Support verifying payment_secrets for inbound static invoice payments

Add a new payment type for this, because normally the payment hash is factored
into the payment secrets we create for invoices, but static invoices don't have
a payment hash since they are paid via keysend.

Author: valentinewallace

lightning/src/ln/inbound_payment.rs

@@ -103,6 +103,7 @@ enum Method {
 	UserPaymentHash = 1,
 	LdkPaymentHashCustomFinalCltv = 2,
 	UserPaymentHashCustomFinalCltv = 3,
+	SpontaneousPayment = 4,
 }
 
 impl Method {
@@ -112,6 +113,7 @@ impl Method {
 			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),
 		}
 	}
@@ -191,6 +193,26 @@ pub fn create_from_hash(keys: &ExpandedKey, min_value_msat: Option<u64>, payment
 	Ok(construct_payment_secret(&iv_bytes, &metadata_bytes, &keys.metadata_key))
 }
 
+#[cfg(async_payments)]
+pub(super) 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 metadata_bytes = construct_metadata_bytes(
+		min_value_msat, Method::SpontaneousPayment, invoice_expiry_delta_secs, current_time,
+		min_final_cltv_expiry_delta
+	)?;
+
+	let mut hmac = HmacEngine::<Sha256>::new(&keys.offers_base_key);
+	hmac.input(&metadata_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, &metadata_bytes, &keys.metadata_key))
+}
+
 fn construct_metadata_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; METADATA_LEN], ()> {
 	if min_value_msat.is_some() && min_value_msat.unwrap() > MAX_VALUE_MSAT {
@@ -320,6 +342,14 @@ pub(super) fn verify<L: Deref>(payment_hash: PaymentHash, payment_data: &msgs::F
 				}
 			}
 		},
+		Ok(Method::SpontaneousPayment) => {
+			let mut hmac = HmacEngine::<Sha256>::new(&keys.offers_base_key);
+			hmac.input(&metadata_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(());
@@ -365,6 +395,9 @@ pub(super) fn get_payment_preimage(payment_hash: PaymentHash, payment_secret: Pa
 		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) }),
 	}
 }