructions::WithoutReplyPath { destination };
+
+	nodes[0].messenger.send_onion_message(test_msg, instructions).unwrap();
+	nodes[3].custom_message_handler.expect_message(TestCustomMessage::Pong);
+	pass_along_path(&nodes);
+}
+
+#[test]
+fn test_blinded_path_no_padding_for_compact_path() {
+	// Check that for a compact blinded path, no padding is applied.
+	let nodes = create_nodes(4);
+	let secp_ctx = Secp256k1::new();
+
+	// Include some short_channel_id, so that MessageRouter uses this to create compact blinded paths.
+	let intermediate_nodes = [
+		MessageForwardNode { node_id: nodes[1].node_id, short_channel_id: Some(24) },
+		MessageForwardNode { node_id: nodes[2].node_id, short_channel_id: Some(25) },
+	];
+	// Update the context to create a larger final receive TLVs, ensuring that
+	// the hop sizes vary before padding.
+	let context = MessageContext::Custom(vec![0u8; 42]);
+	let blinded_path = BlindedMessagePath::new(
+		&intermediate_nodes,
+		nodes[3].node_id,
+		context,
+		&*nodes[3].entropy_source,
+		&secp_ctx,
+	)
+	.unwrap();
+
+	assert!(!is_padded(&blinded_path.blinded_hops(), MESSAGE_PADDING_ROUND_OFF));
+}
+
 #[test]
 fn we_are_intro_node() {
 	// If we are sending straight to a blinded path and we are the introduction node, we need to