signrpc: add schnorr sig to sign and validate msg

lntest/mock/secretkeyring.go

@@ -3,7 +3,9 @@ package mock
 import (
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
+	"github.com/btcsuite/btcd/btcec/v2/schnorr"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/btcsuite/btcd/txscript"
 	"github.com/lightningnetwork/lnd/keychain"
 )
 
@@ -69,3 +71,23 @@ func (s *SecretKeyRing) SignMessageCompact(_ keychain.KeyLocator,
 	}
 	return ecdsa.SignCompact(s.RootKey, digest, true)
 }
+
+// SignMessageSchnorr signs the passed message and ignores the KeyDescriptor.
+func (s *SecretKeyRing) SignMessageSchnorr(_ keychain.KeyLocator,
+	msg []byte, doubleHash bool, taprootTweak []byte) (*schnorr.Signature,
+	error) {
+
+	var digest []byte
+	if doubleHash {
+		digest = chainhash.DoubleHashB(msg)
+	} else {
+		digest = chainhash.HashB(msg)
+	}
+
+	privKey := s.RootKey
+	if len(taprootTweak) > 0 {
+		privKey = txscript.TweakTaprootPrivKey(privKey, taprootTweak)
+	}
+
+	return schnorr.Sign(privKey, digest)
+}