Add test hash odd bytes for js compatibility

To ensure compatibility with https://github.com/arnaucube/blindsecp256k1-js/

blindsecp256k1.go

@@ -157,8 +157,8 @@ type UserSecretData struct {
 
 // Blind performs the blinding operation on m using signerR parameter
 func Blind(m *big.Int, signerR *Point) (*big.Int, *UserSecretData, error) {
-	if !btcec.S256().IsOnCurve(signerR.X, signerR.Y) {
-		return nil, nil, fmt.Errorf("signerR point is not on secp256k1")
+	if err := signerR.isValid(); err != nil {
+		return nil, nil, fmt.Errorf("signerR %s", err)
 	}
 
 	u := &UserSecretData{}
@@ -172,7 +172,7 @@ func Blind(m *big.Int, signerR *Point) (*big.Int, *UserSecretData, error) {
 
 	// TODO check that F != O (point at infinity)
 	if err := u.F.isValid(); err != nil {
-		return nil, nil, err
+		return nil, nil, fmt.Errorf("u.F %s", err)
 	}
 
 	rx := new(big.Int).Mod(u.F.X, N)

blindsecp256k1_test.go

@@ -4,6 +4,7 @@ import (
 	"math/big"
 	"testing"
 
+	"github.com/ethereum/go-ethereum/crypto"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -38,6 +39,28 @@ func TestFlow(t *testing.T) {
 	assert.True(t, verified)
 }
 
+func TestHashMOddBytes(t *testing.T) {
+	// This test is made with same values than
+	// https://github.com/arnaucube/blindsecp256k1-js to ensure
+	// compatibility
+	mStr := "3024162961766929396601888431330224482373544644288322432261208139289299439809"
+	m, ok := new(big.Int).SetString(mStr, 10)
+	require.True(t, ok)
+	mBytes := m.Bytes()
+
+	hBytes := crypto.Keccak256(mBytes[3:])
+	h := new(big.Int).SetBytes(hBytes)
+	assert.Equal(t,
+		"57523339312508913023232057765773019244858443678197951618720342803494056599369",
+		h.String())
+
+	hBytes = crypto.Keccak256(append(mBytes, []byte{0x12, 0x34}...))
+	h = new(big.Int).SetBytes(hBytes)
+	assert.Equal(t,
+		"9697834584560956691445940439424778243200861871421750951058436814122640359156",
+		h.String())
+}
+
 // func newBigIntWithBitLen(n int) *big.Int {
 //         b := make([]byte, n/8)
 //         for i := 0; i < len(b); i++ {