[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 crate::blinded_path::BlindedPath;
use crate::blinded_path::message::{advance_path_by_one, ForwardTlvs, ReceiveTlvs};
use crate::blinded_path::utils;
use crate::sign::{EntropySource, KeysManager, NodeSigner, Recipient};
#[cfg(not(c_bindings))]
use crate::ln::channelmanager::{SimpleArcChannelManager, SimpleRefChannelManager};
use crate::ln::features::{InitFeatures, NodeFeatures};
use crate::ln::msgs::{self, OnionMessage, OnionMessageHandler};
use crate::ln::onion_utils;
use crate::ln::peer_handler::IgnoringMessageHandler;
pub use super::packet::OnionMessageContents;
use super::packet::ParsedOnionMessageContents;
use super::offers::OffersMessageHandler;
use super::packet::{BIG_PACKET_HOP_DATA_LEN, ForwardControlTlvs, Packet, Payload, ReceiveControlTlvs, SMALL_PACKET_HOP_DATA_LEN};
use crate::util::logger::Logger;
use crate::util::ser::Writeable;

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

/// A sender, receiver and forwarder of [`OnionMessage`]s.
///
/// # Handling Messages
///
/// `OnionMessenger` implements [`OnionMessageHandler`], making it responsible for either forwarding
/// messages to peers or delegating to the appropriate handler for the message type. Currently, the
/// available handlers are:
/// * [`OffersMessageHandler`], for responding to [`InvoiceRequest`]s and paying [`Bolt12Invoice`]s
/// * [`CustomOnionMessageHandler`], for handling user-defined message types
///
/// # Sending Messages
///
/// [`OnionMessage`]s are sent initially using [`OnionMessenger::send_onion_message`]. When handling
/// a message, the matched handler may return a response message which `OnionMessenger` will send
/// on its behalf.
///
/// # Example
///
/// ```
/// # extern crate bitcoin;
/// # use bitcoin::hashes::_export::_core::time::Duration;
/// # use bitcoin::secp256k1::{PublicKey, Secp256k1, SecretKey};
/// # use lightning::blinded_path::BlindedPath;
/// # use lightning::sign::KeysManager;
/// # use lightning::ln::peer_handler::IgnoringMessageHandler;
/// # use lightning::onion_message::{OnionMessageContents, Destination, MessageRouter, OnionMessagePath, OnionMessenger};
/// # use lightning::util::logger::{Logger, Record};
/// # use lightning::util::ser::{Writeable, Writer};
/// # use lightning::io;
/// # use std::sync::Arc;
/// # struct FakeLogger;
/// # impl Logger for FakeLogger {
/// #     fn log(&self, record: &Record) { unimplemented!() }
/// # }
/// # struct FakeMessageRouter {}
/// # impl MessageRouter for FakeMessageRouter {
/// #     fn find_path(&self, sender: PublicKey, peers: Vec<PublicKey>, destination: Destination) -> Result<OnionMessagePath, ()> {
/// #         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 message_router = Arc::new(FakeMessageRouter {});
/// # let custom_message_handler = IgnoringMessageHandler {};
/// # let offers_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, &keys_manager, logger, message_router, &offers_message_handler,
///     &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 OnionMessageContents for YourCustomMessage {
///     fn tlv_type(&self) -> u64 {
///             # let your_custom_message_type = 42;
///             your_custom_message_type
///     }
/// }
/// // Send a custom onion message to a node id.
/// let path = OnionMessagePath {
///     intermediate_nodes: vec![hop_node_id1, hop_node_id2],
///     destination: Destination::Node(destination_node_id),
/// };
/// let reply_path = None;
/// # let message = YourCustomMessage {};
/// onion_messenger.send_onion_message(path, message, reply_path);
///
/// // Create a blinded path 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_path = BlindedPath::new_for_message(&hops, &keys_manager, &secp_ctx).unwrap();
///
/// // Send a custom onion message to a blinded path.
/// let path = OnionMessagePath {
///     intermediate_nodes: vec![hop_node_id1, hop_node_id2],
///     destination: Destination::BlindedPath(blinded_path),
/// };
/// let reply_path = None;
/// # let message = YourCustomMessage {};
/// onion_messenger.send_onion_message(path, message, reply_path);
/// ```
///
/// [`InvoiceRequest`]: crate::offers::invoice_request::InvoiceRequest
/// [`Bolt12Invoice`]: crate::offers::invoice::Bolt12Invoice
pub struct OnionMessenger<ES: Deref, NS: Deref, L: Deref, MR: Deref, OMH: Deref, CMH: Deref>
where
        ES::Target: EntropySource,
        NS::Target: NodeSigner,
        L::Target: Logger,
        MR::Target: MessageRouter,
        OMH::Target: OffersMessageHandler,
        CMH:: Target: CustomOnionMessageHandler,
{
        entropy_source: ES,
        node_signer: NS,
        logger: L,
        pending_messages: Mutex<HashMap<PublicKey, VecDeque<OnionMessage>>>,
        secp_ctx: Secp256k1<secp256k1::All>,
        message_router: MR,
        offers_handler: OMH,
        custom_handler: CMH,
}

/// An [`OnionMessage`] for [`OnionMessenger`] to send.
///
/// These are obtained when released from [`OnionMessenger`]'s handlers after which they are
/// enqueued for sending.
pub struct PendingOnionMessage<T: OnionMessageContents> {
        /// The message contents to send in an [`OnionMessage`].
        pub contents: T,

        /// The destination of the message.
        pub destination: Destination,

        /// A reply path to include in the [`OnionMessage`] for a response.
        pub reply_path: Option<BlindedPath>,
}

/// A trait defining behavior for routing an [`OnionMessage`].
pub trait MessageRouter {
        /// Returns a route for sending an [`OnionMessage`] to the given [`Destination`].
        fn find_path(
                &self, sender: PublicKey, peers: Vec<PublicKey>, destination: Destination
        ) -> Result<OnionMessagePath, ()>;
}

/// A [`MessageRouter`] that can only route to a directly connected [`Destination`].
pub struct DefaultMessageRouter;

impl MessageRouter for DefaultMessageRouter {
        fn find_path(
                &self, _sender: PublicKey, peers: Vec<PublicKey>, destination: Destination
        ) -> Result<OnionMessagePath, ()> {
                if peers.contains(&destination.first_node()) {
                        Ok(OnionMessagePath { intermediate_nodes: vec![], destination })
                } else {
                        Err(())
                }
        }
}

/// A path for sending an [`OnionMessage`].
#[derive(Clone)]
pub struct OnionMessagePath {
        /// Nodes on the path between the sender and the destination.
        pub intermediate_nodes: Vec<PublicKey>,

        /// The recipient of the message.
        pub destination: Destination,
}

/// The destination of an onion message.
#[derive(Clone)]
pub enum Destination {
        /// We're sending this onion message to a node.
        Node(PublicKey),
        /// We're sending this onion message to a blinded path.
        BlindedPath(BlindedPath),
}

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

        fn first_node(&self) -> PublicKey {
                match self {
                        Destination::Node(node_id) => *node_id,
                        Destination::BlindedPath(BlindedPath { introduction_node_id: node_id, .. }) => *node_id,
                }
        }
}

/// Errors that may occur when [sending an onion message].
///
/// [sending an onion message]: OnionMessenger::send_onion_message
#[derive(Debug, PartialEq, Eq)]
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 [`BlindedPath`] due to not having any blinded
        /// hops.
        TooFewBlindedHops,
        /// Our next-hop peer was offline or does not support onion message forwarding.
        InvalidFirstHop,
        /// Onion message contents must have a TLV type >= 64.
        InvalidMessage,
        /// Our next-hop peer's buffer was full or our total outbound buffer was full.
        BufferFull,
        /// Failed to retrieve our node id from the provided [`NodeSigner`].
        ///
        /// [`NodeSigner`]: crate::sign::NodeSigner
        GetNodeIdFailed,
        /// We attempted to send to a blinded path where we are the introduction node, and failed to
        /// advance the blinded path to make the second hop the new introduction node. Either
        /// [`NodeSigner::ecdh`] failed, we failed to tweak the current blinding point to get the
        /// new blinding point, or we were attempting to send to ourselves.
        BlindedPathAdvanceFailed,
}

/// 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: OnionMessageContents;

        /// Called with the custom message that was received, returning a response to send, if any.
        ///
        /// The returned [`Self::CustomMessage`], if any, is enqueued to be sent by [`OnionMessenger`].
        fn handle_custom_message(&self, msg: Self::CustomMessage) -> Option<Self::CustomMessage>;

        /// Read a custom message of type `message_type` from `buffer`, returning `Ok(None)` if the
        /// message type is unknown.
        fn read_custom_message<R: io::Read>(&self, message_type: u64, buffer: &mut R) -> Result<Option<Self::CustomMessage>, msgs::DecodeError>;

        /// Releases any [`Self::CustomMessage`]s that need to be sent.
        ///
        /// Typically, this is used for messages initiating a message flow rather than in response to
        /// another message. The latter should use the return value of [`Self::handle_custom_message`].
        fn release_pending_custom_messages(&self) -> Vec<PendingOnionMessage<Self::CustomMessage>>;
}

/// A processed incoming onion message, containing either a Forward (another onion message)
/// or a Receive payload with decrypted contents.
pub enum PeeledOnion<T: OnionMessageContents> {
        /// Forwarded onion, with the next node id and a new onion
        Forward(PublicKey, OnionMessage),
        /// Received onion message, with decrypted contents, path_id, and reply path
        Receive(ParsedOnionMessageContents<T>, Option<[u8; 32]>, Option<BlindedPath>)
}

/// Creates an [`OnionMessage`] with the given `contents` for sending to the destination of
/// `path`.
///
/// Returns both the node id of the peer to send the message to and the message itself.
pub fn create_onion_message<ES: Deref, NS: Deref, T: OnionMessageContents>(
        entropy_source: &ES, node_signer: &NS, secp_ctx: &Secp256k1<secp256k1::All>,
        path: OnionMessagePath, contents: T, reply_path: Option<BlindedPath>,
) -> Result<(PublicKey, OnionMessage), SendError>
where
        ES::Target: EntropySource,
        NS::Target: NodeSigner,
{
        let OnionMessagePath { intermediate_nodes, mut destination } = path;
        if let Destination::BlindedPath(BlindedPath { ref blinded_hops, .. }) = destination {
                if blinded_hops.is_empty() {
                        return Err(SendError::TooFewBlindedHops);
                }
        }

        if contents.tlv_type() < 64 { return Err(SendError::InvalidMessage) }

        // If we are sending straight to a blinded path and we are the introduction node, we need to
        // advance the blinded path by 1 hop so the second hop is the new introduction node.
        if intermediate_nodes.len() == 0 {
                if let Destination::BlindedPath(ref mut blinded_path) = destination {
                        let our_node_id = node_signer.get_node_id(Recipient::Node)
                                .map_err(|()| SendError::GetNodeIdFailed)?;
                        if blinded_path.introduction_node_id == our_node_id {
                                advance_path_by_one(blinded_path, node_signer, &secp_ctx)
                                        .map_err(|()| SendError::BlindedPathAdvanceFailed)?;
                        }
                }
        }

        let blinding_secret_bytes = entropy_source.get_secure_random_bytes();
        let blinding_secret = SecretKey::from_slice(&blinding_secret_bytes[..]).expect("RNG is busted");
        let (first_node_id, blinding_point) = if let Some(first_node_id) = intermediate_nodes.first() {
                (*first_node_id, PublicKey::from_secret_key(&secp_ctx, &blinding_secret))
        } else {
                match destination {
                        Destination::Node(pk) => (pk, PublicKey::from_secret_key(&secp_ctx, &blinding_secret)),
                        Destination::BlindedPath(BlindedPath { introduction_node_id, blinding_point, .. }) =>
                                (introduction_node_id, blinding_point),
                }
        };
        let (packet_payloads, packet_keys) = packet_payloads_and_keys(
                &secp_ctx, &intermediate_nodes, destination, contents, reply_path, &blinding_secret)
                .map_err(|e| SendError::Secp256k1(e))?;

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

        Ok((first_node_id, OnionMessage {
                blinding_point,
                onion_routing_packet
        }))
}

/// Decode one layer of an incoming [`OnionMessage`].
///
/// Returns either the next layer of the onion for forwarding or the decrypted content for the
/// receiver.
pub fn peel_onion_message<NS: Deref, L: Deref, CMH: Deref>(
        msg: &OnionMessage, secp_ctx: &Secp256k1<secp256k1::All>, node_signer: NS, logger: L,
        custom_handler: CMH,
) -> Result<PeeledOnion<<<CMH>::Target as CustomOnionMessageHandler>::CustomMessage>, ()>
where
        NS::Target: NodeSigner,
        L::Target: Logger,
        CMH::Target: CustomOnionMessageHandler,
{
        let control_tlvs_ss = match node_signer.ecdh(Recipient::Node, &msg.blinding_point, None) {
                Ok(ss) => ss,
                Err(e) =>  {
                        log_error!(logger, "Failed to retrieve node secret: {:?}", e);
                        return Err(());
                }
        };
        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 node_signer.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!(logger, "Failed to compute onion packet shared secret");
                                return Err(());
                        }
                }
        };
        match onion_utils::decode_next_untagged_hop(
                onion_decode_ss, &msg.onion_routing_packet.hop_data[..], msg.onion_routing_packet.hmac,
                (control_tlvs_ss, custom_handler.deref(), logger.deref())
        ) {
                Ok((Payload::Receive::<ParsedOnionMessageContents<<<CMH as Deref>::Target as CustomOnionMessageHandler>::CustomMessage>> {
                        message, control_tlvs: ReceiveControlTlvs::Unblinded(ReceiveTlvs { path_id }), reply_path,
                }, None)) => {
                        Ok(PeeledOnion::Receive(message, path_id, reply_path))
                },
                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 paths with dummy hops currently, we should be ok to not handle this
                        // for now.
                        let new_pubkey = match onion_utils::next_hop_pubkey(&secp_ctx, msg.onion_routing_packet.public_key, &onion_decode_ss) {
                                Ok(pk) => pk,
                                Err(e) => {
                                        log_trace!(logger, "Failed to compute next hop packet pubkey: {}", e);
                                        return Err(())
                                }
                        };
                        let outgoing_packet = Packet {
                                version: 0,
                                public_key: new_pubkey,
                                hop_data: new_packet_bytes,
                                hmac: next_hop_hmac,
                        };
                        let onion_message = OnionMessage {
                                blinding_point: match next_blinding_override {
                                        Some(blinding_point) => blinding_point,
                                        None => {
                                                match onion_utils::next_hop_pubkey(
                                                        &secp_ctx, msg.blinding_point, control_tlvs_ss.as_ref()
                                                ) {
                                                        Ok(bp) => bp,
                                                        Err(e) => {
                                                                log_trace!(logger, "Failed to compute next blinding point: {}", e);
                                                                return Err(())
                                                        }
                                                }
                                        }
                                },
                                onion_routing_packet: outgoing_packet,
                        };

                        Ok(PeeledOnion::Forward(next_node_id, onion_message))
                },
                Err(e) => {
                        log_trace!(logger, "Errored decoding onion message packet: {:?}", e);
                        Err(())
                },
                _ => {
                        log_trace!(logger, "Received bogus onion message packet, either the sender encoded a final hop as a forwarding hop or vice versa");
                        Err(())
                },
        }
}

impl<ES: Deref, NS: Deref, L: Deref, MR: Deref, OMH: Deref, CMH: Deref>
OnionMessenger<ES, NS, L, MR, OMH, CMH>
where
        ES::Target: EntropySource,
        NS::Target: NodeSigner,
        L::Target: Logger,
        MR::Target: MessageRouter,
        OMH::Target: OffersMessageHandler,
        CMH::Target: CustomOnionMessageHandler,
{
        /// Constructs a new `OnionMessenger` to send, forward, and delegate received onion messages to
        /// their respective handlers.
        pub fn new(
                entropy_source: ES, node_signer: NS, logger: L, message_router: MR, offers_handler: OMH,
                custom_handler: CMH
        ) -> Self {
                let mut secp_ctx = Secp256k1::new();
                secp_ctx.seeded_randomize(&entropy_source.get_secure_random_bytes());
                OnionMessenger {
                        entropy_source,
                        node_signer,
                        pending_messages: Mutex::new(HashMap::new()),
                        secp_ctx,
                        logger,
                        message_router,
                        offers_handler,
                        custom_handler,
                }
        }

        /// Sends an [`OnionMessage`] with the given `contents` for sending to the destination of
        /// `path`.
        ///
        /// See [`OnionMessenger`] for example usage.
        pub fn send_onion_message<T: OnionMessageContents>(
                &self, path: OnionMessagePath, contents: T, reply_path: Option<BlindedPath>
        ) -> Result<(), SendError> {
                let (first_node_id, onion_msg) = create_onion_message(
                        &self.entropy_source, &self.node_signer, &self.secp_ctx, path, contents, reply_path
                )?;

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

        fn handle_onion_message_response<T: OnionMessageContents>(
                &self, response: Option<T>, reply_path: Option<BlindedPath>, log_suffix: fmt::Arguments
        ) {
                if let Some(response) = response {
                        match reply_path {
                                Some(reply_path) => {
                                        self.find_path_and_enqueue_onion_message(
                                                response, Destination::BlindedPath(reply_path), None, log_suffix
                                        );
                                },
                                None => {
                                        log_trace!(self.logger, "Missing reply path {}", log_suffix);
                                },
                        }
                }
        }

        fn find_path_and_enqueue_onion_message<T: OnionMessageContents>(
                &self, contents: T, destination: Destination, reply_path: Option<BlindedPath>,
                log_suffix: fmt::Arguments
        ) {
                let sender = match self.node_signer.get_node_id(Recipient::Node) {
                        Ok(node_id) => node_id,
                        Err(_) => {
                                log_warn!(self.logger, "Unable to retrieve node id {}", log_suffix);
                                return;
                        }
                };

                let peers = self.pending_messages.lock().unwrap().keys().copied().collect();
                let path = match self.message_router.find_path(sender, peers, destination) {
                        Ok(path) => path,
                        Err(()) => {
                                log_trace!(self.logger, "Failed to find path {}", log_suffix);
                                return;
                        },
                };

                log_trace!(self.logger, "Sending onion message {}", log_suffix);

                if let Err(e) = self.send_onion_message(path, contents, reply_path) {
                        log_trace!(self.logger, "Failed sending onion message {}: {:?}", log_suffix, e);
                        return;
                }
        }

        #[cfg(test)]
        pub(super) fn release_pending_msgs(&self) -> HashMap<PublicKey, VecDeque<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<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<ES: Deref, NS: Deref, L: Deref, MR: Deref, OMH: Deref, CMH: Deref> OnionMessageHandler
for OnionMessenger<ES, NS, L, MR, OMH, CMH>
where
        ES::Target: EntropySource,
        NS::Target: NodeSigner,
        L::Target: Logger,
        MR::Target: MessageRouter,
        OMH::Target: OffersMessageHandler,
        CMH::Target: CustomOnionMessageHandler,
{
        fn handle_onion_message(&self, _peer_node_id: &PublicKey, msg: &OnionMessage) {
                match peel_onion_message(
                        msg, &self.secp_ctx, &*self.node_signer, &*self.logger, &*self.custom_handler
                ) {
                        Ok(PeeledOnion::Receive(message, path_id, reply_path)) => {
                                log_trace!(self.logger,
                                        "Received an onion message with path_id {:02x?} and {} reply_path",
                                                path_id, if reply_path.is_some() { "a" } else { "no" });

                                match message {
                                        ParsedOnionMessageContents::Offers(msg) => {
                                                let response = self.offers_handler.handle_message(msg);
                                                self.handle_onion_message_response(
                                                        response, reply_path, format_args!(
                                                                "when responding to Offers onion message with path_id {:02x?}",
                                                                path_id
                                                        )
                                                );
                                        },
                                        ParsedOnionMessageContents::Custom(msg) => {
                                                let response = self.custom_handler.handle_custom_message(msg);
                                                self.handle_onion_message_response(
                                                        response, reply_path, format_args!(
                                                                "when responding to Custom onion message with path_id {:02x?}",
                                                                path_id
                                                        )
                                                );
                                        },
                                }
                        },
                        Ok(PeeledOnion::Forward(next_node_id, onion_message)) => {
                                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_error!(self.logger, "Failed to process onion message {:?}", e);
                        }
                }
        }

        fn peer_connected(&self, their_node_id: &PublicKey, init: &msgs::Init, _inbound: bool) -> 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) {
                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
        }

        // Before returning any messages to send for the peer, this method will see if any messages were
        // enqueued in the handler by users, find a path to the corresponding blinded path's introduction
        // node, and then enqueue the message for sending to the first peer in the full path.
        fn next_onion_message_for_peer(&self, peer_node_id: PublicKey) -> Option<OnionMessage> {
                // Enqueue any initiating `OffersMessage`s to send.
                for message in self.offers_handler.release_pending_messages() {
                        let PendingOnionMessage { contents, destination, reply_path } = message;
                        self.find_path_and_enqueue_onion_message(
                                contents, destination, reply_path, format_args!("when sending OffersMessage")
                        );
                }

                // Enqueue any initiating `CustomMessage`s to send.
                for message in self.custom_handler.release_pending_custom_messages() {
                        let PendingOnionMessage { contents, destination, reply_path } = message;
                        self.find_path_and_enqueue_onion_message(
                                contents, destination, reply_path, format_args!("when sending CustomMessage")
                        );
                }

                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.
///
/// This is not exported to bindings users as type aliases aren't supported in most languages.
///
/// [`SimpleArcChannelManager`]: crate::ln::channelmanager::SimpleArcChannelManager
/// [`SimpleArcPeerManager`]: crate::ln::peer_handler::SimpleArcPeerManager
#[cfg(not(c_bindings))]
pub type SimpleArcOnionMessenger<M, T, F, L> = OnionMessenger<
        Arc<KeysManager>,
        Arc<KeysManager>,
        Arc<L>,
        Arc<DefaultMessageRouter>,
        Arc<SimpleArcChannelManager<M, T, F, L>>,
        IgnoringMessageHandler
>;

/// Useful for simplifying the parameters of [`SimpleRefChannelManager`] and
/// [`SimpleRefPeerManager`]. See their docs for more details.
///
/// This is not exported to bindings users as type aliases aren't supported in most languages.
///
/// [`SimpleRefChannelManager`]: crate::ln::channelmanager::SimpleRefChannelManager
/// [`SimpleRefPeerManager`]: crate::ln::peer_handler::SimpleRefPeerManager
#[cfg(not(c_bindings))]
pub type SimpleRefOnionMessenger<
        'a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, M, T, F, L
> = OnionMessenger<
        &'a KeysManager,
        &'a KeysManager,
        &'b L,
        &'i DefaultMessageRouter,
        &'j SimpleRefChannelManager<'a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, M, T, F, 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: OnionMessageContents, S: secp256k1::Signing + secp256k1::Verification>(
        secp_ctx: &Secp256k1<S>, unblinded_path: &[PublicKey], destination: Destination, message: T,
        mut reply_path: Option<BlindedPath>, session_priv: &SecretKey
) -> Result<(Vec<(Payload<T>, [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::BlindedPath(BlindedPath {
                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;
        let mut final_control_tlvs = None;
        utils::construct_keys_callback(secp_ctx, unblinded_path.iter(), 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 blinded_path_idx < num_blinded_hops.saturating_sub(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 {
                                final_control_tlvs = Some(ReceiveControlTlvs::Blinded(encrypted_payload));
                                prev_control_tlvs_ss = Some(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) = final_control_tlvs {
                payloads.push((Payload::Receive {
                        control_tlvs,
                        reply_path: reply_path.take(),
                        message,
                }, prev_control_tlvs_ss.unwrap()));
        } else {
                payloads.push((Payload::Receive {
                        control_tlvs: ReceiveControlTlvs::Unblinded(ReceiveTlvs { path_id: None, }),
                        reply_path: reply_path.take(),
                        message,
                }, prev_control_tlvs_ss.unwrap()));
        }

        Ok((payloads, onion_packet_keys))
}

/// Errors if the serialized payload size exceeds onion_message::BIG_PACKET_HOP_DATA_LEN
fn construct_onion_message_packet<T: OnionMessageContents>(payloads: Vec<(Payload<T>, [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(()) };

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