lightning 0.3.0-beta1

// 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.

//! Utilities to generate inbound payment information in service of invoice creation.

use bitcoin::hashes::cmp::fixed_time_eq;
use bitcoin::hashes::hmac::{Hmac, HmacEngine};
use bitcoin::hashes::sha256::Hash as Sha256;
use bitcoin::hashes::{Hash, HashEngine};

use crate::crypto::utils::{apply_chacha20, hkdf_extract_expand_8x};
use crate::ln::msgs;
use crate::ln::msgs::MAX_VALUE_MSAT;
use crate::offers::nonce::Nonce as LocalNonce;
use crate::sign::EntropySource;
use crate::types::payment::{PaymentHash, PaymentPreimage, PaymentSecret};
use crate::util::errors::APIError;
use crate::util::logger::Logger;

#[allow(unused_imports)]
use crate::prelude::*;

pub(crate) const IV_LEN: usize = 16;
const INFO_LEN: usize = 16;
const INFO_KEY_LEN: usize = 32;
const AMT_MSAT_LEN: usize = 8;
// Used to shift the payment type bits to take up the top 3 bits of the info bytes, or to
// retrieve said payment type bits.
const METHOD_TYPE_OFFSET: usize = 5;

/// A set of keys that were HKDF-expanded. Returned by [`NodeSigner::get_expanded_key`].
///
/// [`NodeSigner::get_expanded_key`]: crate::sign::NodeSigner::get_expanded_key
#[derive(Hash, Copy, Clone, PartialEq, Eq, Debug)]
pub struct ExpandedKey {
	/// The key used to encrypt the bytes containing the payment info (i.e. the amount and
	/// expiry, included for payment verification on decryption).
	info_key: [u8; 32],
	/// The key used to authenticate an LDK-provided payment hash and info as previously
	/// registered with LDK.
	ldk_pmt_hash_key: [u8; 32],
	/// The key used to authenticate a user-provided payment hash and info as previously
	/// registered with LDK.
	user_pmt_hash_key: [u8; 32],
	/// The base key used to derive signing keys and authenticate messages for BOLT 12 Offers.
	offers_base_key: [u8; 32],
	/// The key used to encrypt message metadata for BOLT 12 Offers.
	offers_encryption_key: [u8; 32],
	/// The key used to authenticate spontaneous payments' info as previously registered with LDK
	/// for inclusion in a blinded path.
	spontaneous_pmt_key: [u8; 32],
	/// The key used to authenticate phantom-node-shared blinded paths as generated by us. Note
	/// that this is not used for blinded paths that are not expected to be shared across nodes
	/// participating in a "phantom node".
	pub(crate) phantom_node_blinded_path_key: [u8; 32],
	/// The key used to encrypt payment metadata.
	metadata_enc_key: [u8; 32],
}

impl ExpandedKey {
	/// Create a  new [`ExpandedKey`] for generating an inbound payment hash and secret.
	///
	/// It is recommended to cache this value and not regenerate it for each new inbound payment.
	pub fn new(key_material: [u8; 32]) -> ExpandedKey {
		let (
			info_key,
			ldk_pmt_hash_key,
			user_pmt_hash_key,
			offers_base_key,
			offers_encryption_key,
			spontaneous_pmt_key,
			phantom_node_blinded_path_key,
			metadata_enc_key,
		) = hkdf_extract_expand_8x(b"LDK Inbound Payment Key Expansion", &key_material);
		Self {
			info_key,
			ldk_pmt_hash_key,
			user_pmt_hash_key,
			offers_base_key,
			offers_encryption_key,
			spontaneous_pmt_key,
			phantom_node_blinded_path_key,
			metadata_enc_key,
		}
	}

	/// Returns an [`HmacEngine`] used to construct [`Offer::metadata`].
	///
	/// [`Offer::metadata`]: crate::offers::offer::Offer::metadata
	pub(crate) fn hmac_for_offer(&self) -> HmacEngine<Sha256> {
		HmacEngine::<Sha256>::new(&self.offers_base_key)
	}

	/// Encrypts or decrypts the given `bytes`. Used for data included in an offer message's
	/// metadata (e.g., payment id).
	pub(crate) fn crypt_for_offer(&self, mut bytes: [u8; 32], nonce: LocalNonce) -> [u8; 32] {
		apply_chacha20(self.offers_encryption_key, nonce.0, &mut bytes);
		bytes
	}
}

/// We currently set aside 3 bits for the `Method` in the `PaymentSecret`.
enum Method {
	LdkPaymentHash = 0,
	UserPaymentHash = 1,
	LdkPaymentHashCustomFinalCltv = 2,
	UserPaymentHashCustomFinalCltv = 3,
	SpontaneousPayment = 4,
}

impl Method {
	fn from_bits(bits: u8) -> Result<Method, u8> {
		match bits {
			bits if bits == Method::LdkPaymentHash as u8 => Ok(Method::LdkPaymentHash),
			bits if bits == Method::UserPaymentHash as u8 => Ok(Method::UserPaymentHash),
			bits if bits == Method::LdkPaymentHashCustomFinalCltv as u8 => {
				Ok(Method::LdkPaymentHashCustomFinalCltv)
			},
			bits if bits == Method::UserPaymentHashCustomFinalCltv as u8 => {
				Ok(Method::UserPaymentHashCustomFinalCltv)
			},
			bits if bits == Method::SpontaneousPayment as u8 => Ok(Method::SpontaneousPayment),
			unknown => Err(unknown),
		}
	}
}

fn min_final_cltv_expiry_delta_from_info(bytes: [u8; INFO_LEN]) -> u16 {
	let expiry_bytes = &bytes[AMT_MSAT_LEN..];
	u16::from_be_bytes([expiry_bytes[0], expiry_bytes[1]])
}

/// Equivalent to [`crate::ln::channelmanager::ChannelManager::create_inbound_payment`], but no
/// `ChannelManager` is required. Useful for generating invoices for [phantom node payments] without
/// a `ChannelManager`.
///
/// `keys` is generated by calling [`NodeSigner::get_expanded_key`]. It is recommended to
/// cache this value and not regenerate it for each new inbound payment.
///
/// `current_time` is a Unix timestamp representing the current time.
///
/// Note that if `min_final_cltv_expiry_delta` is set to some value, then the payment will not be receivable
/// on versions of LDK prior to 0.0.114.
///
/// Returns an encrypted copy of the `payment_metadata` (if any) which must be included as a part of
/// validation.
///
/// [phantom node payments]: crate::sign::PhantomKeysManager
/// [`NodeSigner::get_expanded_key`]: crate::sign::NodeSigner::get_expanded_key
pub fn create<ES: EntropySource>(
	keys: &ExpandedKey, min_value_msat: Option<u64>, invoice_expiry_delta_secs: u32,
	entropy_source: &ES, current_time: u64, min_final_cltv_expiry_delta: Option<u16>,
	mut payment_metadata: Option<Vec<u8>>,
) -> Result<(PaymentHash, PaymentSecret, Option<Vec<u8>>), ()> {
	let info_bytes = construct_info_bytes(
		min_value_msat,
		if min_final_cltv_expiry_delta.is_some() {
			Method::LdkPaymentHashCustomFinalCltv
		} else {
			Method::LdkPaymentHash
		},
		invoice_expiry_delta_secs,
		current_time,
		min_final_cltv_expiry_delta,
	)?;

	let mut iv_bytes = [0 as u8; IV_LEN];
	let rand_bytes = entropy_source.get_secure_random_bytes();
	iv_bytes.copy_from_slice(&rand_bytes[..IV_LEN]);

	if let Some(metadata) = payment_metadata.as_mut() {
		apply_chacha20(keys.metadata_enc_key, iv_bytes, metadata.as_mut_slice());
	}

	let mut hmac = HmacEngine::<Sha256>::new(&keys.ldk_pmt_hash_key);
	hmac.input(&iv_bytes);
	hmac.input(&info_bytes);
	if let Some(metadata) = payment_metadata.as_ref() {
		hmac.input(&(metadata.len() as u64).to_le_bytes());
		hmac.input(metadata);
	}
	let payment_preimage_bytes = Hmac::from_engine(hmac).to_byte_array();

	let ldk_pmt_hash = PaymentHash(Sha256::hash(&payment_preimage_bytes).to_byte_array());
	let payment_secret = construct_payment_secret(&iv_bytes, &info_bytes, &keys.info_key);
	Ok((ldk_pmt_hash, payment_secret, payment_metadata))
}

/// Equivalent to [`crate::ln::channelmanager::ChannelManager::create_inbound_payment_for_hash`],
/// but no `ChannelManager` is required. Useful for generating invoices for [phantom node payments]
/// without a `ChannelManager`.
///
/// See [`create`] for information on the `keys` and `current_time` parameters.
///
/// Note that if `min_final_cltv_expiry_delta` is set to some value, then the payment will not be receivable
/// on versions of LDK prior to 0.0.114.
///
/// Returns an encrypted copy of the `payment_metadata` (if any) which must be included as a part of
/// validation.
///
/// [phantom node payments]: crate::sign::PhantomKeysManager
pub fn create_from_hash<ES: EntropySource>(
	keys: &ExpandedKey, min_value_msat: Option<u64>, payment_hash: PaymentHash,
	invoice_expiry_delta_secs: u32, entropy_source: &ES, current_time: u64,
	min_final_cltv_expiry_delta: Option<u16>, mut payment_metadata: Option<Vec<u8>>,
) -> Result<(PaymentSecret, Option<Vec<u8>>), ()> {
	let info_bytes = construct_info_bytes(
		min_value_msat,
		if min_final_cltv_expiry_delta.is_some() {
			Method::UserPaymentHashCustomFinalCltv
		} else {
			Method::UserPaymentHash
		},
		invoice_expiry_delta_secs,
		current_time,
		min_final_cltv_expiry_delta,
	)?;

	if let Some(metadata) = payment_metadata.as_mut() {
		let mut iv_bytes = [0 as u8; IV_LEN];
		let rand_bytes = entropy_source.get_secure_random_bytes();
		iv_bytes.copy_from_slice(&rand_bytes[..IV_LEN]);

		apply_chacha20(keys.metadata_enc_key, iv_bytes, metadata.as_mut_slice());
		metadata.extend_from_slice(&iv_bytes);
	}

	let mut hmac = HmacEngine::<Sha256>::new(&keys.user_pmt_hash_key);
	hmac.input(&info_bytes);
	hmac.input(&payment_hash.0);
	if let Some(metadata) = payment_metadata.as_ref() {
		hmac.input(&(metadata.len() as u64).to_le_bytes());
		hmac.input(metadata);
	}
	let hmac_bytes = Hmac::from_engine(hmac).to_byte_array();

	let mut iv_bytes = [0 as u8; IV_LEN];
	iv_bytes.copy_from_slice(&hmac_bytes[..IV_LEN]);

	Ok((construct_payment_secret(&iv_bytes, &info_bytes, &keys.info_key), payment_metadata))
}

pub(crate) fn create_for_spontaneous_payment(
	keys: &ExpandedKey, min_value_msat: Option<u64>, invoice_expiry_delta_secs: u32,
	current_time: u64, min_final_cltv_expiry_delta: Option<u16>,
) -> Result<PaymentSecret, ()> {
	let info_bytes = construct_info_bytes(
		min_value_msat,
		Method::SpontaneousPayment,
		invoice_expiry_delta_secs,
		current_time,
		min_final_cltv_expiry_delta,
	)?;

	let mut hmac = HmacEngine::<Sha256>::new(&keys.spontaneous_pmt_key);
	hmac.input(&info_bytes);
	let hmac_bytes = Hmac::from_engine(hmac).to_byte_array();

	let mut iv_bytes = [0 as u8; IV_LEN];
	iv_bytes.copy_from_slice(&hmac_bytes[..IV_LEN]);

	Ok(construct_payment_secret(&iv_bytes, &info_bytes, &keys.info_key))
}

pub(crate) fn calculate_absolute_expiry(
	highest_seen_timestamp: u64, invoice_expiry_delta_secs: u32,
) -> u64 {
	// We assume that highest_seen_timestamp is pretty close to the current time - it's updated when
	// we receive a new block with the maximum time we've seen in a header. It should never be more
	// than two hours in the future.  Thus, we add two hours here as a buffer to ensure we
	// absolutely never fail a payment too early.
	// Note that we assume that received blocks have reasonably up-to-date timestamps.
	highest_seen_timestamp + invoice_expiry_delta_secs as u64 + 7200
}

fn construct_info_bytes(
	min_value_msat: Option<u64>, payment_type: Method, invoice_expiry_delta_secs: u32,
	highest_seen_timestamp: u64, min_final_cltv_expiry_delta: Option<u16>,
) -> Result<[u8; INFO_LEN], ()> {
	if min_value_msat.is_some() && min_value_msat.unwrap() > MAX_VALUE_MSAT {
		return Err(());
	}

	let mut min_amt_msat_bytes: [u8; AMT_MSAT_LEN] = match min_value_msat {
		Some(amt) => amt.to_be_bytes(),
		None => [0; AMT_MSAT_LEN],
	};
	min_amt_msat_bytes[0] |= (payment_type as u8) << METHOD_TYPE_OFFSET;

	let expiry_timestamp =
		calculate_absolute_expiry(highest_seen_timestamp, invoice_expiry_delta_secs);
	let mut expiry_bytes = expiry_timestamp.to_be_bytes();

	// `min_value_msat` should fit in (64 bits - 3 payment type bits =) 61 bits as an unsigned integer.
	// This should leave us with a maximum value greater than the 21M BTC supply cap anyway.
	if min_value_msat.is_some() && min_value_msat.unwrap() > ((1u64 << 61) - 1) {
		return Err(());
	}

	// `expiry_timestamp` should fit in (64 bits - 2 delta bytes =) 48 bits as an unsigned integer.
	// Bitcoin's block header timestamps are actually `u32`s, so we're technically already limited to
	// the much smaller maximum timestamp of `u32::MAX` for now, but we check the u64 `expiry_timestamp`
	// for future-proofing.
	if min_final_cltv_expiry_delta.is_some() && expiry_timestamp > ((1u64 << 48) - 1) {
		return Err(());
	}

	if let Some(min_final_cltv_expiry_delta) = min_final_cltv_expiry_delta {
		let bytes = min_final_cltv_expiry_delta.to_be_bytes();
		expiry_bytes[0] |= bytes[0];
		expiry_bytes[1] |= bytes[1];
	}

	let mut info_bytes: [u8; INFO_LEN] = [0; INFO_LEN];

	info_bytes[..AMT_MSAT_LEN].copy_from_slice(&min_amt_msat_bytes);
	info_bytes[AMT_MSAT_LEN..].copy_from_slice(&expiry_bytes);

	Ok(info_bytes)
}

fn construct_payment_secret(
	iv_bytes: &[u8; IV_LEN], info_bytes: &[u8; INFO_LEN], info_key: &[u8; INFO_KEY_LEN],
) -> PaymentSecret {
	let mut payment_secret_bytes: [u8; 32] = [0; 32];
	let (iv_slice, encrypted_info_slice) = payment_secret_bytes.split_at_mut(IV_LEN);
	iv_slice.copy_from_slice(iv_bytes);

	encrypted_info_slice.copy_from_slice(info_bytes);
	apply_chacha20(*info_key, *iv_bytes, encrypted_info_slice);

	PaymentSecret(payment_secret_bytes)
}

/// Check that an inbound payment's `payment_data` field is sane.
///
/// LDK does not store any data for pending inbound payments. Instead, we construct our payment
/// secret (and, if supplied by LDK, our payment preimage) to include encrypted information about
/// the payment.
///
/// For payments without a custom `min_final_cltv_expiry_delta`, the payment info is:
///   payment method (3 bits) || payment amount (8 bytes - 3 bits) || expiry (8 bytes)
///
/// For payments including a custom `min_final_cltv_expiry_delta`, the payment info is:
///   payment method (3 bits) || payment amount (8 bytes - 3 bits) || min_final_cltv_expiry_delta (2 bytes) || expiry (6 bytes)
///
/// In both cases the result is then encrypted using a key derived from [`NodeSigner::get_expanded_key`].
///
/// Then on payment receipt, we verify in this method that the payment preimage and payment secret
/// match what was constructed.
///
/// [`create_inbound_payment`] and [`create_inbound_payment_for_hash`] are called by the user to
/// construct the payment secret and/or payment hash that this method is verifying. If the former
/// method is called, then the payment method bits mentioned above are represented internally as
/// [`Method::LdkPaymentHash`]. If the latter, [`Method::UserPaymentHash`].
///
/// For the former method, the payment preimage is constructed as an HMAC of payment info and
/// random bytes. Because the payment secret is also encoded with these random bytes and info
/// (with the info encrypted with a block cipher), we're able to authenticate the preimage on
/// payment receipt.
///
/// For the latter, the payment secret instead contains an HMAC of the user-provided payment hash
/// and payment info (encrypted with a block cipher), allowing us to authenticate the payment
/// hash and info on payment receipt.
///
/// See [`ExpandedKey`] docs for more info on the individual keys used.
///
/// [`NodeSigner::get_expanded_key`]: crate::sign::NodeSigner::get_expanded_key
/// [`create_inbound_payment`]: crate::ln::channelmanager::ChannelManager::create_inbound_payment
/// [`create_inbound_payment_for_hash`]: crate::ln::channelmanager::ChannelManager::create_inbound_payment_for_hash
pub(super) fn verify<L: Logger>(
	payment_hash: PaymentHash, payment_data: &msgs::FinalOnionHopData,
	mut payment_metadata: Option<&mut Vec<u8>>, highest_seen_timestamp: u64, keys: &ExpandedKey,
	logger: &L,
) -> Result<(Option<PaymentPreimage>, Option<u16>), ()> {
	let (iv_bytes, info_bytes) = decrypt_info(payment_data.payment_secret, keys);

	let payment_type_res = Method::from_bits((info_bytes[0] & 0b1110_0000) >> METHOD_TYPE_OFFSET);
	let mut amt_msat_bytes = [0; AMT_MSAT_LEN];
	let mut expiry_bytes = [0; INFO_LEN - AMT_MSAT_LEN];
	amt_msat_bytes.copy_from_slice(&info_bytes[..AMT_MSAT_LEN]);
	expiry_bytes.copy_from_slice(&info_bytes[AMT_MSAT_LEN..]);
	// Zero out the bits reserved to indicate the payment type.
	amt_msat_bytes[0] &= 0b00011111;
	let mut min_final_cltv_expiry_delta = None;

	// Make sure to check the HMAC before doing the other checks below, to mitigate timing attacks.
	let mut payment_preimage = None;

	match payment_type_res {
		Ok(Method::UserPaymentHash) | Ok(Method::UserPaymentHashCustomFinalCltv) => {
			let mut hmac = HmacEngine::<Sha256>::new(&keys.user_pmt_hash_key);
			hmac.input(&info_bytes[..]);
			hmac.input(&payment_hash.0);
			if let Some(metadata) = payment_metadata.as_deref() {
				hmac.input(&(metadata.len() as u64).to_le_bytes());
				hmac.input(metadata);
			}
			if !fixed_time_eq(
				&iv_bytes,
				&Hmac::from_engine(hmac).to_byte_array().split_at_mut(IV_LEN).0,
			) {
				log_trace!(
					logger,
					"Failing HTLC with user-generated payment_hash {}: unexpected payment_secret",
					&payment_hash
				);
				return Err(());
			};

			if let Some(metadata) = payment_metadata.as_mut() {
				if metadata.len() < IV_LEN {
					log_trace!(logger, "payment_metadata was shorter than expected IV. Failing HTLC with payment_hash {payment_hash}");
					return Err(());
				}
				let new_len = metadata.len() - IV_LEN;
				let (metadata_enc, metadata_iv) = metadata.split_at_mut(new_len);
				let metadata_iv: [u8; IV_LEN] = metadata_iv.try_into().expect("len checked");

				apply_chacha20(keys.metadata_enc_key, metadata_iv, metadata_enc);
				metadata.truncate(new_len);
			}
		},
		Ok(Method::LdkPaymentHash) | Ok(Method::LdkPaymentHashCustomFinalCltv) => {
			match derive_ldk_payment_preimage(
				payment_hash,
				&iv_bytes,
				&info_bytes,
				payment_metadata.as_deref().map(Vec::as_slice),
				keys,
			) {
				Ok(preimage) => payment_preimage = Some(preimage),
				Err(bad_preimage_bytes) => {
					log_trace!(
						logger,
						"Failing HTLC with payment_hash {} due to mismatching preimage {}",
						&payment_hash,
						log_bytes!(bad_preimage_bytes)
					);
					return Err(());
				},
			}

			if let Some(metadata) = payment_metadata {
				apply_chacha20(keys.metadata_enc_key, iv_bytes, metadata);
			}
		},
		Ok(Method::SpontaneousPayment) => {
			if payment_metadata.is_some() {
				log_trace!(logger, "Shouldn't have a payment_metadata for a spontaneous payment, failing payment with hash {payment_hash}");
				return Err(());
			}
			let mut hmac = HmacEngine::<Sha256>::new(&keys.spontaneous_pmt_key);
			hmac.input(&info_bytes[..]);
			if !fixed_time_eq(
				&iv_bytes,
				&Hmac::from_engine(hmac).to_byte_array().split_at_mut(IV_LEN).0,
			) {
				log_trace!(logger, "Failing async payment HTLC with sender-generated payment_hash {}: unexpected payment_secret", &payment_hash);
				return Err(());
			}
		},
		Err(unknown_bits) => {
			log_trace!(
				logger,
				"Failing HTLC with payment hash {} due to unknown payment type {}",
				&payment_hash,
				unknown_bits
			);
			return Err(());
		},
	}

	match payment_type_res {
		Ok(Method::UserPaymentHashCustomFinalCltv) | Ok(Method::LdkPaymentHashCustomFinalCltv) => {
			min_final_cltv_expiry_delta = Some(min_final_cltv_expiry_delta_from_info(info_bytes));
			// Zero out first two bytes of expiry reserved for `min_final_cltv_expiry_delta`.
			expiry_bytes[0] &= 0;
			expiry_bytes[1] &= 0;
		},
		_ => {},
	}

	let min_amt_msat: u64 = u64::from_be_bytes(amt_msat_bytes.into());
	let expiry = u64::from_be_bytes(expiry_bytes.try_into().unwrap());

	if payment_data.total_msat < min_amt_msat {
		log_trace!(logger, "Failing HTLC with payment_hash {} due to total_msat {} being less than the minimum amount of {} msat", &payment_hash, payment_data.total_msat, min_amt_msat);
		return Err(());
	}

	if expiry < highest_seen_timestamp {
		log_trace!(logger, "Failing HTLC with payment_hash {}: expired payment", &payment_hash);
		return Err(());
	}

	Ok((payment_preimage, min_final_cltv_expiry_delta))
}

pub(super) fn get_payment_preimage(
	payment_hash: PaymentHash, payment_secret: PaymentSecret, payment_metadata: Option<&mut [u8]>,
	keys: &ExpandedKey,
) -> Result<PaymentPreimage, APIError> {
	let (iv_bytes, info_bytes) = decrypt_info(payment_secret, keys);

	match Method::from_bits((info_bytes[0] & 0b1110_0000) >> METHOD_TYPE_OFFSET) {
		Ok(Method::LdkPaymentHash) | Ok(Method::LdkPaymentHashCustomFinalCltv) => {
			let preimage = derive_ldk_payment_preimage(
				payment_hash,
				&iv_bytes,
				&info_bytes,
				payment_metadata.as_deref(),
				keys,
			)
			.map_err(|bad_preimage_bytes| APIError::APIMisuseError {
				err: format!(
					"Payment hash {} did not match decoded preimage {}",
					&payment_hash,
					log_bytes!(bad_preimage_bytes)
				),
			})?;

			if let Some(metadata) = payment_metadata {
				apply_chacha20(keys.metadata_enc_key, iv_bytes, metadata);
			}
			Ok(preimage)
		},
		Ok(Method::UserPaymentHash) | Ok(Method::UserPaymentHashCustomFinalCltv) => {
			Err(APIError::APIMisuseError {
				err: "Expected payment type to be LdkPaymentHash, instead got UserPaymentHash"
					.to_string(),
			})
		},
		Ok(Method::SpontaneousPayment) => Err(APIError::APIMisuseError {
			err: "Can't extract payment preimage for spontaneous payments".to_string(),
		}),
		Err(other) => {
			Err(APIError::APIMisuseError { err: format!("Unknown payment type: {}", other) })
		},
	}
}

fn decrypt_info(
	payment_secret: PaymentSecret, keys: &ExpandedKey,
) -> ([u8; IV_LEN], [u8; INFO_LEN]) {
	let mut iv_bytes = [0; IV_LEN];
	let (iv_slice, encrypted_info_bytes) = payment_secret.0.split_at(IV_LEN);
	iv_bytes.copy_from_slice(iv_slice);

	let mut info_bytes: [u8; INFO_LEN] = [0; INFO_LEN];
	info_bytes.copy_from_slice(encrypted_info_bytes);
	apply_chacha20(keys.info_key, iv_bytes, &mut info_bytes);

	(iv_bytes, info_bytes)
}

// Errors if the payment preimage doesn't match `payment_hash`. Returns the bad preimage bytes in
// this case.
fn derive_ldk_payment_preimage(
	payment_hash: PaymentHash, iv_bytes: &[u8; IV_LEN], info_bytes: &[u8; INFO_LEN],
	payment_metadata: Option<&[u8]>, keys: &ExpandedKey,
) -> Result<PaymentPreimage, [u8; 32]> {
	let mut hmac = HmacEngine::<Sha256>::new(&keys.ldk_pmt_hash_key);
	hmac.input(iv_bytes);
	hmac.input(info_bytes);
	if let Some(metadata) = payment_metadata {
		hmac.input(&(metadata.len() as u64).to_le_bytes());
		hmac.input(metadata);
	}
	let decoded_payment_preimage = Hmac::from_engine(hmac).to_byte_array();
	if !fixed_time_eq(&payment_hash.0, &Sha256::hash(&decoded_payment_preimage).to_byte_array()) {
		return Err(decoded_payment_preimage);
	}
	return Ok(PaymentPreimage(decoded_payment_preimage));
}