btcec/schnorr: use private key copy for BIP-340 signatures

This is a fix similar to #1905.
We'll always make a copy of the key in the local scope before passing it
around elsewhere. Depending on the parity of the public key, the private
key itself might need to be negated.

A similar test is added here that fails without the patch to the
signature.go file.

btcec/schnorr/signature.go

@@ -51,8 +51,9 @@ func NewSignature(r *btcec.FieldVal, s *btcec.ModNScalar) *Signature {
 // Serialize returns the Schnorr signature in the more strict format.
 //
 // The signatures are encoded as
-//   sig[0:32]  x coordinate of the point R, encoded as a big-endian uint256
-//   sig[32:64] s, encoded also as big-endian uint256
+//
+//	sig[0:32]  x coordinate of the point R, encoded as a big-endian uint256
+//	sig[32:64] s, encoded also as big-endian uint256
 func (sig Signature) Serialize() []byte {
 	// Total length of returned signature is the length of r and s.
 	var b [SignatureSize]byte
@@ -441,7 +442,8 @@ func Sign(privKey *btcec.PrivateKey, hash []byte,
 	// Step 1.
 	//
 	// d' = int(d)
-	privKeyScalar := &privKey.Key
+	var privKeyScalar btcec.ModNScalar
+	privKeyScalar.Set(&privKey.Key)
 
 	// Step 2.
 	//
@@ -512,7 +514,7 @@ func Sign(privKey *btcec.PrivateKey, hash []byte,
 			return nil, signatureError(ecdsa_schnorr.ErrSchnorrHashValue, str)
 		}
 
-		sig, err := schnorrSign(privKeyScalar, &kPrime, pub, hash, opts)
+		sig, err := schnorrSign(&privKeyScalar, &kPrime, pub, hash, opts)
 		kPrime.Zero()
 		if err != nil {
 			return nil, err
@@ -535,7 +537,7 @@ func Sign(privKey *btcec.PrivateKey, hash []byte,
 		)
 
 		// Steps 10-15.
-		sig, err := schnorrSign(privKeyScalar, k, pub, hash, opts)
+		sig, err := schnorrSign(&privKeyScalar, k, pub, hash, opts)
 		k.Zero()
 		if err != nil {
 			// Try again with a new nonce.

btcec/schnorr/signature_test.go

@@ -10,8 +10,10 @@ import (
 	"errors"
 	"strings"
 	"testing"
+	"testing/quick"
 
 	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/davecgh/go-spew/spew"
 	secp_ecdsa "github.com/decred/dcrd/dcrec/secp256k1/v4"
 	ecdsa_schnorr "github.com/decred/dcrd/dcrec/secp256k1/v4/schnorr"
 )
@@ -254,3 +256,38 @@ func TestSchnorrVerify(t *testing.T) {
 		}
 	}
 }
+
+// TestSchnorrSignNoMutate tests that generating a schnorr signature doesn't
+// modify/mutate the underlying private key.
+func TestSchnorrSignNoMutate(t *testing.T) {
+	t.Parallel()
+
+	// Assert that given a random private key and message, we can generate
+	// a signature from that w/o modifying the underlying private key.
+	f := func(privBytes, msg [32]byte) bool {
+		privBytesCopy := privBytes
+		privKey, _ := btcec.PrivKeyFromBytes(privBytesCopy[:])
+
+		// Generate a signature for private key with our message.
+		_, err := Sign(privKey, msg[:])
+		if err != nil {
+			t.Logf("unable to gen sig: %v", err)
+			return false
+		}
+
+		// We should be able to re-derive the private key from raw
+		// bytes and have that match up again.
+		privKeyCopy, _ := btcec.PrivKeyFromBytes(privBytes[:])
+		if *privKey != *privKeyCopy {
+			t.Logf("private doesn't match: expected %v, got %v",
+				spew.Sdump(privKeyCopy), spew.Sdump(privKey))
+			return false
+		}
+
+		return true
+	}
+
+	if err := quick.Check(f, nil); err != nil {
+		t.Fatalf("private key modified: %v", err)
+	}
+}