Allow outer session priv overrides [expound]

For test vectors

Author: arik-so

lightning/src/ln/onion_utils.rs

@@ -942,7 +942,7 @@ pub(crate) struct DecodedOnionFailure {
 #[inline]
 pub(super) fn process_onion_failure<T: secp256k1::Signing, L: Deref>(
 	secp_ctx: &Secp256k1<T>, logger: &L, htlc_source: &HTLCSource,
-	mut encrypted_packet: OnionErrorPacket,
+	mut encrypted_packet: OnionErrorPacket, secondary_session_priv: Option<SecretKey>,
 ) -> DecodedOnionFailure
 where
 	L::Target: Logger,
@@ -1004,8 +1004,10 @@ where
 
 	let outer_session_priv = path.has_trampoline_hops().then(|| {
 		// if we have Trampoline hops, the outer onion session_priv is a hash of the inner one
-		let session_priv_hash = Sha256::hash(&session_priv.secret_bytes()).to_byte_array();
-		SecretKey::from_slice(&session_priv_hash[..]).expect("You broke SHA-256!")
+		secondary_session_priv.unwrap_or_else(|| {
+			let session_priv_hash = Sha256::hash(&session_priv.secret_bytes()).to_byte_array();
+			SecretKey::from_slice(&session_priv_hash[..]).expect("You broke SHA-256!")
+		})
 	});
 
 	let mut onion_keys = Vec::with_capacity(path.hops.len());
@@ -1469,7 +1471,7 @@ impl HTLCFailReason {
 	{
 		match self.0 {
 			HTLCFailReasonRepr::LightningError { ref err } => {
-				process_onion_failure(secp_ctx, logger, &htlc_source, err.clone())
+				process_onion_failure(secp_ctx, logger, &htlc_source, err.clone(), None)
 			},
 			#[allow(unused)]
 			HTLCFailReasonRepr::Reason { ref failure_code, ref data, .. } => {
@@ -2397,7 +2399,7 @@ mod tests {
 
 		// Assert that the original failure can be retrieved and that all hmacs check out.
 		let decrypted_failure =
-			process_onion_failure(&ctx_full, &logger, &htlc_source, onion_error);
+			process_onion_failure(&ctx_full, &logger, &htlc_source, onion_error, None);
 
 		assert_eq!(decrypted_failure.onion_error_code, Some(0x2002));
 	}
@@ -2487,7 +2489,8 @@ mod tests {
 			payment_id: PaymentId([1; 32]),
 		};
 
-		let decrypted_failure = process_onion_failure(&ctx_full, &logger, &htlc_source, packet);
+		let decrypted_failure =
+			process_onion_failure(&ctx_full, &logger, &htlc_source, packet, None);
 
 		decrypted_failure
 	}