[rust-lightning] / lightning / src / onion_message / messenger.rs

// This file is Copyright its original authors, visible in version control
// history.
//
// This file is licensed under the Apache License, Version 2.0 <LICENSE-APACHE
// or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
// You may not use this file except in accordance with one or both of these
// licenses.

//! LDK sends, receives, and forwards onion messages via the [`OnionMessenger`]. See its docs for
//! more information.

use bitcoin::hashes::{Hash, HashEngine};
use bitcoin::hashes::hmac::{Hmac, HmacEngine};
use bitcoin::hashes::sha256::Hash as Sha256;
use bitcoin::secp256k1::{self, PublicKey, Scalar, Secp256k1, SecretKey};

use chain::keysinterface::{InMemorySigner, KeysInterface, KeysManager, Recipient, Sign};
use ln::features::{InitFeatures, NodeFeatures};
use ln::msgs::{self, OnionMessageHandler};
use ln::onion_utils;
use ln::peer_handler::IgnoringMessageHandler;
use super::blinded_route::{BlindedRoute, ForwardTlvs, ReceiveTlvs};
pub use super::packet::CustomOnionMessageContents;
use super::packet::{BIG_PACKET_HOP_DATA_LEN, ForwardControlTlvs, Packet, Payload, ReceiveControlTlvs, SMALL_PACKET_HOP_DATA_LEN};
use super::utils;
use util::events::OnionMessageProvider;
use util::logger::Logger;
use util::ser::Writeable;

use core::ops::Deref;
use sync::{Arc, Mutex};
use prelude::*;

/// A sender, receiver and forwarder of onion messages. In upcoming releases, this object will be
/// used to retrieve invoices and fulfill invoice requests from [offers]. Currently, only sending
/// and receiving empty onion messages is supported.
///
/// # Example
///
/// ```
/// # extern crate bitcoin;
/// # use bitcoin::hashes::_export::_core::time::Duration;
/// # use bitcoin::secp256k1::{PublicKey, Secp256k1, SecretKey};
/// # use lightning::chain::keysinterface::{InMemorySigner, KeysManager, KeysInterface};
/// # use lightning::ln::msgs::DecodeError;
/// # use lightning::ln::peer_handler::IgnoringMessageHandler;
/// # use lightning::onion_message::{BlindedRoute, CustomOnionMessageContents, Destination, OnionMessenger};
/// # use lightning::util::logger::{Logger, Record};
/// # use lightning::util::ser::{MaybeReadableArgs, Writeable, Writer};
/// # use lightning::io;
/// # use std::sync::Arc;
/// # struct FakeLogger {};
/// # impl Logger for FakeLogger {
/// #     fn log(&self, record: &Record) { unimplemented!() }
/// # }
/// # let seed = [42u8; 32];
/// # let time = Duration::from_secs(123456);
/// # let keys_manager = KeysManager::new(&seed, time.as_secs(), time.subsec_nanos());
/// # let logger = Arc::new(FakeLogger {});
/// # let node_secret = SecretKey::from_slice(&hex::decode("0101010101010101010101010101010101010101010101010101010101010101").unwrap()[..]).unwrap();
/// # let secp_ctx = Secp256k1::new();
/// # let hop_node_id1 = PublicKey::from_secret_key(&secp_ctx, &node_secret);
/// # let (hop_node_id2, hop_node_id3, hop_node_id4) = (hop_node_id1, hop_node_id1,
/// hop_node_id1);
/// # let destination_node_id = hop_node_id1;
/// # let your_custom_message_handler = IgnoringMessageHandler {};
/// // Create the onion messenger. This must use the same `keys_manager` as is passed to your
/// // ChannelManager.
/// let onion_messenger = OnionMessenger::new(&keys_manager, logger, your_custom_message_handler);
///
/// # struct YourCustomMessage {}
/// impl Writeable for YourCustomMessage {
///     fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
///             # Ok(())
///             // Write your custom onion message to `w`
///     }
/// }
/// impl CustomOnionMessageContents for YourCustomMessage {
///     fn tlv_type(&self) -> u64 {
///             # let your_custom_message_type = 42;
///             your_custom_message_type
///     }
/// }
/// impl MaybeReadableArgs<u64> for YourCustomMessage {
///     fn read<R: io::Read>(r: &mut R, message_type: u64) -> Result<Option<Self>, DecodeError> {
///             # unreachable!()
///             // Read your custom onion message of type `message_type` from `r`, or return `None`
///             // if the message type is unknown
///     }
/// }
/// let intermediate_hops = [hop_node_id1, hop_node_id2];
/// let reply_path = None;
/// onion_messenger.send_onion_message(&intermediate_hops, Destination::Node(destination_node_id), reply_path);
///
/// // Create a blinded route to yourself, for someone to send an onion message to.
/// # let your_node_id = hop_node_id1;
/// let hops = [hop_node_id3, hop_node_id4, your_node_id];
/// let blinded_route = BlindedRoute::new(&hops, &keys_manager, &secp_ctx).unwrap();
///
/// // Send an empty onion message to a blinded route.
/// # let intermediate_hops = [hop_node_id1, hop_node_id2];
/// let reply_path = None;
/// onion_messenger.send_onion_message(&intermediate_hops, Destination::BlindedRoute(blinded_route), reply_path);
/// ```
///
/// [offers]: <https://github.com/lightning/bolts/pull/798>
/// [`OnionMessenger`]: crate::onion_message::OnionMessenger
pub struct OnionMessenger<Signer: Sign, K: Deref, L: Deref, CMH: Deref>
        where K::Target: KeysInterface<Signer = Signer>,
              L::Target: Logger,
              CMH:: Target: CustomOnionMessageHandler,
{
        keys_manager: K,
        logger: L,
        pending_messages: Mutex<HashMap<PublicKey, VecDeque<msgs::OnionMessage>>>,
        secp_ctx: Secp256k1<secp256k1::All>,
        custom_handler: CMH,
        // Coming soon:
        // invoice_handler: InvoiceHandler,
}

/// The destination of an onion message.
pub enum Destination {
        /// We're sending this onion message to a node.
        Node(PublicKey),
        /// We're sending this onion message to a blinded route.
        BlindedRoute(BlindedRoute),
}

impl Destination {
        pub(super) fn num_hops(&self) -> usize {
                match self {
                        Destination::Node(_) => 1,
                        Destination::BlindedRoute(BlindedRoute { blinded_hops, .. }) => blinded_hops.len(),
                }
        }
}

/// Errors that may occur when [sending an onion message].
///
/// [sending an onion message]: OnionMessenger::send_onion_message
#[derive(Debug, PartialEq)]
pub enum SendError {
        /// Errored computing onion message packet keys.
        Secp256k1(secp256k1::Error),
        /// Because implementations such as Eclair will drop onion messages where the message packet
        /// exceeds 32834 bytes, we refuse to send messages where the packet exceeds this size.
        TooBigPacket,
        /// The provided [`Destination`] was an invalid [`BlindedRoute`], due to having fewer than two
        /// blinded hops.
        TooFewBlindedHops,
        /// Our next-hop peer was offline or does not support onion message forwarding.
        InvalidFirstHop,
        /// Our next-hop peer's buffer was full or our total outbound buffer was full.
        BufferFull,
}

/// Handler for custom onion messages. If you are using [`SimpleArcOnionMessenger`],
/// [`SimpleRefOnionMessenger`], or prefer to ignore inbound custom onion messages,
/// [`IgnoringMessageHandler`] must be provided to [`OnionMessenger::new`]. Otherwise, a custom
/// implementation of this trait must be provided, with [`CustomMessage`] specifying the supported
/// message types.
///
/// See [`OnionMessenger`] for example usage.
///
/// [`IgnoringMessageHandler`]: crate::ln::peer_handler::IgnoringMessageHandler
/// [`CustomMessage`]: Self::CustomMessage
pub trait CustomOnionMessageHandler {
        /// The message known to the handler. To support multiple message types, you may want to make this
        /// an enum with a variant for each supported message.
        type CustomMessage: CustomOnionMessageContents;
        /// Called with the custom message that was received.
        fn handle_custom_message(&self, msg: Self::CustomMessage);
}

impl<Signer: Sign, K: Deref, L: Deref, CMH: Deref> OnionMessenger<Signer, K, L, CMH>
        where K::Target: KeysInterface<Signer = Signer>,
              L::Target: Logger,
              CMH::Target: CustomOnionMessageHandler,
{
        /// Constructs a new `OnionMessenger` to send, forward, and delegate received onion messages to
        /// their respective handlers.
        pub fn new(keys_manager: K, logger: L, custom_handler: CMH) -> Self {
                let mut secp_ctx = Secp256k1::new();
                secp_ctx.seeded_randomize(&keys_manager.get_secure_random_bytes());
                OnionMessenger {
                        keys_manager,
                        pending_messages: Mutex::new(HashMap::new()),
                        secp_ctx,
                        logger,
                        custom_handler,
                }
        }

        /// Send an empty onion message to `destination`, routing it through `intermediate_nodes`.
        /// See [`OnionMessenger`] for example usage.
        pub fn send_onion_message(&self, intermediate_nodes: &[PublicKey], destination: Destination, reply_path: Option<BlindedRoute>) -> Result<(), SendError> {
                if let Destination::BlindedRoute(BlindedRoute { ref blinded_hops, .. }) = destination {
                        if blinded_hops.len() < 2 {
                                return Err(SendError::TooFewBlindedHops);
                        }
                }
                let blinding_secret_bytes = self.keys_manager.get_secure_random_bytes();
                let blinding_secret = SecretKey::from_slice(&blinding_secret_bytes[..]).expect("RNG is busted");
                let (introduction_node_id, blinding_point) = if intermediate_nodes.len() != 0 {
                        (intermediate_nodes[0], PublicKey::from_secret_key(&self.secp_ctx, &blinding_secret))
                } else {
                        match destination {
                                Destination::Node(pk) => (pk, PublicKey::from_secret_key(&self.secp_ctx, &blinding_secret)),
                                Destination::BlindedRoute(BlindedRoute { introduction_node_id, blinding_point, .. }) =>
                                        (introduction_node_id, blinding_point),
                        }
                };
                let (packet_payloads, packet_keys) = packet_payloads_and_keys(
                        &self.secp_ctx, intermediate_nodes, destination, reply_path, &blinding_secret)
                        .map_err(|e| SendError::Secp256k1(e))?;

                let prng_seed = self.keys_manager.get_secure_random_bytes();
                let onion_routing_packet = construct_onion_message_packet(
                        packet_payloads, packet_keys, prng_seed).map_err(|()| SendError::TooBigPacket)?;

                let mut pending_per_peer_msgs = self.pending_messages.lock().unwrap();
                if outbound_buffer_full(&introduction_node_id, &pending_per_peer_msgs) { return Err(SendError::BufferFull) }
                match pending_per_peer_msgs.entry(introduction_node_id) {
                        hash_map::Entry::Vacant(_) => Err(SendError::InvalidFirstHop),
                        hash_map::Entry::Occupied(mut e) => {
                                e.get_mut().push_back(msgs::OnionMessage { blinding_point, onion_routing_packet });
                                Ok(())
                        }
                }
        }

        #[cfg(test)]
        pub(super) fn release_pending_msgs(&self) -> HashMap<PublicKey, VecDeque<msgs::OnionMessage>> {
                let mut pending_msgs = self.pending_messages.lock().unwrap();
                let mut msgs = HashMap::new();
                // We don't want to disconnect the peers by removing them entirely from the original map, so we
                // swap the pending message buffers individually.
                for (peer_node_id, pending_messages) in &mut *pending_msgs {
                        msgs.insert(*peer_node_id, core::mem::take(pending_messages));
                }
                msgs
        }
}

fn outbound_buffer_full(peer_node_id: &PublicKey, buffer: &HashMap<PublicKey, VecDeque<msgs::OnionMessage>>) -> bool {
        const MAX_TOTAL_BUFFER_SIZE: usize = (1 << 20) * 128;
        const MAX_PER_PEER_BUFFER_SIZE: usize = (1 << 10) * 256;
        let mut total_buffered_bytes = 0;
        let mut peer_buffered_bytes = 0;
        for (pk, peer_buf) in buffer {
                for om in peer_buf {
                        let om_len = om.serialized_length();
                        if pk == peer_node_id {
                                peer_buffered_bytes += om_len;
                        }
                        total_buffered_bytes += om_len;

                        if total_buffered_bytes >= MAX_TOTAL_BUFFER_SIZE ||
                                peer_buffered_bytes >= MAX_PER_PEER_BUFFER_SIZE
                        {
                                return true
                        }
                }
        }
        false
}

impl<Signer: Sign, K: Deref, L: Deref, CMH: Deref> OnionMessageHandler for OnionMessenger<Signer, K, L, CMH>
        where K::Target: KeysInterface<Signer = Signer>,
              L::Target: Logger,
              CMH::Target: CustomOnionMessageHandler,
{
        /// Handle an incoming onion message. Currently, if a message was destined for us we will log, but
        /// soon we'll delegate the onion message to a handler that can generate invoices or send
        /// payments.
        fn handle_onion_message(&self, _peer_node_id: &PublicKey, msg: &msgs::OnionMessage) {
                let control_tlvs_ss = match self.keys_manager.ecdh(Recipient::Node, &msg.blinding_point, None) {
                        Ok(ss) => ss,
                        Err(e) =>  {
                                log_error!(self.logger, "Failed to retrieve node secret: {:?}", e);
                                return
                        }
                };
                let onion_decode_ss = {
                        let blinding_factor = {
                                let mut hmac = HmacEngine::<Sha256>::new(b"blinded_node_id");
                                hmac.input(control_tlvs_ss.as_ref());
                                Hmac::from_engine(hmac).into_inner()
                        };
                        match self.keys_manager.ecdh(Recipient::Node, &msg.onion_routing_packet.public_key,
                                Some(&Scalar::from_be_bytes(blinding_factor).unwrap()))
                        {
                                Ok(ss) => ss.secret_bytes(),
                                Err(()) => {
                                        log_trace!(self.logger, "Failed to compute onion packet shared secret");
                                        return
                                }
                        }
                };
                match onion_utils::decode_next_hop(onion_decode_ss, &msg.onion_routing_packet.hop_data[..],
                        msg.onion_routing_packet.hmac, control_tlvs_ss)
                {
                        Ok((Payload::Receive {
                                control_tlvs: ReceiveControlTlvs::Unblinded(ReceiveTlvs { path_id }), reply_path,
                        }, None)) => {
                                log_info!(self.logger,
                                        "Received an onion message with path_id: {:02x?} and {}reply_path",
                                                path_id, if reply_path.is_some() { "" } else { "no " });
                        },
                        Ok((Payload::Forward(ForwardControlTlvs::Unblinded(ForwardTlvs {
                                next_node_id, next_blinding_override
                        })), Some((next_hop_hmac, new_packet_bytes)))) => {
                                // TODO: we need to check whether `next_node_id` is our node, in which case this is a dummy
                                // blinded hop and this onion message is destined for us. In this situation, we should keep
                                // unwrapping the onion layers to get to the final payload. Since we don't have the option
                                // of creating blinded routes with dummy hops currently, we should be ok to not handle this
                                // for now.
                                let new_pubkey = match onion_utils::next_hop_packet_pubkey(&self.secp_ctx, msg.onion_routing_packet.public_key, &onion_decode_ss) {
                                        Ok(pk) => pk,
                                        Err(e) => {
                                                log_trace!(self.logger, "Failed to compute next hop packet pubkey: {}", e);
                                                return
                                        }
                                };
                                let outgoing_packet = Packet {
                                        version: 0,
                                        public_key: new_pubkey,
                                        hop_data: new_packet_bytes,
                                        hmac: next_hop_hmac,
                                };
                                let onion_message = msgs::OnionMessage {
                                        blinding_point: match next_blinding_override {
                                                Some(blinding_point) => blinding_point,
                                                None => {
                                                        let blinding_factor = {
                                                                let mut sha = Sha256::engine();
                                                                sha.input(&msg.blinding_point.serialize()[..]);
                                                                sha.input(control_tlvs_ss.as_ref());
                                                                Sha256::from_engine(sha).into_inner()
                                                        };
                                                        let next_blinding_point = msg.blinding_point;
                                                        match next_blinding_point.mul_tweak(&self.secp_ctx, &Scalar::from_be_bytes(blinding_factor).unwrap()) {
                                                                Ok(bp) => bp,
                                                                Err(e) => {
                                                                        log_trace!(self.logger, "Failed to compute next blinding point: {}", e);
                                                                        return
                                                                }
                                                        }
                                                },
                                        },
                                        onion_routing_packet: outgoing_packet,
                                };

                                let mut pending_per_peer_msgs = self.pending_messages.lock().unwrap();
                                if outbound_buffer_full(&next_node_id, &pending_per_peer_msgs) {
                                        log_trace!(self.logger, "Dropping forwarded onion message to peer {:?}: outbound buffer full", next_node_id);
                                        return
                                }

                                #[cfg(fuzzing)]
                                pending_per_peer_msgs.entry(next_node_id).or_insert_with(VecDeque::new);

                                match pending_per_peer_msgs.entry(next_node_id) {
                                        hash_map::Entry::Vacant(_) => {
                                                log_trace!(self.logger, "Dropping forwarded onion message to disconnected peer {:?}", next_node_id);
                                                return
                                        },
                                        hash_map::Entry::Occupied(mut e) => {
                                                e.get_mut().push_back(onion_message);
                                                log_trace!(self.logger, "Forwarding an onion message to peer {}", next_node_id);
                                        }
                                };
                        },
                        Err(e) => {
                                log_trace!(self.logger, "Errored decoding onion message packet: {:?}", e);
                        },
                        _ => {
                                log_trace!(self.logger, "Received bogus onion message packet, either the sender encoded a final hop as a forwarding hop or vice versa");
                        },
                };
        }

        fn peer_connected(&self, their_node_id: &PublicKey, init: &msgs::Init) -> Result<(), ()> {
                if init.features.supports_onion_messages() {
                        let mut peers = self.pending_messages.lock().unwrap();
                        peers.insert(their_node_id.clone(), VecDeque::new());
                }
                Ok(())
        }

        fn peer_disconnected(&self, their_node_id: &PublicKey, _no_connection_possible: bool) {
                let mut pending_msgs = self.pending_messages.lock().unwrap();
                pending_msgs.remove(their_node_id);
        }

        fn provided_node_features(&self) -> NodeFeatures {
                let mut features = NodeFeatures::empty();
                features.set_onion_messages_optional();
                features
        }

        fn provided_init_features(&self, _their_node_id: &PublicKey) -> InitFeatures {
                let mut features = InitFeatures::empty();
                features.set_onion_messages_optional();
                features
        }
}

impl<Signer: Sign, K: Deref, L: Deref, CMH: Deref> OnionMessageProvider for OnionMessenger<Signer, K, L, CMH>
        where K::Target: KeysInterface<Signer = Signer>,
              L::Target: Logger,
              CMH::Target: CustomOnionMessageHandler,
{
        fn next_onion_message_for_peer(&self, peer_node_id: PublicKey) -> Option<msgs::OnionMessage> {
                let mut pending_msgs = self.pending_messages.lock().unwrap();
                if let Some(msgs) = pending_msgs.get_mut(&peer_node_id) {
                        return msgs.pop_front()
                }
                None
        }
}

// TODO: parameterize the below Simple* types with OnionMessenger and handle the messages it
// produces
/// Useful for simplifying the parameters of [`SimpleArcChannelManager`] and
/// [`SimpleArcPeerManager`]. See their docs for more details.
///
/// (C-not exported) as `Arc`s don't make sense in bindings.
///
/// [`SimpleArcChannelManager`]: crate::ln::channelmanager::SimpleArcChannelManager
/// [`SimpleArcPeerManager`]: crate::ln::peer_handler::SimpleArcPeerManager
pub type SimpleArcOnionMessenger<L> = OnionMessenger<InMemorySigner, Arc<KeysManager>, Arc<L>, IgnoringMessageHandler>;
/// Useful for simplifying the parameters of [`SimpleRefChannelManager`] and
/// [`SimpleRefPeerManager`]. See their docs for more details.
///
/// (C-not exported) as general type aliases don't make sense in bindings.
///
/// [`SimpleRefChannelManager`]: crate::ln::channelmanager::SimpleRefChannelManager
/// [`SimpleRefPeerManager`]: crate::ln::peer_handler::SimpleRefPeerManager
pub type SimpleRefOnionMessenger<'a, 'b, L> = OnionMessenger<InMemorySigner, &'a KeysManager, &'b L, IgnoringMessageHandler>;

/// Construct onion packet payloads and keys for sending an onion message along the given
/// `unblinded_path` to the given `destination`.
fn packet_payloads_and_keys<T: secp256k1::Signing + secp256k1::Verification>(
        secp_ctx: &Secp256k1<T>, unblinded_path: &[PublicKey], destination: Destination, mut reply_path:
        Option<BlindedRoute>, session_priv: &SecretKey
) -> Result<(Vec<(Payload, [u8; 32])>, Vec<onion_utils::OnionKeys>), secp256k1::Error> {
        let num_hops = unblinded_path.len() + destination.num_hops();
        let mut payloads = Vec::with_capacity(num_hops);
        let mut onion_packet_keys = Vec::with_capacity(num_hops);

        let (mut intro_node_id_blinding_pt, num_blinded_hops) = if let Destination::BlindedRoute(BlindedRoute {
                introduction_node_id, blinding_point, blinded_hops }) = &destination {
                (Some((*introduction_node_id, *blinding_point)), blinded_hops.len()) } else { (None, 0) };
        let num_unblinded_hops = num_hops - num_blinded_hops;

        let mut unblinded_path_idx = 0;
        let mut blinded_path_idx = 0;
        let mut prev_control_tlvs_ss = None;
        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| {
                if num_unblinded_hops != 0 && unblinded_path_idx < num_unblinded_hops {
                        if let Some(ss) = prev_control_tlvs_ss.take() {
                                payloads.push((Payload::Forward(ForwardControlTlvs::Unblinded(
                                        ForwardTlvs {
                                                next_node_id: unblinded_pk_opt.unwrap(),
                                                next_blinding_override: None,
                                        }
                                )), ss));
                        }
                        prev_control_tlvs_ss = Some(control_tlvs_ss);
                        unblinded_path_idx += 1;
                } else if let Some((intro_node_id, blinding_pt)) = intro_node_id_blinding_pt.take() {
                        if let Some(control_tlvs_ss) = prev_control_tlvs_ss.take() {
                                payloads.push((Payload::Forward(ForwardControlTlvs::Unblinded(ForwardTlvs {
                                        next_node_id: intro_node_id,
                                        next_blinding_override: Some(blinding_pt),
                                })), control_tlvs_ss));
                        }
                        if let Some(encrypted_payload) = enc_payload_opt {
                                payloads.push((Payload::Forward(ForwardControlTlvs::Blinded(encrypted_payload)),
                                        control_tlvs_ss));
                        } else { debug_assert!(false); }
                        blinded_path_idx += 1;
                } else if blinded_path_idx < num_blinded_hops - 1 && enc_payload_opt.is_some() {
                        payloads.push((Payload::Forward(ForwardControlTlvs::Blinded(enc_payload_opt.unwrap())),
                                control_tlvs_ss));
                        blinded_path_idx += 1;
                } else if let Some(encrypted_payload) = enc_payload_opt {
                        payloads.push((Payload::Receive {
                                control_tlvs: ReceiveControlTlvs::Blinded(encrypted_payload),
                                reply_path: reply_path.take(),
                        }, control_tlvs_ss));
                }

                let (rho, mu) = onion_utils::gen_rho_mu_from_shared_secret(onion_packet_ss.as_ref());
                onion_packet_keys.push(onion_utils::OnionKeys {
                        #[cfg(test)]
                        shared_secret: onion_packet_ss,
                        #[cfg(test)]
                        blinding_factor: [0; 32],
                        ephemeral_pubkey,
                        rho,
                        mu,
                });
        })?;

        if let Some(control_tlvs_ss) = prev_control_tlvs_ss {
                payloads.push((Payload::Receive {
                        control_tlvs: ReceiveControlTlvs::Unblinded(ReceiveTlvs { path_id: None, }),
                        reply_path: reply_path.take(),
                }, control_tlvs_ss));
        }

        Ok((payloads, onion_packet_keys))
}

/// Errors if the serialized payload size exceeds onion_message::BIG_PACKET_HOP_DATA_LEN
fn construct_onion_message_packet(payloads: Vec<(Payload, [u8; 32])>, onion_keys: Vec<onion_utils::OnionKeys>, prng_seed: [u8; 32]) -> Result<Packet, ()> {
        // Spec rationale:
        // "`len` allows larger messages to be sent than the standard 1300 bytes allowed for an HTLC
        // onion, but this should be used sparingly as it is reduces anonymity set, hence the
        // recommendation that it either look like an HTLC onion, or if larger, be a fixed size."
        let payloads_ser_len = onion_utils::payloads_serialized_length(&payloads);
        let hop_data_len = if payloads_ser_len <= SMALL_PACKET_HOP_DATA_LEN {
                SMALL_PACKET_HOP_DATA_LEN
        } else if payloads_ser_len <= BIG_PACKET_HOP_DATA_LEN {
                BIG_PACKET_HOP_DATA_LEN
        } else { return Err(()) };

        Ok(onion_utils::construct_onion_message_packet::<_, _>(
                payloads, onion_keys, prng_seed, hop_data_len))
}