_ Git - rust-lightning/blob - lightning/src/blinded_path/payment.rs

   1 //! Data structures and methods for constructing [`BlindedPath`]s to send a payment over.
   2 //!
   3 //! [`BlindedPath`]: crate::blinded_path::BlindedPath
   4
   5 use bitcoin::secp256k1::{self, PublicKey, Secp256k1, SecretKey};
   6
   7 use crate::blinded_path::BlindedHop;
   8 use crate::blinded_path::utils;
   9 use crate::io;
  10 use crate::ln::PaymentSecret;
  11 use crate::ln::channelmanager::CounterpartyForwardingInfo;
  12 use crate::ln::features::BlindedHopFeatures;
  13 use crate::ln::msgs::DecodeError;
  14 use crate::offers::invoice::BlindedPayInfo;
  15 use crate::util::ser::{HighZeroBytesDroppedBigSize, Readable, Writeable, Writer};
  16
  17 #[allow(unused_imports)]
  18 use crate::prelude::*;
  19
  20 /// An intermediate node, its outbound channel, and relay parameters.
  21 #[derive(Clone, Debug)]
  22 pub struct ForwardNode {
  23         /// The TLVs for this node's [`BlindedHop`], where the fee parameters contained within are also
  24         /// used for [`BlindedPayInfo`] construction.
  25         pub tlvs: ForwardTlvs,
  26         /// This node's pubkey.
  27         pub node_id: PublicKey,
  28         /// The maximum value, in msat, that may be accepted by this node.
  29         pub htlc_maximum_msat: u64,
  30 }
  31
  32 /// Data to construct a [`BlindedHop`] for forwarding a payment.
  33 #[derive(Clone, Debug)]
  34 pub struct ForwardTlvs {
  35         /// The short channel id this payment should be forwarded out over.
  36         pub short_channel_id: u64,
  37         /// Payment parameters for relaying over [`Self::short_channel_id`].
  38         pub payment_relay: PaymentRelay,
  39         /// Payment constraints for relaying over [`Self::short_channel_id`].
  40         pub payment_constraints: PaymentConstraints,
  41         /// Supported and required features when relaying a payment onion containing this object's
  42         /// corresponding [`BlindedHop::encrypted_payload`].
  43         ///
  44         /// [`BlindedHop::encrypted_payload`]: crate::blinded_path::BlindedHop::encrypted_payload
  45         pub features: BlindedHopFeatures,
  46 }
  47
  48 /// Data to construct a [`BlindedHop`] for receiving a payment. This payload is custom to LDK and
  49 /// may not be valid if received by another lightning implementation.
  50 #[derive(Clone, Debug)]
  51 pub struct ReceiveTlvs {
  52         /// Used to authenticate the sender of a payment to the receiver and tie MPP HTLCs together.
  53         pub payment_secret: PaymentSecret,
  54         /// Constraints for the receiver of this payment.
  55         pub payment_constraints: PaymentConstraints,
  56         /// Context for the receiver of this payment.
  57         pub payment_context: PaymentContext,
  58 }
  59
  60 /// Data to construct a [`BlindedHop`] for sending a payment over.
  61 ///
  62 /// [`BlindedHop`]: crate::blinded_path::BlindedHop
  63 pub(crate) enum BlindedPaymentTlvs {
  64         /// This blinded payment data is for a forwarding node.
  65         Forward(ForwardTlvs),
  66         /// This blinded payment data is for the receiving node.
  67         Receive(ReceiveTlvs),
  68 }
  69
  70 // Used to include forward and receive TLVs in the same iterator for encoding.
  71 enum BlindedPaymentTlvsRef<'a> {
  72         Forward(&'a ForwardTlvs),
  73         Receive(&'a ReceiveTlvs),
  74 }
  75
  76 /// Parameters for relaying over a given [`BlindedHop`].
  77 ///
  78 /// [`BlindedHop`]: crate::blinded_path::BlindedHop
  79 #[derive(Clone, Debug)]
  80 pub struct PaymentRelay {
  81         /// Number of blocks subtracted from an incoming HTLC's `cltv_expiry` for this [`BlindedHop`].
  82         pub cltv_expiry_delta: u16,
  83         /// Liquidity fee charged (in millionths of the amount transferred) for relaying a payment over
  84         /// this [`BlindedHop`], (i.e., 10,000 is 1%).
  85         pub fee_proportional_millionths: u32,
  86         /// Base fee charged (in millisatoshi) for relaying a payment over this [`BlindedHop`].
  87         pub fee_base_msat: u32,
  88 }
  89
  90 /// Constraints for relaying over a given [`BlindedHop`].
  91 ///
  92 /// [`BlindedHop`]: crate::blinded_path::BlindedHop
  93 #[derive(Clone, Debug)]
  94 pub struct PaymentConstraints {
  95         /// The maximum total CLTV that is acceptable when relaying a payment over this [`BlindedHop`].
  96         pub max_cltv_expiry: u32,
  97         /// The minimum value, in msat, that may be accepted by the node corresponding to this
  98         /// [`BlindedHop`].
  99         pub htlc_minimum_msat: u64,
 100 }
 101
 102 /// The context of an inbound payment, which is included in a [`BlindedPath`] via [`ReceiveTlvs`]
 103 /// and surfaced in [`PaymentPurpose`].
 104 ///
 105 /// [`BlindedPath`]: crate::blinded_path::BlindedPath
 106 /// [`PaymentPurpose`]: crate::events::PaymentPurpose
 107 #[derive(Clone, Debug)]
 108 pub enum PaymentContext {
 109         /// The payment context was unknown.
 110         Unknown(UnknownPaymentContext),
 111 }
 112
 113 /// An unknown payment context.
 114 #[derive(Clone, Debug)]
 115 pub struct UnknownPaymentContext(());
 116
 117 impl PaymentContext {
 118         pub(crate) fn unknown() -> Self {
 119                 PaymentContext::Unknown(UnknownPaymentContext(()))
 120         }
 121 }
 122
 123 impl TryFrom<CounterpartyForwardingInfo> for PaymentRelay {
 124         type Error = ();
 125
 126         fn try_from(info: CounterpartyForwardingInfo) -> Result<Self, ()> {
 127                 let CounterpartyForwardingInfo {
 128                         fee_base_msat, fee_proportional_millionths, cltv_expiry_delta
 129                 } = info;
 130
 131                 // Avoid exposing esoteric CLTV expiry deltas
 132                 let cltv_expiry_delta = match cltv_expiry_delta {
 133                         0..=40 => 40,
 134                         41..=80 => 80,
 135                         81..=144 => 144,
 136                         145..=216 => 216,
 137                         _ => return Err(()),
 138                 };
 139
 140                 Ok(Self { cltv_expiry_delta, fee_proportional_millionths, fee_base_msat })
 141         }
 142 }
 143
 144 impl Writeable for ForwardTlvs {
 145         fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
 146                 let features_opt =
 147                         if self.features == BlindedHopFeatures::empty() { None }
 148                         else { Some(&self.features) };
 149                 encode_tlv_stream!(w, {
 150                         (2, self.short_channel_id, required),
 151                         (10, self.payment_relay, required),
 152                         (12, self.payment_constraints, required),
 153                         (14, features_opt, option)
 154                 });
 155                 Ok(())
 156         }
 157 }
 158
 159 impl Writeable for ReceiveTlvs {
 160         fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
 161                 encode_tlv_stream!(w, {
 162                         (12, self.payment_constraints, required),
 163                         (65536, self.payment_secret, required),
 164                         (65537, self.payment_context, required)
 165                 });
 166                 Ok(())
 167         }
 168 }
 169
 170 impl<'a> Writeable for BlindedPaymentTlvsRef<'a> {
 171         fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
 172                 // TODO: write padding
 173                 match self {
 174                         Self::Forward(tlvs) => tlvs.write(w)?,
 175                         Self::Receive(tlvs) => tlvs.write(w)?,
 176                 }
 177                 Ok(())
 178         }
 179 }
 180
 181 impl Readable for BlindedPaymentTlvs {
 182         fn read<R: io::Read>(r: &mut R) -> Result<Self, DecodeError> {
 183                 _init_and_read_tlv_stream!(r, {
 184                         (1, _padding, option),
 185                         (2, scid, option),
 186                         (10, payment_relay, option),
 187                         (12, payment_constraints, required),
 188                         (14, features, option),
 189                         (65536, payment_secret, option),
 190                         (65537, payment_context, (default_value, PaymentContext::unknown())),
 191                 });
 192                 let _padding: Option<utils::Padding> = _padding;
 193
 194                 if let Some(short_channel_id) = scid {
 195                         if payment_secret.is_some() {
 196                                 return Err(DecodeError::InvalidValue)
 197                         }
 198                         Ok(BlindedPaymentTlvs::Forward(ForwardTlvs {
 199                                 short_channel_id,
 200                                 payment_relay: payment_relay.ok_or(DecodeError::InvalidValue)?,
 201                                 payment_constraints: payment_constraints.0.unwrap(),
 202                                 features: features.unwrap_or_else(BlindedHopFeatures::empty),
 203                         }))
 204                 } else {
 205                         if payment_relay.is_some() || features.is_some() { return Err(DecodeError::InvalidValue) }
 206                         Ok(BlindedPaymentTlvs::Receive(ReceiveTlvs {
 207                                 payment_secret: payment_secret.ok_or(DecodeError::InvalidValue)?,
 208                                 payment_constraints: payment_constraints.0.unwrap(),
 209                                 payment_context: payment_context.0.unwrap(),
 210                         }))
 211                 }
 212         }
 213 }
 214
 215 /// Construct blinded payment hops for the given `intermediate_nodes` and payee info.
 216 pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 217         secp_ctx: &Secp256k1<T>, intermediate_nodes: &[ForwardNode],
 218         payee_node_id: PublicKey, payee_tlvs: ReceiveTlvs, session_priv: &SecretKey
 219 ) -> Result<Vec<BlindedHop>, secp256k1::Error> {
 220         let pks = intermediate_nodes.iter().map(|node| &node.node_id)
 221                 .chain(core::iter::once(&payee_node_id));
 222         let tlvs = intermediate_nodes.iter().map(|node| BlindedPaymentTlvsRef::Forward(&node.tlvs))
 223                 .chain(core::iter::once(BlindedPaymentTlvsRef::Receive(&payee_tlvs)));
 224         utils::construct_blinded_hops(secp_ctx, pks, tlvs, session_priv)
 225 }
 226
 227 /// `None` if underflow occurs.
 228 pub(crate) fn amt_to_forward_msat(inbound_amt_msat: u64, payment_relay: &PaymentRelay) -> Option<u64> {
 229         let inbound_amt = inbound_amt_msat as u128;
 230         let base = payment_relay.fee_base_msat as u128;
 231         let prop = payment_relay.fee_proportional_millionths as u128;
 232
 233         let post_base_fee_inbound_amt =
 234                 if let Some(amt) = inbound_amt.checked_sub(base) { amt } else { return None };
 235         let mut amt_to_forward =
 236                 (post_base_fee_inbound_amt * 1_000_000 + 1_000_000 + prop - 1) / (prop + 1_000_000);
 237
 238         let fee = ((amt_to_forward * prop) / 1_000_000) + base;
 239         if inbound_amt - fee < amt_to_forward {
 240                 // Rounding up the forwarded amount resulted in underpaying this node, so take an extra 1 msat
 241                 // in fee to compensate.
 242                 amt_to_forward -= 1;
 243         }
 244         debug_assert_eq!(amt_to_forward + fee, inbound_amt);
 245         u64::try_from(amt_to_forward).ok()
 246 }
 247
 248 pub(super) fn compute_payinfo(
 249         intermediate_nodes: &[ForwardNode], payee_tlvs: &ReceiveTlvs, payee_htlc_maximum_msat: u64,
 250         min_final_cltv_expiry_delta: u16
 251 ) -> Result<BlindedPayInfo, ()> {
 252         let mut curr_base_fee: u64 = 0;
 253         let mut curr_prop_mil: u64 = 0;
 254         let mut cltv_expiry_delta: u16 = min_final_cltv_expiry_delta;
 255         for tlvs in intermediate_nodes.iter().rev().map(|n| &n.tlvs) {
 256                 // In the future, we'll want to take the intersection of all supported features for the
 257                 // `BlindedPayInfo`, but there are no features in that context right now.
 258                 if tlvs.features.requires_unknown_bits_from(&BlindedHopFeatures::empty()) { return Err(()) }
 259
 260                 let next_base_fee = tlvs.payment_relay.fee_base_msat as u64;
 261                 let next_prop_mil = tlvs.payment_relay.fee_proportional_millionths as u64;
 262                 // Use integer arithmetic to compute `ceil(a/b)` as `(a+b-1)/b`
 263                 // ((curr_base_fee * (1_000_000 + next_prop_mil)) / 1_000_000) + next_base_fee
 264                 curr_base_fee = curr_base_fee.checked_mul(1_000_000 + next_prop_mil)
 265                         .and_then(|f| f.checked_add(1_000_000 - 1))
 266                         .map(|f| f / 1_000_000)
 267                         .and_then(|f| f.checked_add(next_base_fee))
 268                         .ok_or(())?;
 269                 // ceil(((curr_prop_mil + 1_000_000) * (next_prop_mil + 1_000_000)) / 1_000_000) - 1_000_000
 270                 curr_prop_mil = curr_prop_mil.checked_add(1_000_000)
 271                         .and_then(|f1| next_prop_mil.checked_add(1_000_000).and_then(|f2| f2.checked_mul(f1)))
 272                         .and_then(|f| f.checked_add(1_000_000 - 1))
 273                         .map(|f| f / 1_000_000)
 274                         .and_then(|f| f.checked_sub(1_000_000))
 275                         .ok_or(())?;
 276
 277                 cltv_expiry_delta = cltv_expiry_delta.checked_add(tlvs.payment_relay.cltv_expiry_delta).ok_or(())?;
 278         }
 279
 280         let mut htlc_minimum_msat: u64 = 1;
 281         let mut htlc_maximum_msat: u64 = 21_000_000 * 100_000_000 * 1_000; // Total bitcoin supply
 282         for node in intermediate_nodes.iter() {
 283                 // The min htlc for an intermediate node is that node's min minus the fees charged by all of the
 284                 // following hops for forwarding that min, since that fee amount will automatically be included
 285                 // in the amount that this node receives and contribute towards reaching its min.
 286                 htlc_minimum_msat = amt_to_forward_msat(
 287                         core::cmp::max(node.tlvs.payment_constraints.htlc_minimum_msat, htlc_minimum_msat),
 288                         &node.tlvs.payment_relay
 289                 ).unwrap_or(1); // If underflow occurs, we definitely reached this node's min
 290                 htlc_maximum_msat = amt_to_forward_msat(
 291                         core::cmp::min(node.htlc_maximum_msat, htlc_maximum_msat), &node.tlvs.payment_relay
 292                 ).ok_or(())?; // If underflow occurs, we cannot send to this hop without exceeding their max
 293         }
 294         htlc_minimum_msat = core::cmp::max(
 295                 payee_tlvs.payment_constraints.htlc_minimum_msat, htlc_minimum_msat
 296         );
 297         htlc_maximum_msat = core::cmp::min(payee_htlc_maximum_msat, htlc_maximum_msat);
 298
 299         if htlc_maximum_msat < htlc_minimum_msat { return Err(()) }
 300         Ok(BlindedPayInfo {
 301                 fee_base_msat: u32::try_from(curr_base_fee).map_err(|_| ())?,
 302                 fee_proportional_millionths: u32::try_from(curr_prop_mil).map_err(|_| ())?,
 303                 cltv_expiry_delta,
 304                 htlc_minimum_msat,
 305                 htlc_maximum_msat,
 306                 features: BlindedHopFeatures::empty(),
 307         })
 308 }
 309
 310 impl Writeable for PaymentRelay {
 311         fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
 312                 self.cltv_expiry_delta.write(w)?;
 313                 self.fee_proportional_millionths.write(w)?;
 314                 HighZeroBytesDroppedBigSize(self.fee_base_msat).write(w)
 315         }
 316 }
 317 impl Readable for PaymentRelay {
 318         fn read<R: io::Read>(r: &mut R) -> Result<Self, DecodeError> {
 319                 let cltv_expiry_delta: u16 = Readable::read(r)?;
 320                 let fee_proportional_millionths: u32 = Readable::read(r)?;
 321                 let fee_base_msat: HighZeroBytesDroppedBigSize<u32> = Readable::read(r)?;
 322                 Ok(Self { cltv_expiry_delta, fee_proportional_millionths, fee_base_msat: fee_base_msat.0 })
 323         }
 324 }
 325
 326 impl Writeable for PaymentConstraints {
 327         fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
 328                 self.max_cltv_expiry.write(w)?;
 329                 HighZeroBytesDroppedBigSize(self.htlc_minimum_msat).write(w)
 330         }
 331 }
 332 impl Readable for PaymentConstraints {
 333         fn read<R: io::Read>(r: &mut R) -> Result<Self, DecodeError> {
 334                 let max_cltv_expiry: u32 = Readable::read(r)?;
 335                 let htlc_minimum_msat: HighZeroBytesDroppedBigSize<u64> = Readable::read(r)?;
 336                 Ok(Self { max_cltv_expiry, htlc_minimum_msat: htlc_minimum_msat.0 })
 337         }
 338 }
 339
 340 impl_writeable_tlv_based_enum!(PaymentContext,
 341         ;
 342         (0, Unknown),
 343 );
 344
 345 impl Writeable for UnknownPaymentContext {
 346         fn write<W: Writer>(&self, _w: &mut W) -> Result<(), io::Error> {
 347                 Ok(())
 348         }
 349 }
 350
 351 impl Readable for UnknownPaymentContext {
 352         fn read<R: io::Read>(_r: &mut R) -> Result<Self, DecodeError> {
 353                 Ok(UnknownPaymentContext(()))
 354         }
 355 }
 356
 357 #[cfg(test)]
 358 mod tests {
 359         use bitcoin::secp256k1::PublicKey;
 360         use crate::blinded_path::payment::{ForwardNode, ForwardTlvs, ReceiveTlvs, PaymentConstraints, PaymentContext, PaymentRelay};
 361         use crate::ln::PaymentSecret;
 362         use crate::ln::features::BlindedHopFeatures;
 363         use crate::ln::functional_test_utils::TEST_FINAL_CLTV;
 364
 365         #[test]
 366         fn compute_payinfo() {
 367                 // Taken from the spec example for aggregating blinded payment info. See
 368                 // https://github.com/lightning/bolts/blob/master/proposals/route-blinding.md#blinded-payments
 369                 let dummy_pk = PublicKey::from_slice(&[2; 33]).unwrap();
 370                 let intermediate_nodes = vec![ForwardNode {
 371                         node_id: dummy_pk,
 372                         tlvs: ForwardTlvs {
 373                                 short_channel_id: 0,
 374                                 payment_relay: PaymentRelay {
 375                                         cltv_expiry_delta: 144,
 376                                         fee_proportional_millionths: 500,
 377                                         fee_base_msat: 100,
 378                                 },
 379                                 payment_constraints: PaymentConstraints {
 380                                         max_cltv_expiry: 0,
 381                                         htlc_minimum_msat: 100,
 382                                 },
 383                                 features: BlindedHopFeatures::empty(),
 384                         },
 385                         htlc_maximum_msat: u64::max_value(),
 386                 }, ForwardNode {
 387                         node_id: dummy_pk,
 388                         tlvs: ForwardTlvs {
 389                                 short_channel_id: 0,
 390                                 payment_relay: PaymentRelay {
 391                                         cltv_expiry_delta: 144,
 392                                         fee_proportional_millionths: 500,
 393                                         fee_base_msat: 100,
 394                                 },
 395                                 payment_constraints: PaymentConstraints {
 396                                         max_cltv_expiry: 0,
 397                                         htlc_minimum_msat: 1_000,
 398                                 },
 399                                 features: BlindedHopFeatures::empty(),
 400                         },
 401                         htlc_maximum_msat: u64::max_value(),
 402                 }];
 403                 let recv_tlvs = ReceiveTlvs {
 404                         payment_secret: PaymentSecret([0; 32]),
 405                         payment_constraints: PaymentConstraints {
 406                                 max_cltv_expiry: 0,
 407                                 htlc_minimum_msat: 1,
 408                         },
 409                         payment_context: PaymentContext::unknown(),
 410                 };
 411                 let htlc_maximum_msat = 100_000;
 412                 let blinded_payinfo = super::compute_payinfo(&intermediate_nodes[..], &recv_tlvs, htlc_maximum_msat, 12).unwrap();
 413                 assert_eq!(blinded_payinfo.fee_base_msat, 201);
 414                 assert_eq!(blinded_payinfo.fee_proportional_millionths, 1001);
 415                 assert_eq!(blinded_payinfo.cltv_expiry_delta, 300);
 416                 assert_eq!(blinded_payinfo.htlc_minimum_msat, 900);
 417                 assert_eq!(blinded_payinfo.htlc_maximum_msat, htlc_maximum_msat);
 418         }
 419
 420         #[test]
 421         fn compute_payinfo_1_hop() {
 422                 let recv_tlvs = ReceiveTlvs {
 423                         payment_secret: PaymentSecret([0; 32]),
 424                         payment_constraints: PaymentConstraints {
 425                                 max_cltv_expiry: 0,
 426                                 htlc_minimum_msat: 1,
 427                         },
 428                         payment_context: PaymentContext::unknown(),
 429                 };
 430                 let blinded_payinfo = super::compute_payinfo(&[], &recv_tlvs, 4242, TEST_FINAL_CLTV as u16).unwrap();
 431                 assert_eq!(blinded_payinfo.fee_base_msat, 0);
 432                 assert_eq!(blinded_payinfo.fee_proportional_millionths, 0);
 433                 assert_eq!(blinded_payinfo.cltv_expiry_delta, TEST_FINAL_CLTV as u16);
 434                 assert_eq!(blinded_payinfo.htlc_minimum_msat, 1);
 435                 assert_eq!(blinded_payinfo.htlc_maximum_msat, 4242);
 436         }
 437
 438         #[test]
 439         fn simple_aggregated_htlc_min() {
 440                 // If no hops charge fees, the htlc_minimum_msat should just be the maximum htlc_minimum_msat
 441                 // along the path.
 442                 let dummy_pk = PublicKey::from_slice(&[2; 33]).unwrap();
 443                 let intermediate_nodes = vec![ForwardNode {
 444                         node_id: dummy_pk,
 445                         tlvs: ForwardTlvs {
 446                                 short_channel_id: 0,
 447                                 payment_relay: PaymentRelay {
 448                                         cltv_expiry_delta: 0,
 449                                         fee_proportional_millionths: 0,
 450                                         fee_base_msat: 0,
 451                                 },
 452                                 payment_constraints: PaymentConstraints {
 453                                         max_cltv_expiry: 0,
 454                                         htlc_minimum_msat: 1,
 455                                 },
 456                                 features: BlindedHopFeatures::empty(),
 457                         },
 458                         htlc_maximum_msat: u64::max_value()
 459                 }, ForwardNode {
 460                         node_id: dummy_pk,
 461                         tlvs: ForwardTlvs {
 462                                 short_channel_id: 0,
 463                                 payment_relay: PaymentRelay {
 464                                         cltv_expiry_delta: 0,
 465                                         fee_proportional_millionths: 0,
 466                                         fee_base_msat: 0,
 467                                 },
 468                                 payment_constraints: PaymentConstraints {
 469                                         max_cltv_expiry: 0,
 470                                         htlc_minimum_msat: 2_000,
 471                                 },
 472                                 features: BlindedHopFeatures::empty(),
 473                         },
 474                         htlc_maximum_msat: u64::max_value()
 475                 }];
 476                 let recv_tlvs = ReceiveTlvs {
 477                         payment_secret: PaymentSecret([0; 32]),
 478                         payment_constraints: PaymentConstraints {
 479                                 max_cltv_expiry: 0,
 480                                 htlc_minimum_msat: 3,
 481                         },
 482                         payment_context: PaymentContext::unknown(),
 483                 };
 484                 let htlc_maximum_msat = 100_000;
 485                 let blinded_payinfo = super::compute_payinfo(&intermediate_nodes[..], &recv_tlvs, htlc_maximum_msat, TEST_FINAL_CLTV as u16).unwrap();
 486                 assert_eq!(blinded_payinfo.htlc_minimum_msat, 2_000);
 487         }
 488
 489         #[test]
 490         fn aggregated_htlc_min() {
 491                 // Create a path with varying fees and htlc_mins, and make sure htlc_minimum_msat ends up as the
 492                 // max (htlc_min - following_fees) along the path.
 493                 let dummy_pk = PublicKey::from_slice(&[2; 33]).unwrap();
 494                 let intermediate_nodes = vec![ForwardNode {
 495                         node_id: dummy_pk,
 496                         tlvs: ForwardTlvs {
 497                                 short_channel_id: 0,
 498                                 payment_relay: PaymentRelay {
 499                                         cltv_expiry_delta: 0,
 500                                         fee_proportional_millionths: 500,
 501                                         fee_base_msat: 1_000,
 502                                 },
 503                                 payment_constraints: PaymentConstraints {
 504                                         max_cltv_expiry: 0,
 505                                         htlc_minimum_msat: 5_000,
 506                                 },
 507                                 features: BlindedHopFeatures::empty(),
 508                         },
 509                         htlc_maximum_msat: u64::max_value()
 510                 }, ForwardNode {
 511                         node_id: dummy_pk,
 512                         tlvs: ForwardTlvs {
 513                                 short_channel_id: 0,
 514                                 payment_relay: PaymentRelay {
 515                                         cltv_expiry_delta: 0,
 516                                         fee_proportional_millionths: 500,
 517                                         fee_base_msat: 200,
 518                                 },
 519                                 payment_constraints: PaymentConstraints {
 520                                         max_cltv_expiry: 0,
 521                                         htlc_minimum_msat: 2_000,
 522                                 },
 523                                 features: BlindedHopFeatures::empty(),
 524                         },
 525                         htlc_maximum_msat: u64::max_value()
 526                 }];
 527                 let recv_tlvs = ReceiveTlvs {
 528                         payment_secret: PaymentSecret([0; 32]),
 529                         payment_constraints: PaymentConstraints {
 530                                 max_cltv_expiry: 0,
 531                                 htlc_minimum_msat: 1,
 532                         },
 533                         payment_context: PaymentContext::unknown(),
 534                 };
 535                 let htlc_minimum_msat = 3798;
 536                 assert!(super::compute_payinfo(&intermediate_nodes[..], &recv_tlvs, htlc_minimum_msat - 1, TEST_FINAL_CLTV as u16).is_err());
 537
 538                 let htlc_maximum_msat = htlc_minimum_msat + 1;
 539                 let blinded_payinfo = super::compute_payinfo(&intermediate_nodes[..], &recv_tlvs, htlc_maximum_msat, TEST_FINAL_CLTV as u16).unwrap();
 540                 assert_eq!(blinded_payinfo.htlc_minimum_msat, htlc_minimum_msat);
 541                 assert_eq!(blinded_payinfo.htlc_maximum_msat, htlc_maximum_msat);
 542         }
 543
 544         #[test]
 545         fn aggregated_htlc_max() {
 546                 // Create a path with varying fees and `htlc_maximum_msat`s, and make sure the aggregated max
 547                 // htlc ends up as the min (htlc_max - following_fees) along the path.
 548                 let dummy_pk = PublicKey::from_slice(&[2; 33]).unwrap();
 549                 let intermediate_nodes = vec![ForwardNode {
 550                         node_id: dummy_pk,
 551                         tlvs: ForwardTlvs {
 552                                 short_channel_id: 0,
 553                                 payment_relay: PaymentRelay {
 554                                         cltv_expiry_delta: 0,
 555                                         fee_proportional_millionths: 500,
 556                                         fee_base_msat: 1_000,
 557                                 },
 558                                 payment_constraints: PaymentConstraints {
 559                                         max_cltv_expiry: 0,
 560                                         htlc_minimum_msat: 1,
 561                                 },
 562                                 features: BlindedHopFeatures::empty(),
 563                         },
 564                         htlc_maximum_msat: 5_000,
 565                 }, ForwardNode {
 566                         node_id: dummy_pk,
 567                         tlvs: ForwardTlvs {
 568                                 short_channel_id: 0,
 569                                 payment_relay: PaymentRelay {
 570                                         cltv_expiry_delta: 0,
 571                                         fee_proportional_millionths: 500,
 572                                         fee_base_msat: 1,
 573                                 },
 574                                 payment_constraints: PaymentConstraints {
 575                                         max_cltv_expiry: 0,
 576                                         htlc_minimum_msat: 1,
 577                                 },
 578                                 features: BlindedHopFeatures::empty(),
 579                         },
 580                         htlc_maximum_msat: 10_000
 581                 }];
 582                 let recv_tlvs = ReceiveTlvs {
 583                         payment_secret: PaymentSecret([0; 32]),
 584                         payment_constraints: PaymentConstraints {
 585                                 max_cltv_expiry: 0,
 586                                 htlc_minimum_msat: 1,
 587                         },
 588                         payment_context: PaymentContext::unknown(),
 589                 };
 590
 591                 let blinded_payinfo = super::compute_payinfo(&intermediate_nodes[..], &recv_tlvs, 10_000, TEST_FINAL_CLTV as u16).unwrap();
 592                 assert_eq!(blinded_payinfo.htlc_maximum_msat, 3997);
 593         }
 594 }