Implement receiving and forwarding onion messages

This required adapting `onion_utils::decode_next_hop` to work for both payments
and onion messages.

Currently we just print out the path_id of any onion messages we receive. In
the future, these received onion messages will be redirected to their
respective handlers: i.e. an invoice_request will go to an InvoiceHandler,
custom onion messages will go to a custom handler, etc.

lightning/src/ln/channelmanager.rs

@@ -2139,7 +2139,7 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 			}
 		}
 
-		let next_hop = match onion_utils::decode_next_hop(shared_secret, &msg.onion_routing_packet.hop_data[..], msg.onion_routing_packet.hmac, msg.payment_hash) {
+		let next_hop = match onion_utils::decode_next_payment_hop(shared_secret, &msg.onion_routing_packet.hop_data[..], msg.onion_routing_packet.hmac, msg.payment_hash) {
 			Ok(res) => res,
 			Err(onion_utils::OnionDecodeErr::Malformed { err_msg, err_code }) => {
 				return_malformed_err!(err_msg, err_code);
@@ -2955,7 +2955,7 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 												let phantom_secret_res = self.keys_manager.get_node_secret(Recipient::PhantomNode);
 												if phantom_secret_res.is_ok() && fake_scid::is_valid_phantom(&self.fake_scid_rand_bytes, short_chan_id) {
 													let phantom_shared_secret = SharedSecret::new(&onion_packet.public_key.unwrap(), &phantom_secret_res.unwrap()).secret_bytes();
-													let next_hop = match onion_utils::decode_next_hop(phantom_shared_secret, &onion_packet.hop_data, onion_packet.hmac, payment_hash) {
+													let next_hop = match onion_utils::decode_next_payment_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 }) => {
 															let sha256_of_onion = Sha256::hash(&onion_packet.hop_data).into_inner();

lightning/src/ln/onion_message.rs

@@ -64,6 +64,13 @@ impl Writeable for (Payload, SharedSecret) {
 	}
 }
 
+/// Reads of `Payload`s are parameterized by the `rho` of a `SharedSecret`, which is used to decrypt
+/// the onion message payload's `encrypted_data` field.
+impl ReadableArgs<SharedSecret> for Payload {
+	fn read<R: Read>(mut r: &mut R, encrypted_tlvs_ss: SharedSecret) -> Result<Self, DecodeError> {
+	}
+}
+
 /// Onion messages contain an encrypted TLV stream. This can be supplied by someone else, in the
 /// case that we're sending to a blinded route, or created by us if we're constructing payloads for
 /// unblinded hops in the onion message's path.

lightning/src/ln/onion_utils.rs

@@ -573,7 +573,37 @@ pub(super) fn process_onion_failure<T: secp256k1::Signing, L: Deref>(secp_ctx: &
 	} else { unreachable!(); }
 }
 
-/// Data decrypted from the onion payload.
+/// Used in the decoding of inbound payments' and onion messages' routing packets. This enum allows
+/// us to use `decode_next_hop` to return the payloads and next hop packet bytes of both payments
+/// and onion messages.
+enum Payload {
+	/// This payload was for an incoming payment.
+	Payment(msgs::OnionHopData),
+	/// This payload was for an incoming onion message.
+	Message(onion_message::Payload),
+}
+
+enum NextPacketBytes {
+	Payment([u8; 20*65]),
+	Message(Vec<u8>),
+}
+
+/// Data decrypted from an onion message's onion payload.
+pub(crate) enum MessageHop {
+	/// This onion payload was for us, not for forwarding to a next-hop.
+	Receive(onion_message::Payload),
+	/// This onion payload needs to be forwarded to a next-hop.
+	Forward {
+		/// Onion payload data used in forwarding the onion message.
+		next_hop_data: onion_message::Payload,
+		/// HMAC of the next hop's onion packet.
+		next_hop_hmac: [u8; 32],
+		/// Bytes of the onion packet we're forwarding.
+		new_packet_bytes: Vec<u8>,
+	},
+}
+
+/// Data decrypted from a payment's onion payload.
 pub(crate) enum Hop {
 	/// This onion payload was for us, not for forwarding to a next-hop. Contains information for
 	/// verifying the incoming payment.
@@ -603,7 +633,13 @@ pub(crate) enum OnionDecodeErr {
 	},
 }
 
-pub(crate) fn decode_next_hop(shared_secret: [u8; 32], hop_data: &[u8], hmac_bytes: [u8; 32], payment_hash: PaymentHash) -> Result<Hop, OnionDecodeErr> {
+pub(crate) fn decode_next_message_hop(shared_secret: [u8; 32], hop_data: &[u8], hmac_bytes: [u8; 32], encrypted_tlvs_ss: SharedSecret) -> Result<MessageHop, OnionDecodeErr> {
+}
+
+pub(crate) fn decode_next_payment_hop(shared_secret: [u8; 32], hop_data: &[u8], hmac_bytes: [u8; 32], payment_hash: PaymentHash) -> Result<Hop, OnionDecodeErr> {
+}
+
+fn decode_next_hop(shared_secret: [u8; 32], hop_data: &[u8], hmac_bytes: [u8; 32], payment_hash: Option<PaymentHash>, encrypted_tlv_ss: Option<SharedSecret>) -> Result<(Payload, Option<([u8; 32], NextPacketBytes)>), OnionDecodeErr> {
 	let (rho, mu) = gen_rho_mu_from_shared_secret(&shared_secret);
 	let mut hmac = HmacEngine::<Sha256>::new(&mu);
 	hmac.input(hop_data);