lnwallet: support combined tweak to private key

Previously we'd define either a single or a double tweak for the sign
descriptor. We introduce the option to apply both consecutively (double
tweak first, single tweak second) if both tweak parameters are set. For
callers who define only one of the two parameters we maintain the old
behavior.

Author: GeorgeTsagk

lnwallet/btcwallet/signer.go

@@ -238,13 +238,32 @@ func (b *BtcWallet) fetchPrivKey(
 
 // maybeTweakPrivKey examines the single and double tweak parameters on the
 // passed sign descriptor and may perform a mapping on the passed private key
-// in order to utilize the tweaks, if populated.
+// in order to utilize the tweaks, if populated. If both tweak parameters are
+// set, then both are applied in the following order:
+//
+//	a) double tweak
+//	b) single tweak
 func maybeTweakPrivKey(signDesc *input.SignDescriptor,
 	privKey *btcec.PrivateKey) (*btcec.PrivateKey, error) {
 
 	var retPriv *btcec.PrivateKey
-	switch {
 
+	// If both tweak parameters are set, then apply both.
+	if signDesc.DoubleTweak != nil && signDesc.SingleTweak != nil {
+		// First apply the double tweak.
+		retPriv = input.DeriveRevocationPrivKey(privKey,
+			signDesc.DoubleTweak)
+
+		// Then apply the single tweak.
+		retPriv = input.TweakPrivKey(retPriv,
+			signDesc.SingleTweak)
+
+		return retPriv, nil
+	}
+
+	// If only one tweak parameter is set, apply it respectively over the
+	// provided private key.
+	switch {
 	case signDesc.SingleTweak != nil:
 		retPriv = input.TweakPrivKey(privKey,
 			signDesc.SingleTweak)