Add phantom shared secret to HTLCPreviousHopData

This also fixes a bug where we were failing back phantom payments with the
wrong scid, causing them to never actually be failed backwards (L3022 in
channelmanager.rs)

This new field will be used in upcoming commit(s) to encrypt phantom payment failure
packets.

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -420,6 +420,7 @@ pub(crate) struct HTLCPreviousHopData {
 	short_channel_id: u64,
 	htlc_id: u64,
 	incoming_packet_shared_secret: [u8; 32],
+	phantom_shared_secret: Option<[u8; 32]>,
 
 	// This field is consumed by `claim_funds_from_hop()` when updating a force-closed backwards
 	// channel with a preimage provided by the forward channel.
@@ -3014,17 +3015,18 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 										routing, incoming_shared_secret, payment_hash, amt_to_forward, outgoing_cltv_value },
 										prev_funding_outpoint } => {
 											macro_rules! fail_forward {
-												($msg: expr, $err_code: expr, $err_data: expr) => {
+												($msg: expr, $err_code: expr, $err_data: expr, $phantom_ss: expr) => {
 													{
 														log_info!(self.logger, "Failed to accept/forward incoming HTLC: {}", $msg);
 														let htlc_source = HTLCSource::PreviousHopData(HTLCPreviousHopData {
-															short_channel_id: short_chan_id,
+															short_channel_id: prev_short_channel_id,
 															outpoint: prev_funding_outpoint,
 															htlc_id: prev_htlc_id,
 															incoming_packet_shared_secret: incoming_shared_secret,
+															phantom_shared_secret: $phantom_ss,
 														});
 														failed_forwards.push((htlc_source, payment_hash,
-																HTLCFailReason::Reason { failure_code: $err_code, data: $err_data }
+															HTLCFailReason::Reason { failure_code: $err_code, data: $err_data }
 														));
 														continue;
 													}
@@ -3041,26 +3043,26 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 													let next_hop = match onion_utils::decode_next_hop(phantom_shared_secret, &onion_packet.hop_data, onion_packet.hmac, payment_hash) {
 														Ok(res) => res,
 														Err(onion_utils::OnionDecodeErr::Malformed { err_msg, err_code }) => {
-															fail_forward!(err_msg, err_code, Vec::new());
+															fail_forward!(err_msg, err_code, Vec::new(), None);
 														},
 														Err(onion_utils::OnionDecodeErr::Relay { err_msg, err_code }) => {
-															fail_forward!(err_msg, err_code, Vec::new());
+															fail_forward!(err_msg, err_code, Vec::new(), Some(phantom_shared_secret));
 														},
 													};
 													match next_hop {
 														onion_utils::Hop::Receive(hop_data) => {
 															match self.construct_recv_pending_htlc_info(hop_data, incoming_shared_secret, payment_hash, amt_to_forward, outgoing_cltv_value, Some(phantom_shared_secret)) {
 																Ok(info) => phantom_receives.push((prev_short_channel_id, prev_funding_outpoint, vec![(info, prev_htlc_id)])),
-																Err(ReceiveError { err_code, err_data, msg }) => fail_forward!(msg, err_code, err_data)
+																Err(ReceiveError { err_code, err_data, msg }) => fail_forward!(msg, err_code, err_data, Some(phantom_shared_secret))
 															}
 														},
 														_ => panic!(),
 													}
 												} else {
-													fail_forward!(format!("Unknown short channel id {} for forward HTLC", short_chan_id), 0x4000 | 10, Vec::new());
+													fail_forward!(format!("Unknown short channel id {} for forward HTLC", short_chan_id), 0x4000 | 10, Vec::new(), None);
 												}
 											} else {
-												fail_forward!(format!("Unknown short channel id {} for forward HTLC", short_chan_id), 0x4000 | 10, Vec::new());
+												fail_forward!(format!("Unknown short channel id {} for forward HTLC", short_chan_id), 0x4000 | 10, Vec::new(), None);
 											}
 										},
 									HTLCForwardInfo::FailHTLC { .. } => {
@@ -3093,6 +3095,8 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 										outpoint: prev_funding_outpoint,
 										htlc_id: prev_htlc_id,
 										incoming_packet_shared_secret: incoming_shared_secret,
+										// Phantom payments are only PendingHTLCRouting::Receive.
+										phantom_shared_secret: None,
 									});
 									match chan.get_mut().send_htlc(amt_to_forward, payment_hash, outgoing_cltv_value, htlc_source.clone(), onion_packet, &self.logger) {
 										Err(e) => {
@@ -3224,6 +3228,7 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 										outpoint: prev_funding_outpoint,
 										htlc_id: prev_htlc_id,
 										incoming_packet_shared_secret: incoming_shared_secret,
+										phantom_shared_secret,
 									},
 									value: amt_to_forward,
 									cltv_expiry,
@@ -3241,6 +3246,7 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 												outpoint: prev_funding_outpoint,
 												htlc_id: $htlc.prev_hop.htlc_id,
 												incoming_packet_shared_secret: $htlc.prev_hop.incoming_packet_shared_secret,
+												phantom_shared_secret,
 											}), payment_hash,
 											HTLCFailReason::Reason { failure_code: 0x4000 | 15, data: htlc_msat_height_data }
 										));
@@ -6073,6 +6079,7 @@ impl_writeable_tlv_based_enum!(PendingHTLCStatus, ;
 
 impl_writeable_tlv_based!(HTLCPreviousHopData, {
 	(0, short_channel_id, required),
+	(1, phantom_shared_secret, option),
 	(2, outpoint, required),
 	(4, htlc_id, required),
 	(6, incoming_packet_shared_secret, required)