lnwire: add onion message type

diff --git a/lnwire/fuzz_test.go b/lnwire/fuzz_test.go
index a143ce6..6bbc867 100644
--- a/lnwire/fuzz_test.go
+++ b/lnwire/fuzz_test.go
@@ -498,6 +498,12 @@ func FuzzCustomMessage(f *testing.F) {
 	})
 }
 
+func FuzzOnionMessage(f *testing.F) {
+	f.Fuzz(func(t *testing.T, data []byte) {
+		wireMsgHarness(t, data, MsgOnionMessage)
+	})
+}
+
 // FuzzParseRawSignature tests that our DER-encoded signature parsing does not
 // panic for arbitrary inputs and that serializing and reparsing the signatures
 // does not mutate them.
diff --git a/lnwire/message.go b/lnwire/message.go
index c64b09b..428f06b 100644
--- a/lnwire/message.go
+++ b/lnwire/message.go
@@ -66,6 +66,7 @@ const (
 	MsgChannelAnnouncement2                = 267
 	MsgNodeAnnouncement2                   = 269
 	MsgChannelUpdate2                      = 271
+	MsgOnionMessage                        = 513
 	MsgKickoffSig                          = 777
 
 	// MsgEnd defines the end of the official message range of the protocol.
@@ -198,6 +199,8 @@ func (t MessageType) String() string {
 		return "NodeAnnouncement2"
 	case MsgChannelUpdate2:
 		return "ChannelUpdate2"
+	case MsgOnionMessage:
+		return "OnionMessage"
 	default:
 		return "<unknown>"
 	}
@@ -362,6 +365,8 @@ func makeEmptyMessage(msgType MessageType) (Message, error) {
 		msg = &NodeAnnouncement2{}
 	case MsgChannelUpdate2:
 		msg = &ChannelUpdate2{}
+	case MsgOnionMessage:
+		msg = &OnionMessage{}
 	default:
 		// If the message is not within our custom range and has not
 		// specifically been overridden, return an unknown message.
diff --git a/lnwire/onion_message.go b/lnwire/onion_message.go
new file mode 100644
index 0000000..cabbc21
--- /dev/null
+++ b/lnwire/onion_message.go
@@ -0,0 +1,81 @@
+package lnwire
+
+import (
+	"bytes"
+	"io"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+)
+
+// OnionMessage is a message that carries an onion-encrypted payload.
+// This is used for BOLT12 messages.
+type OnionMessage struct {
+	// PathKey is the route blinding ephemeral pubkey to be used for
+	// the onion message.
+	PathKey *btcec.PublicKey
+
+	// OnionBlob contains the onion_message_packet, the raw serialized
+	// Sphinx onion packet (BOLT 4) containing the layered, per-hop
+	// encrypted payloads and routing instructions used to forward this
+	// message along its designated path. This blob should be handled in the
+	// same manner as onion_routing_packet used to route HTLCs, with the
+	// exception that it uses blinded routes by default.
+	OnionBlob []byte
+}
+
+// NewOnionMessage creates a new OnionMessage.
+func NewOnionMessage(pathKey *btcec.PublicKey,
+	onion []byte) *OnionMessage {
+
+	return &OnionMessage{
+		PathKey:   pathKey,
+		OnionBlob: onion,
+	}
+}
+
+// A compile-time check to ensure OnionMessage implements the Message interface.
+var _ Message = (*OnionMessage)(nil)
+
+// Decode reads the bytes stream and converts it to the object.
+func (o *OnionMessage) Decode(r io.Reader, _ uint32) error {
+	if err := ReadElement(r, &o.PathKey); err != nil {
+		return err
+	}
+
+	var onionLen uint16
+	if err := ReadElement(r, &onionLen); err != nil {
+		return err
+	}
+
+	o.OnionBlob = make([]byte, onionLen)
+	if err := ReadElement(r, o.OnionBlob); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// Encode converts object to the bytes stream and write it into the
+// write buffer.
+func (o *OnionMessage) Encode(w *bytes.Buffer, _ uint32) error {
+	if err := WritePublicKey(w, o.PathKey); err != nil {
+		return err
+	}
+
+	onionLen := len(o.OnionBlob)
+	if err := WriteUint16(w, uint16(onionLen)); err != nil {
+		return err
+	}
+
+	if err := WriteBytes(w, o.OnionBlob); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// MsgType returns the integer uniquely identifying this message type on the
+// wire.
+func (o *OnionMessage) MsgType() MessageType {
+	return MsgOnionMessage
+}
diff --git a/lnwire/test_message.go b/lnwire/test_message.go
index 4e62bee..9aae7d2 100644
--- a/lnwire/test_message.go
+++ b/lnwire/test_message.go
@@ -822,6 +822,28 @@ func (c *Custom) RandTestMessage(t *rapid.T) Message {
 	return msg
 }
 
+// A compile time check to ensure OnionMessage implements the lnwire.TestMessage
+// interface.
+var _ TestMessage = (*OnionMessage)(nil)
+
+// RandTestMessage populates the message with random data suitable for testing.
+// It uses the rapid testing framework to generate random values.
+//
+// This is part of the TestMessage interface.
+func (o *OnionMessage) RandTestMessage(t *rapid.T) Message {
+	// Generate random compressed public key for node ID
+	pathKey := RandPubKey(t)
+
+	dataLen := rapid.IntRange(0, 1000).Draw(t, "onionMessageDataLength")
+	data := rapid.SliceOfN(rapid.Byte(), dataLen, dataLen).Draw(
+		t, "onionMessageData",
+	)
+
+	msg := NewOnionMessage(pathKey, data)
+
+	return msg
+}
+
 // A compile time check to ensure DynAck implements the lnwire.TestMessage
 // interface.
 var _ TestMessage = (*DynAck)(nil)