Propagate Trampoline blinding errors

When a blinded hop fails to decode, we send a special malformed error.
However, we previously simply checked the presence of a blinding point
within the `UpdateAddHTLC` message, which is not necessarily applicable
to Trampoline, so now we additionally return a flag if the error stemmed
from an inner onion's blinded hop decoding failure.

Author: arik-so

lightning/src/ln/onion_payment.rs

@@ -475,7 +475,7 @@ where
 {
 	let encode_malformed_error = |message: &str, err_code: u16| {
 		log_info!(logger, "Failed to accept/forward incoming HTLC: {}", message);
-		let (sha256_of_onion, failure_code) = if msg.blinding_point.is_some() {
+		let (sha256_of_onion, failure_code) = if msg.blinding_point.is_some() || err_code == INVALID_ONION_BLINDING {
 			([0; 32], INVALID_ONION_BLINDING)
 		} else {
 			(Sha256::hash(&msg.onion_routing_packet.hop_data).to_byte_array(), err_code)

lightning/src/ln/onion_utils.rs

@@ -1828,15 +1828,54 @@ where
 							trampoline_shared_secret,
 						),
 					}),
-					Ok((_, None)) => Err(OnionDecodeErr::Malformed {
+					Ok((msgs::InboundTrampolinePayload::BlindedForward(hop_data), None)) => {
+						if hop_data.intro_node_blinding_point.is_some() {
+							return Err(OnionDecodeErr::Relay {
+								err_msg: "Non-final intro node Trampoline onion data provided to us as last hop",
+								err_code: 0x4000 | 22,
+								shared_secret,
+								trampoline_shared_secret: Some(SharedSecret::from_bytes(
+									trampoline_shared_secret,
+								)),
+							});
+						}
+						Err(OnionDecodeErr::Malformed {
+							err_msg: "Non-final Trampoline onion data provided to us as last hop",
+							err_code: INVALID_ONION_BLINDING,
+						})
+					},
+					Ok((msgs::InboundTrampolinePayload::BlindedReceive(hop_data), Some(_))) => {
+						if hop_data.intro_node_blinding_point.is_some() {
+							return Err(OnionDecodeErr::Relay {
+								err_msg: "Final Trampoline intro node onion data provided to us as intermediate hop",
+								err_code: 0x4000 | 22,
+								shared_secret,
+								trampoline_shared_secret: Some(SharedSecret::from_bytes(
+									trampoline_shared_secret,
+								)),
+							});
+						}
+						Err(OnionDecodeErr::Malformed {
+							err_msg:
+								"Final Trampoline onion data provided to us as intermediate hop",
+							err_code: INVALID_ONION_BLINDING,
+						})
+					},
+					Ok((_, None)) => Err(OnionDecodeErr::Relay {
 						err_msg: "Non-final Trampoline onion data provided to us as last hop",
-						// todo: find more suitable error code
 						err_code: 0x4000 | 22,
+						shared_secret,
+						trampoline_shared_secret: Some(SharedSecret::from_bytes(
+							trampoline_shared_secret,
+						)),
 					}),
-					Ok((_, Some(_))) => Err(OnionDecodeErr::Malformed {
+					Ok((_, Some(_))) => Err(OnionDecodeErr::Relay {
 						err_msg: "Final Trampoline onion data provided to us as intermediate hop",
-						// todo: find more suitable error code
 						err_code: 0x4000 | 22,
+						shared_secret,
+						trampoline_shared_secret: Some(SharedSecret::from_bytes(
+							trampoline_shared_secret,
+						)),
 					}),
 					Err(e) => Err(e),
 				}