X-Git-Url: http://git.bitcoin.ninja/index.cgi?a=blobdiff_plain;f=lightning%2Fsrc%2Fonion_message%2Fmessenger.rs;h=ff20f19a7fc16a6bfea57e59f1b00eca27dc1d77;hb=71739dbe0495b3aed4ce907fea88c122df9ac6a1;hp=7eba3cdd254af6418132fc29bfac3cde59bd2292;hpb=736c0b9e7f3dfe022e9bcd73717d17c21fe68355;p=rust-lightning diff --git a/lightning/src/onion_message/messenger.rs b/lightning/src/onion_message/messenger.rs index 7eba3cdd..ff20f19a 100644 --- a/lightning/src/onion_message/messenger.rs +++ b/lightning/src/onion_message/messenger.rs @@ -13,35 +13,42 @@ use bitcoin::hashes::{Hash, HashEngine}; use bitcoin::hashes::hmac::{Hmac, HmacEngine}; use bitcoin::hashes::sha256::Hash as Sha256; -use bitcoin::secp256k1::{self, PublicKey, Secp256k1, SecretKey}; +use bitcoin::secp256k1::{self, PublicKey, Scalar, Secp256k1, SecretKey}; -use chain::keysinterface::{InMemorySigner, KeysInterface, KeysManager, Recipient, Sign}; -use ln::msgs; -use ln::onion_utils; +use crate::chain::keysinterface::{InMemorySigner, KeysInterface, KeysManager, Recipient, Sign}; +use crate::ln::features::{InitFeatures, NodeFeatures}; +use crate::ln::msgs::{self, OnionMessageHandler}; +use crate::ln::onion_utils; +use crate::ln::peer_handler::IgnoringMessageHandler; use super::blinded_route::{BlindedRoute, ForwardTlvs, ReceiveTlvs}; +pub use super::packet::{CustomOnionMessageContents, OnionMessageContents}; use super::packet::{BIG_PACKET_HOP_DATA_LEN, ForwardControlTlvs, Packet, Payload, ReceiveControlTlvs, SMALL_PACKET_HOP_DATA_LEN}; use super::utils; -use util::logger::Logger; +use crate::util::events::OnionMessageProvider; +use crate::util::logger::Logger; +use crate::util::ser::Writeable; use core::ops::Deref; -use sync::{Arc, Mutex}; -use prelude::*; +use crate::sync::{Arc, Mutex}; +use crate::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. +/// and receiving custom onion messages is supported. /// /// # Example /// -// Needs to be `ignore` until the `onion_message` module is made public, otherwise this is a test -// failure. -/// ```ignore +/// ``` /// # 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::onion_message::{BlindedRoute, Destination, OnionMessenger}; +/// # use lightning::ln::msgs::DecodeError; +/// # use lightning::ln::peer_handler::IgnoringMessageHandler; +/// # use lightning::onion_message::{BlindedRoute, CustomOnionMessageContents, Destination, OnionMessageContents, 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 { @@ -54,41 +61,67 @@ use prelude::*; /// # 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 (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); +/// let onion_messenger = OnionMessenger::new(&keys_manager, logger, your_custom_message_handler); /// -/// // Send an empty onion message to a node id. +/// # struct YourCustomMessage {} +/// impl Writeable for YourCustomMessage { +/// fn write(&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 for YourCustomMessage { +/// fn read(r: &mut R, message_type: u64) -> Result, DecodeError> { +/// # unreachable!() +/// // Read your custom onion message of type `message_type` from `r`, or return `None` +/// // if the message type is unknown +/// } +/// } +/// // Send a custom onion message to a node id. /// let intermediate_hops = [hop_node_id1, hop_node_id2]; -/// onion_messenger.send_onion_message(&intermediate_hops, Destination::Node(destination_node_id)); +/// let reply_path = None; +/// # let your_custom_message = YourCustomMessage {}; +/// let message = OnionMessageContents::Custom(your_custom_message); +/// onion_messenger.send_onion_message(&intermediate_hops, Destination::Node(destination_node_id), message, 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(); +/// let blinded_route = BlindedRoute::new(&hops, &keys_manager, &secp_ctx).unwrap(); /// -/// // Send an empty onion message to a blinded route. +/// // Send a custom onion message to a blinded route. /// # let intermediate_hops = [hop_node_id1, hop_node_id2]; -/// onion_messenger.send_onion_message(&intermediate_hops, Destination::BlindedRoute(blinded_route)); +/// let reply_path = None; +/// # let your_custom_message = YourCustomMessage {}; +/// let message = OnionMessageContents::Custom(your_custom_message); +/// onion_messenger.send_onion_message(&intermediate_hops, Destination::BlindedRoute(blinded_route), message, reply_path); /// ``` /// /// [offers]: /// [`OnionMessenger`]: crate::onion_message::OnionMessenger -pub struct OnionMessenger +pub struct OnionMessenger where K::Target: KeysInterface, L::Target: Logger, + CMH:: Target: CustomOnionMessageHandler, { keys_manager: K, logger: L, - pending_messages: Mutex>>, + pending_messages: Mutex>>, secp_ctx: Secp256k1, + custom_handler: CMH, // Coming soon: // invoice_handler: InvoiceHandler, - // custom_handler: CustomHandler, // handles custom onion messages } /// The destination of an onion message. @@ -111,7 +144,7 @@ impl Destination { /// Errors that may occur when [sending an onion message]. /// /// [sending an onion message]: OnionMessenger::send_onion_message -#[derive(Debug, PartialEq)] +#[derive(Debug, PartialEq, Eq)] pub enum SendError { /// Errored computing onion message packet keys. Secp256k1(secp256k1::Error), @@ -121,15 +154,40 @@ pub enum SendError { /// 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, + /// 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, +} + +/// 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 OnionMessenger +impl OnionMessenger where K::Target: KeysInterface, 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) -> Self { + 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 { @@ -137,17 +195,21 @@ impl OnionMessenger pending_messages: Mutex::new(HashMap::new()), secp_ctx, logger, + custom_handler, } } - /// Send an empty onion message to `destination`, routing it through `intermediate_nodes`. + /// Send an onion message with contents `message` to `destination`, routing it through `intermediate_nodes`. /// See [`OnionMessenger`] for example usage. - pub fn send_onion_message(&self, intermediate_nodes: &[PublicKey], destination: Destination) -> Result<(), SendError> { + pub fn send_onion_message(&self, intermediate_nodes: &[PublicKey], destination: Destination, message: OnionMessageContents, reply_path: Option) -> Result<(), SendError> { if let Destination::BlindedRoute(BlindedRoute { ref blinded_hops, .. }) = destination { if blinded_hops.len() < 2 { return Err(SendError::TooFewBlindedHops); } } + let OnionMessageContents::Custom(ref msg) = message; + if msg.tlv_type() < 64 { return Err(SendError::InvalidMessage) } + 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 { @@ -160,28 +222,69 @@ impl OnionMessenger } }; let (packet_payloads, packet_keys) = packet_payloads_and_keys( - &self.secp_ctx, intermediate_nodes, destination, &blinding_secret) + &self.secp_ctx, intermediate_nodes, destination, message, reply_path, &blinding_secret) .map_err(|e| SendError::Secp256k1(e))?; let prng_seed = self.keys_manager.get_secure_random_bytes(); - let onion_packet = construct_onion_message_packet( + 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(); - let pending_msgs = pending_per_peer_msgs.entry(introduction_node_id).or_insert(Vec::new()); - pending_msgs.push( - msgs::OnionMessage { - blinding_point, - onion_routing_packet: onion_packet, + 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(()) } - ); - Ok(()) + } + } + + #[cfg(test)] + pub(super) fn release_pending_msgs(&self) -> HashMap> { + 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>) -> 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 OnionMessageHandler for OnionMessenger + where K::Target: KeysInterface, + 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. - pub fn handle_onion_message(&self, _peer_node_id: &PublicKey, msg: &msgs::OnionMessage) { + 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) => { @@ -196,7 +299,7 @@ impl OnionMessenger Hmac::from_engine(hmac).into_inner() }; match self.keys_manager.ecdh(Recipient::Node, &msg.onion_routing_packet.public_key, - Some(&blinding_factor)) + Some(&Scalar::from_be_bytes(blinding_factor).unwrap())) { Ok(ss) => ss.secret_bytes(), Err(()) => { @@ -208,10 +311,15 @@ impl OnionMessenger 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 }) + Ok((Payload::Receive::<<::Target as CustomOnionMessageHandler>::CustomMessage> { + message, control_tlvs: ReceiveControlTlvs::Unblinded(ReceiveTlvs { path_id }), reply_path, }, None)) => { - log_info!(self.logger, "Received an onion message with path_id: {:02x?}", path_id); + log_info!(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 { + OnionMessageContents::Custom(msg) => self.custom_handler.handle_custom_message(msg), + } }, Ok((Payload::Forward(ForwardControlTlvs::Unblinded(ForwardTlvs { next_node_id, next_blinding_override @@ -234,31 +342,48 @@ impl OnionMessenger hop_data: new_packet_bytes, hmac: next_hop_hmac, }; - - let mut pending_per_peer_msgs = self.pending_messages.lock().unwrap(); - let pending_msgs = pending_per_peer_msgs.entry(next_node_id).or_insert(Vec::new()); - pending_msgs.push( - 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 mut next_blinding_point = msg.blinding_point; - if let Err(e) = next_blinding_point.mul_assign(&self.secp_ctx, &blinding_factor[..]) { + 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 } - next_blinding_point - }, + } }, - onion_routing_packet: outgoing_packet, }, - ); + 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); @@ -269,12 +394,43 @@ impl OnionMessenger }; } - #[cfg(test)] - pub(super) fn release_pending_msgs(&self) -> HashMap> { + 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(); - let mut msgs = HashMap::new(); - core::mem::swap(&mut *pending_msgs, &mut msgs); - msgs + 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 OnionMessageProvider for OnionMessenger + where K::Target: KeysInterface, + L::Target: Logger, + CMH::Target: CustomOnionMessageHandler, +{ + fn next_onion_message_for_peer(&self, peer_node_id: PublicKey) -> Option { + 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 } } @@ -283,21 +439,26 @@ impl OnionMessenger /// Useful for simplifying the parameters of [`SimpleArcChannelManager`] and /// [`SimpleArcPeerManager`]. See their docs for more details. /// -///[`SimpleArcChannelManager`]: crate::ln::channelmanager::SimpleArcChannelManager -///[`SimpleArcPeerManager`]: crate::ln::peer_handler::SimpleArcPeerManager -pub type SimpleArcOnionMessenger = OnionMessenger, Arc>; +/// (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 = OnionMessenger, Arc, IgnoringMessageHandler>; /// Useful for simplifying the parameters of [`SimpleRefChannelManager`] and /// [`SimpleRefPeerManager`]. See their docs for more details. /// -///[`SimpleRefChannelManager`]: crate::ln::channelmanager::SimpleRefChannelManager -///[`SimpleRefPeerManager`]: crate::ln::peer_handler::SimpleRefPeerManager -pub type SimpleRefOnionMessenger<'a, 'b, L> = OnionMessenger; +/// (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; /// 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( - secp_ctx: &Secp256k1, unblinded_path: &[PublicKey], destination: Destination, session_priv: &SecretKey -) -> Result<(Vec<(Payload, [u8; 32])>, Vec), secp256k1::Error> { +fn packet_payloads_and_keys( + secp_ctx: &Secp256k1, unblinded_path: &[PublicKey], destination: Destination, + message: OnionMessageContents, mut reply_path: Option, session_priv: &SecretKey +) -> Result<(Vec<(Payload, [u8; 32])>, Vec), 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); @@ -310,6 +471,7 @@ fn packet_payloads_and_keys( 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, 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() { @@ -339,9 +501,8 @@ fn packet_payloads_and_keys( 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), - }, control_tlvs_ss)); + 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()); @@ -356,17 +517,25 @@ fn packet_payloads_and_keys( }); })?; - if let Some(control_tlvs_ss) = prev_control_tlvs_ss { + 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, }) - }, control_tlvs_ss)); + 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(payloads: Vec<(Payload, [u8; 32])>, onion_keys: Vec, prng_seed: [u8; 32]) -> Result { +fn construct_onion_message_packet(payloads: Vec<(Payload, [u8; 32])>, onion_keys: Vec, prng_seed: [u8; 32]) -> Result { // 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