lnwallet: update the SignDescriptor struct to use keychain.KeyDescriptor

In this commit, we update the SignDescriptor struct to instead use the
keychain.KeyDescriptor struct inplace of a raw public key. We do this
as this allows for a recovery mode, where the caller doesn’t know what
key was used exactly, but remembers the precise KeyDescriptor. A
stateless signer is still able to re-derive the proper key, as it’s
given the full path.

lnwallet/signdescriptor.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"io"
 
+	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/roasbeef/btcd/btcec"
 	"github.com/roasbeef/btcd/txscript"
 	"github.com/roasbeef/btcd/wire"
@@ -20,10 +21,11 @@ var (
 // a given output. This struct is used by the Signer interface in order to gain
 // access to critical data needed to generate a valid signature.
 type SignDescriptor struct {
-	// Pubkey is the public key to which the signature should be generated
-	// over. The Signer should then generate a signature with the private
-	// key corresponding to this public key.
-	PubKey *btcec.PublicKey
+	// KeyDesc is a descriptor that precisely describes *which* key to use
+	// for signing. This may provide the raw public key directly, or
+	// require the Signer to re-derive the key according to the populated
+	// derivation path.
+	KeyDesc keychain.KeyDescriptor
 
 	// SingleTweak is a scalar value that will be added to the private key
 	// corresponding to the above public key to obtain the private key to
@@ -83,10 +85,26 @@ type SignDescriptor struct {
 // yet, since that is usually done just before broadcast by the witness
 // generator.
 func WriteSignDescriptor(w io.Writer, sd *SignDescriptor) error {
-	serializedPubKey := sd.PubKey.SerializeCompressed()
-	if err := wire.WriteVarBytes(w, 0, serializedPubKey); err != nil {
+	err := binary.Write(w, binary.BigEndian, sd.KeyDesc.Family)
+	if err != nil {
 		return err
 	}
+	err = binary.Write(w, binary.BigEndian, sd.KeyDesc.Index)
+	if err != nil {
+		return err
+	}
+
+	err = binary.Write(w, binary.BigEndian, sd.KeyDesc.PubKey != nil)
+	if err != nil {
+		return err
+	}
+
+	if sd.KeyDesc.PubKey != nil {
+		serializedPubKey := sd.KeyDesc.PubKey.SerializeCompressed()
+		if err := wire.WriteVarBytes(w, 0, serializedPubKey); err != nil {
+			return err
+		}
+	}
 
 	if err := wire.WriteVarBytes(w, 0, sd.SingleTweak); err != nil {
 		return err
@@ -120,15 +138,34 @@ func WriteSignDescriptor(w io.Writer, sd *SignDescriptor) error {
 // ReadSignDescriptor deserializes a SignDescriptor struct from the passed
 // io.Reader stream.
 func ReadSignDescriptor(r io.Reader, sd *SignDescriptor) error {
-	pubKeyBytes, err := wire.ReadVarBytes(r, 0, 34, "pubkey")
+	err := binary.Read(r, binary.BigEndian, &sd.KeyDesc.Family)
 	if err != nil {
 		return err
 	}
-	sd.PubKey, err = btcec.ParsePubKey(pubKeyBytes, btcec.S256())
+	err = binary.Read(r, binary.BigEndian, &sd.KeyDesc.Index)
 	if err != nil {
 		return err
 	}
 
+	var hasKey bool
+	err = binary.Read(r, binary.BigEndian, &hasKey)
+	if err != nil {
+		return err
+	}
+
+	if hasKey {
+		pubKeyBytes, err := wire.ReadVarBytes(r, 0, 34, "pubkey")
+		if err != nil {
+			return err
+		}
+		sd.KeyDesc.PubKey, err = btcec.ParsePubKey(
+			pubKeyBytes, btcec.S256(),
+		)
+		if err != nil {
+			return err
+		}
+	}
+
 	singleTweak, err := wire.ReadVarBytes(r, 0, 32, "singleTweak")
 	if err != nil {
 		return err