lnwire: add onion message type

This message type is a message that carries an onion-encrypted payload
used for BOLT12 messages.

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
+}