_ Git - rust-lightning/blob - lightning/src/onion_message/messenger.rs

   1 // This file is Copyright its original authors, visible in version control
   2 // history.
   3 //
   4 // This file is licensed under the Apache License, Version 2.0 <LICENSE-APACHE
   5 // or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
   7 // You may not use this file except in accordance with one or both of these
   8 // licenses.
   9
  10 //! LDK sends, receives, and forwards onion messages via the [`OnionMessenger`]. See its docs for
  11 //! more information.
  12
  13 use bitcoin::hashes::{Hash, HashEngine};
  14 use bitcoin::hashes::hmac::{Hmac, HmacEngine};
  15 use bitcoin::hashes::sha256::Hash as Sha256;
  16 use bitcoin::secp256k1::{self, PublicKey, Scalar, Secp256k1, SecretKey};
  17
  18 use chain::keysinterface::{InMemorySigner, KeysInterface, KeysManager, Recipient, Sign};
  19 use ln::msgs::{self, OnionMessageHandler};
  20 use ln::onion_utils;
  21 use super::blinded_route::{BlindedRoute, ForwardTlvs, ReceiveTlvs};
  22 use super::packet::{BIG_PACKET_HOP_DATA_LEN, ForwardControlTlvs, Packet, Payload, ReceiveControlTlvs, SMALL_PACKET_HOP_DATA_LEN};
  23 use super::utils;
  24 use util::events::OnionMessageProvider;
  25 use util::logger::Logger;
  26 use util::ser::Writeable;
  27
  28 use core::ops::Deref;
  29 use sync::{Arc, Mutex};
  30 use prelude::*;
  31
  32 /// A sender, receiver and forwarder of onion messages. In upcoming releases, this object will be
  33 /// used to retrieve invoices and fulfill invoice requests from [offers]. Currently, only sending
  34 /// and receiving empty onion messages is supported.
  35 ///
  36 /// # Example
  37 ///
  38 /// ```
  39 /// # extern crate bitcoin;
  40 /// # use bitcoin::hashes::_export::_core::time::Duration;
  41 /// # use bitcoin::secp256k1::{PublicKey, Secp256k1, SecretKey};
  42 /// # use lightning::chain::keysinterface::{InMemorySigner, KeysManager, KeysInterface};
  43 /// # use lightning::onion_message::{BlindedRoute, Destination, OnionMessenger};
  44 /// # use lightning::util::logger::{Logger, Record};
  45 /// # use std::sync::Arc;
  46 /// # struct FakeLogger {};
  47 /// # impl Logger for FakeLogger {
  48 /// #     fn log(&self, record: &Record) { unimplemented!() }
  49 /// # }
  50 /// # let seed = [42u8; 32];
  51 /// # let time = Duration::from_secs(123456);
  52 /// # let keys_manager = KeysManager::new(&seed, time.as_secs(), time.subsec_nanos());
  53 /// # let logger = Arc::new(FakeLogger {});
  54 /// # let node_secret = SecretKey::from_slice(&hex::decode("0101010101010101010101010101010101010101010101010101010101010101").unwrap()[..]).unwrap();
  55 /// # let secp_ctx = Secp256k1::new();
  56 /// # let hop_node_id1 = PublicKey::from_secret_key(&secp_ctx, &node_secret);
  57 /// # let (hop_node_id2, hop_node_id3, hop_node_id4) = (hop_node_id1, hop_node_id1,
  58 /// hop_node_id1);
  59 /// # let destination_node_id = hop_node_id1;
  60 /// #
  61 /// // Create the onion messenger. This must use the same `keys_manager` as is passed to your
  62 /// // ChannelManager.
  63 /// let onion_messenger = OnionMessenger::new(&keys_manager, logger);
  64 ///
  65 /// // Send an empty onion message to a node id.
  66 /// let intermediate_hops = [hop_node_id1, hop_node_id2];
  67 /// let reply_path = None;
  68 /// onion_messenger.send_onion_message(&intermediate_hops, Destination::Node(destination_node_id), reply_path);
  69 ///
  70 /// // Create a blinded route to yourself, for someone to send an onion message to.
  71 /// # let your_node_id = hop_node_id1;
  72 /// let hops = [hop_node_id3, hop_node_id4, your_node_id];
  73 /// let blinded_route = BlindedRoute::new::<InMemorySigner, _, _>(&hops, &keys_manager, &secp_ctx).unwrap();
  74 ///
  75 /// // Send an empty onion message to a blinded route.
  76 /// # let intermediate_hops = [hop_node_id1, hop_node_id2];
  77 /// let reply_path = None;
  78 /// onion_messenger.send_onion_message(&intermediate_hops, Destination::BlindedRoute(blinded_route), reply_path);
  79 /// ```
  80 ///
  81 /// [offers]: <https://github.com/lightning/bolts/pull/798>
  82 /// [`OnionMessenger`]: crate::onion_message::OnionMessenger
  83 pub struct OnionMessenger<Signer: Sign, K: Deref, L: Deref>
  84         where K::Target: KeysInterface<Signer = Signer>,
  85               L::Target: Logger,
  86 {
  87         keys_manager: K,
  88         logger: L,
  89         pending_messages: Mutex<HashMap<PublicKey, VecDeque<msgs::OnionMessage>>>,
  90         secp_ctx: Secp256k1<secp256k1::All>,
  91         // Coming soon:
  92         // invoice_handler: InvoiceHandler,
  93         // custom_handler: CustomHandler, // handles custom onion messages
  94 }
  95
  96 /// The destination of an onion message.
  97 pub enum Destination {
  98         /// We're sending this onion message to a node.
  99         Node(PublicKey),
 100         /// We're sending this onion message to a blinded route.
 101         BlindedRoute(BlindedRoute),
 102 }
 103
 104 impl Destination {
 105         pub(super) fn num_hops(&self) -> usize {
 106                 match self {
 107                         Destination::Node(_) => 1,
 108                         Destination::BlindedRoute(BlindedRoute { blinded_hops, .. }) => blinded_hops.len(),
 109                 }
 110         }
 111 }
 112
 113 /// Errors that may occur when [sending an onion message].
 114 ///
 115 /// [sending an onion message]: OnionMessenger::send_onion_message
 116 #[derive(Debug, PartialEq)]
 117 pub enum SendError {
 118         /// Errored computing onion message packet keys.
 119         Secp256k1(secp256k1::Error),
 120         /// Because implementations such as Eclair will drop onion messages where the message packet
 121         /// exceeds 32834 bytes, we refuse to send messages where the packet exceeds this size.
 122         TooBigPacket,
 123         /// The provided [`Destination`] was an invalid [`BlindedRoute`], due to having fewer than two
 124         /// blinded hops.
 125         TooFewBlindedHops,
 126         /// Our next-hop peer was offline or does not support onion message forwarding.
 127         InvalidFirstHop,
 128         /// Our next-hop peer's buffer was full or our total outbound buffer was full.
 129         BufferFull,
 130 }
 131
 132 impl<Signer: Sign, K: Deref, L: Deref> OnionMessenger<Signer, K, L>
 133         where K::Target: KeysInterface<Signer = Signer>,
 134               L::Target: Logger,
 135 {
 136         /// Constructs a new `OnionMessenger` to send, forward, and delegate received onion messages to
 137         /// their respective handlers.
 138         pub fn new(keys_manager: K, logger: L) -> Self {
 139                 let mut secp_ctx = Secp256k1::new();
 140                 secp_ctx.seeded_randomize(&keys_manager.get_secure_random_bytes());
 141                 OnionMessenger {
 142                         keys_manager,
 143                         pending_messages: Mutex::new(HashMap::new()),
 144                         secp_ctx,
 145                         logger,
 146                 }
 147         }
 148
 149         /// Send an empty onion message to `destination`, routing it through `intermediate_nodes`.
 150         /// See [`OnionMessenger`] for example usage.
 151         pub fn send_onion_message(&self, intermediate_nodes: &[PublicKey], destination: Destination, reply_path: Option<BlindedRoute>) -> Result<(), SendError> {
 152                 if let Destination::BlindedRoute(BlindedRoute { ref blinded_hops, .. }) = destination {
 153                         if blinded_hops.len() < 2 {
 154                                 return Err(SendError::TooFewBlindedHops);
 155                         }
 156                 }
 157                 let blinding_secret_bytes = self.keys_manager.get_secure_random_bytes();
 158                 let blinding_secret = SecretKey::from_slice(&blinding_secret_bytes[..]).expect("RNG is busted");
 159                 let (introduction_node_id, blinding_point) = if intermediate_nodes.len() != 0 {
 160                         (intermediate_nodes[0], PublicKey::from_secret_key(&self.secp_ctx, &blinding_secret))
 161                 } else {
 162                         match destination {
 163                                 Destination::Node(pk) => (pk, PublicKey::from_secret_key(&self.secp_ctx, &blinding_secret)),
 164                                 Destination::BlindedRoute(BlindedRoute { introduction_node_id, blinding_point, .. }) =>
 165                                         (introduction_node_id, blinding_point),
 166                         }
 167                 };
 168                 let (packet_payloads, packet_keys) = packet_payloads_and_keys(
 169                         &self.secp_ctx, intermediate_nodes, destination, reply_path, &blinding_secret)
 170                         .map_err(|e| SendError::Secp256k1(e))?;
 171
 172                 let prng_seed = self.keys_manager.get_secure_random_bytes();
 173                 let onion_routing_packet = construct_onion_message_packet(
 174                         packet_payloads, packet_keys, prng_seed).map_err(|()| SendError::TooBigPacket)?;
 175
 176                 let mut pending_per_peer_msgs = self.pending_messages.lock().unwrap();
 177                 if outbound_buffer_full(&introduction_node_id, &pending_per_peer_msgs) { return Err(SendError::BufferFull) }
 178                 match pending_per_peer_msgs.entry(introduction_node_id) {
 179                         hash_map::Entry::Vacant(_) => Err(SendError::InvalidFirstHop),
 180                         hash_map::Entry::Occupied(mut e) => {
 181                                 e.get_mut().push_back(msgs::OnionMessage { blinding_point, onion_routing_packet });
 182                                 Ok(())
 183                         }
 184                 }
 185         }
 186
 187         #[cfg(test)]
 188         pub(super) fn release_pending_msgs(&self) -> HashMap<PublicKey, VecDeque<msgs::OnionMessage>> {
 189                 let mut pending_msgs = self.pending_messages.lock().unwrap();
 190                 let mut msgs = HashMap::new();
 191                 // We don't want to disconnect the peers by removing them entirely from the original map, so we
 192                 // swap the pending message buffers individually.
 193                 for (peer_node_id, pending_messages) in &mut *pending_msgs {
 194                         msgs.insert(*peer_node_id, core::mem::take(pending_messages));
 195                 }
 196                 msgs
 197         }
 198 }
 199
 200 fn outbound_buffer_full(peer_node_id: &PublicKey, buffer: &HashMap<PublicKey, VecDeque<msgs::OnionMessage>>) -> bool {
 201         const MAX_TOTAL_BUFFER_SIZE: usize = (1 << 20) * 128;
 202         const MAX_PER_PEER_BUFFER_SIZE: usize = (1 << 10) * 256;
 203         let mut total_buffered_bytes = 0;
 204         let mut peer_buffered_bytes = 0;
 205         for (pk, peer_buf) in buffer {
 206                 for om in peer_buf {
 207                         let om_len = om.serialized_length();
 208                         if pk == peer_node_id {
 209                                 peer_buffered_bytes += om_len;
 210                         }
 211                         total_buffered_bytes += om_len;
 212
 213                         if total_buffered_bytes >= MAX_TOTAL_BUFFER_SIZE ||
 214                                 peer_buffered_bytes >= MAX_PER_PEER_BUFFER_SIZE
 215                         {
 216                                 return true
 217                         }
 218                 }
 219         }
 220         false
 221 }
 222
 223 impl<Signer: Sign, K: Deref, L: Deref> OnionMessageHandler for OnionMessenger<Signer, K, L>
 224         where K::Target: KeysInterface<Signer = Signer>,
 225               L::Target: Logger,
 226 {
 227         /// Handle an incoming onion message. Currently, if a message was destined for us we will log, but
 228         /// soon we'll delegate the onion message to a handler that can generate invoices or send
 229         /// payments.
 230         fn handle_onion_message(&self, _peer_node_id: &PublicKey, msg: &msgs::OnionMessage) {
 231                 let control_tlvs_ss = match self.keys_manager.ecdh(Recipient::Node, &msg.blinding_point, None) {
 232                         Ok(ss) => ss,
 233                         Err(e) =>  {
 234                                 log_error!(self.logger, "Failed to retrieve node secret: {:?}", e);
 235                                 return
 236                         }
 237                 };
 238                 let onion_decode_ss = {
 239                         let blinding_factor = {
 240                                 let mut hmac = HmacEngine::<Sha256>::new(b"blinded_node_id");
 241                                 hmac.input(control_tlvs_ss.as_ref());
 242                                 Hmac::from_engine(hmac).into_inner()
 243                         };
 244                         match self.keys_manager.ecdh(Recipient::Node, &msg.onion_routing_packet.public_key,
 245                                 Some(&Scalar::from_be_bytes(blinding_factor).unwrap()))
 246                         {
 247                                 Ok(ss) => ss.secret_bytes(),
 248                                 Err(()) => {
 249                                         log_trace!(self.logger, "Failed to compute onion packet shared secret");
 250                                         return
 251                                 }
 252                         }
 253                 };
 254                 match onion_utils::decode_next_hop(onion_decode_ss, &msg.onion_routing_packet.hop_data[..],
 255                         msg.onion_routing_packet.hmac, control_tlvs_ss)
 256                 {
 257                         Ok((Payload::Receive {
 258                                 control_tlvs: ReceiveControlTlvs::Unblinded(ReceiveTlvs { path_id }), reply_path,
 259                         }, None)) => {
 260                                 log_info!(self.logger,
 261                                         "Received an onion message with path_id: {:02x?} and {}reply_path",
 262                                                 path_id, if reply_path.is_some() { "" } else { "no " });
 263                         },
 264                         Ok((Payload::Forward(ForwardControlTlvs::Unblinded(ForwardTlvs {
 265                                 next_node_id, next_blinding_override
 266                         })), Some((next_hop_hmac, new_packet_bytes)))) => {
 267                                 // TODO: we need to check whether `next_node_id` is our node, in which case this is a dummy
 268                                 // blinded hop and this onion message is destined for us. In this situation, we should keep
 269                                 // unwrapping the onion layers to get to the final payload. Since we don't have the option
 270                                 // of creating blinded routes with dummy hops currently, we should be ok to not handle this
 271                                 // for now.
 272                                 let new_pubkey = match onion_utils::next_hop_packet_pubkey(&self.secp_ctx, msg.onion_routing_packet.public_key, &onion_decode_ss) {
 273                                         Ok(pk) => pk,
 274                                         Err(e) => {
 275                                                 log_trace!(self.logger, "Failed to compute next hop packet pubkey: {}", e);
 276                                                 return
 277                                         }
 278                                 };
 279                                 let outgoing_packet = Packet {
 280                                         version: 0,
 281                                         public_key: new_pubkey,
 282                                         hop_data: new_packet_bytes,
 283                                         hmac: next_hop_hmac,
 284                                 };
 285                                 let onion_message = msgs::OnionMessage {
 286                                         blinding_point: match next_blinding_override {
 287                                                 Some(blinding_point) => blinding_point,
 288                                                 None => {
 289                                                         let blinding_factor = {
 290                                                                 let mut sha = Sha256::engine();
 291                                                                 sha.input(&msg.blinding_point.serialize()[..]);
 292                                                                 sha.input(control_tlvs_ss.as_ref());
 293                                                                 Sha256::from_engine(sha).into_inner()
 294                                                         };
 295                                                         let next_blinding_point = msg.blinding_point;
 296                                                         match next_blinding_point.mul_tweak(&self.secp_ctx, &Scalar::from_be_bytes(blinding_factor).unwrap()) {
 297                                                                 Ok(bp) => bp,
 298                                                                 Err(e) => {
 299                                                                         log_trace!(self.logger, "Failed to compute next blinding point: {}", e);
 300                                                                         return
 301                                                                 }
 302                                                         }
 303                                                 },
 304                                         },
 305                                         onion_routing_packet: outgoing_packet,
 306                                 };
 307
 308                                 let mut pending_per_peer_msgs = self.pending_messages.lock().unwrap();
 309                                 if outbound_buffer_full(&next_node_id, &pending_per_peer_msgs) {
 310                                         log_trace!(self.logger, "Dropping forwarded onion message to peer {:?}: outbound buffer full", next_node_id);
 311                                         return
 312                                 }
 313
 314                                 #[cfg(fuzzing)]
 315                                 pending_per_peer_msgs.entry(next_node_id).or_insert_with(VecDeque::new);
 316
 317                                 match pending_per_peer_msgs.entry(next_node_id) {
 318                                         hash_map::Entry::Vacant(_) => {
 319                                                 log_trace!(self.logger, "Dropping forwarded onion message to disconnected peer {:?}", next_node_id);
 320                                                 return
 321                                         },
 322                                         hash_map::Entry::Occupied(mut e) => {
 323                                                 e.get_mut().push_back(onion_message);
 324                                                 log_trace!(self.logger, "Forwarding an onion message to peer {}", next_node_id);
 325                                         }
 326                                 };
 327                         },
 328                         Err(e) => {
 329                                 log_trace!(self.logger, "Errored decoding onion message packet: {:?}", e);
 330                         },
 331                         _ => {
 332                                 log_trace!(self.logger, "Received bogus onion message packet, either the sender encoded a final hop as a forwarding hop or vice versa");
 333                         },
 334                 };
 335         }
 336
 337         fn peer_connected(&self, their_node_id: &PublicKey, init: &msgs::Init) {
 338                 if init.features.supports_onion_messages() {
 339                         let mut peers = self.pending_messages.lock().unwrap();
 340                         peers.insert(their_node_id.clone(), VecDeque::new());
 341                 }
 342         }
 343
 344         fn peer_disconnected(&self, their_node_id: &PublicKey, _no_connection_possible: bool) {
 345                 let mut pending_msgs = self.pending_messages.lock().unwrap();
 346                 pending_msgs.remove(their_node_id);
 347         }
 348 }
 349
 350 impl<Signer: Sign, K: Deref, L: Deref> OnionMessageProvider for OnionMessenger<Signer, K, L>
 351         where K::Target: KeysInterface<Signer = Signer>,
 352               L::Target: Logger,
 353 {
 354         fn next_onion_message_for_peer(&self, peer_node_id: PublicKey) -> Option<msgs::OnionMessage> {
 355                 let mut pending_msgs = self.pending_messages.lock().unwrap();
 356                 if let Some(msgs) = pending_msgs.get_mut(&peer_node_id) {
 357                         return msgs.pop_front()
 358                 }
 359                 None
 360         }
 361 }
 362
 363 // TODO: parameterize the below Simple* types with OnionMessenger and handle the messages it
 364 // produces
 365 /// Useful for simplifying the parameters of [`SimpleArcChannelManager`] and
 366 /// [`SimpleArcPeerManager`]. See their docs for more details.
 367 ///
 368 ///[`SimpleArcChannelManager`]: crate::ln::channelmanager::SimpleArcChannelManager
 369 ///[`SimpleArcPeerManager`]: crate::ln::peer_handler::SimpleArcPeerManager
 370 pub type SimpleArcOnionMessenger<L> = OnionMessenger<InMemorySigner, Arc<KeysManager>, Arc<L>>;
 371 /// Useful for simplifying the parameters of [`SimpleRefChannelManager`] and
 372 /// [`SimpleRefPeerManager`]. See their docs for more details.
 373 ///
 374 ///[`SimpleRefChannelManager`]: crate::ln::channelmanager::SimpleRefChannelManager
 375 ///[`SimpleRefPeerManager`]: crate::ln::peer_handler::SimpleRefPeerManager
 376 pub type SimpleRefOnionMessenger<'a, 'b, L> = OnionMessenger<InMemorySigner, &'a KeysManager, &'b L>;
 377
 378 /// Construct onion packet payloads and keys for sending an onion message along the given
 379 /// `unblinded_path` to the given `destination`.
 380 fn packet_payloads_and_keys<T: secp256k1::Signing + secp256k1::Verification>(
 381         secp_ctx: &Secp256k1<T>, unblinded_path: &[PublicKey], destination: Destination, mut reply_path:
 382         Option<BlindedRoute>, session_priv: &SecretKey
 383 ) -> Result<(Vec<(Payload, [u8; 32])>, Vec<onion_utils::OnionKeys>), secp256k1::Error> {
 384         let num_hops = unblinded_path.len() + destination.num_hops();
 385         let mut payloads = Vec::with_capacity(num_hops);
 386         let mut onion_packet_keys = Vec::with_capacity(num_hops);
 387
 388         let (mut intro_node_id_blinding_pt, num_blinded_hops) = if let Destination::BlindedRoute(BlindedRoute {
 389                 introduction_node_id, blinding_point, blinded_hops }) = &destination {
 390                 (Some((*introduction_node_id, *blinding_point)), blinded_hops.len()) } else { (None, 0) };
 391         let num_unblinded_hops = num_hops - num_blinded_hops;
 392
 393         let mut unblinded_path_idx = 0;
 394         let mut blinded_path_idx = 0;
 395         let mut prev_control_tlvs_ss = None;
 396         utils::construct_keys_callback(secp_ctx, unblinded_path, Some(destination), session_priv, |_, onion_packet_ss, ephemeral_pubkey, control_tlvs_ss, unblinded_pk_opt, enc_payload_opt| {
 397                 if num_unblinded_hops != 0 && unblinded_path_idx < num_unblinded_hops {
 398                         if let Some(ss) = prev_control_tlvs_ss.take() {
 399                                 payloads.push((Payload::Forward(ForwardControlTlvs::Unblinded(
 400                                         ForwardTlvs {
 401                                                 next_node_id: unblinded_pk_opt.unwrap(),
 402                                                 next_blinding_override: None,
 403                                         }
 404                                 )), ss));
 405                         }
 406                         prev_control_tlvs_ss = Some(control_tlvs_ss);
 407                         unblinded_path_idx += 1;
 408                 } else if let Some((intro_node_id, blinding_pt)) = intro_node_id_blinding_pt.take() {
 409                         if let Some(control_tlvs_ss) = prev_control_tlvs_ss.take() {
 410                                 payloads.push((Payload::Forward(ForwardControlTlvs::Unblinded(ForwardTlvs {
 411                                         next_node_id: intro_node_id,
 412                                         next_blinding_override: Some(blinding_pt),
 413                                 })), control_tlvs_ss));
 414                         }
 415                         if let Some(encrypted_payload) = enc_payload_opt {
 416                                 payloads.push((Payload::Forward(ForwardControlTlvs::Blinded(encrypted_payload)),
 417                                         control_tlvs_ss));
 418                         } else { debug_assert!(false); }
 419                         blinded_path_idx += 1;
 420                 } else if blinded_path_idx < num_blinded_hops - 1 && enc_payload_opt.is_some() {
 421                         payloads.push((Payload::Forward(ForwardControlTlvs::Blinded(enc_payload_opt.unwrap())),
 422                                 control_tlvs_ss));
 423                         blinded_path_idx += 1;
 424                 } else if let Some(encrypted_payload) = enc_payload_opt {
 425                         payloads.push((Payload::Receive {
 426                                 control_tlvs: ReceiveControlTlvs::Blinded(encrypted_payload),
 427                                 reply_path: reply_path.take(),
 428                         }, control_tlvs_ss));
 429                 }
 430
 431                 let (rho, mu) = onion_utils::gen_rho_mu_from_shared_secret(onion_packet_ss.as_ref());
 432                 onion_packet_keys.push(onion_utils::OnionKeys {
 433                         #[cfg(test)]
 434                         shared_secret: onion_packet_ss,
 435                         #[cfg(test)]
 436                         blinding_factor: [0; 32],
 437                         ephemeral_pubkey,
 438                         rho,
 439                         mu,
 440                 });
 441         })?;
 442
 443         if let Some(control_tlvs_ss) = prev_control_tlvs_ss {
 444                 payloads.push((Payload::Receive {
 445                         control_tlvs: ReceiveControlTlvs::Unblinded(ReceiveTlvs { path_id: None, }),
 446                         reply_path: reply_path.take(),
 447                 }, control_tlvs_ss));
 448         }
 449
 450         Ok((payloads, onion_packet_keys))
 451 }
 452
 453 /// Errors if the serialized payload size exceeds onion_message::BIG_PACKET_HOP_DATA_LEN
 454 fn construct_onion_message_packet(payloads: Vec<(Payload, [u8; 32])>, onion_keys: Vec<onion_utils::OnionKeys>, prng_seed: [u8; 32]) -> Result<Packet, ()> {
 455         // Spec rationale:
 456         // "`len` allows larger messages to be sent than the standard 1300 bytes allowed for an HTLC
 457         // onion, but this should be used sparingly as it is reduces anonymity set, hence the
 458         // recommendation that it either look like an HTLC onion, or if larger, be a fixed size."
 459         let payloads_ser_len = onion_utils::payloads_serialized_length(&payloads);
 460         let hop_data_len = if payloads_ser_len <= SMALL_PACKET_HOP_DATA_LEN {
 461                 SMALL_PACKET_HOP_DATA_LEN
 462         } else if payloads_ser_len <= BIG_PACKET_HOP_DATA_LEN {
 463                 BIG_PACKET_HOP_DATA_LEN
 464         } else { return Err(()) };
 465
 466         Ok(onion_utils::construct_onion_message_packet::<_, _>(
 467                 payloads, onion_keys, prng_seed, hop_data_len))
 468 }