diff --git a/README.md b/README.md
index fdbb26c..69923ee 100644
--- a/README.md
+++ b/README.md
@@ -35,3 +35,10 @@ https://en.wikipedia.org/wiki/Elliptic-curve_cryptography
 - [x] define elliptic curve
 - [x] get point at X
 - [x] Add two points on the elliptic curve
+- [x] Multiply a point n times on the elliptic curve
+
+
+To run all tests:
+```
+go test ./... -v
+```
diff --git a/ecc/coord.go b/ecc/coord.go
index 4ac06a0..c963171 100644
--- a/ecc/coord.go
+++ b/ecc/coord.go
@@ -1,6 +1,9 @@
 package ecc
 
-import "math/big"
+import (
+	"bytes"
+	"math/big"
+)
 
 var (
 	bigZero   = big.NewInt(int64(0))
@@ -13,10 +16,10 @@ type Point struct {
 }
 
 func (c1 *Point) Equal(c2 Point) bool {
-	if c1.X.Int64() != c2.X.Int64() {
+	if !bytes.Equal(c1.X.Bytes(), c2.X.Bytes()) {
 		return false
 	}
-	if c1.Y.Int64() != c2.Y.Int64() {
+	if !bytes.Equal(c1.Y.Bytes(), c2.Y.Bytes()) {
 		return false
 	}
 	return true
diff --git a/ecc/ecc.go b/ecc/ecc.go
index e219b67..4e62e75 100644
--- a/ecc/ecc.go
+++ b/ecc/ecc.go
@@ -1,6 +1,7 @@
 package ecc
 
 import (
+	"bytes"
 	"errors"
 	"math/big"
 )
@@ -11,10 +12,7 @@ type EC struct {
 	Q *big.Int
 }
 
-/*
-	(y^2 = x^3 + Ax + B ) mod Q
-	Q: prime number
-*/
+// NewEC (y^2 = x^3 + ax + b) mod q, where q is a prime number
 func NewEC(a, b, q int) (ec EC) {
 	ec.A = big.NewInt(int64(a))
 	ec.B = big.NewInt(int64(b))
@@ -52,15 +50,41 @@ func (ec *EC) Neg(p Point) Point {
 // Add adds two points p1 and p2 and gets q
 func (ec *EC) Add(p1, p2 Point) (Point, error) {
 	if p1.Equal(zeroPoint) {
-		return p2, errors.New("p1==(0, 0)")
+		return p2, nil
 	}
 	if p2.Equal(zeroPoint) {
-		return p1, errors.New("p1==(0, 0)")
+		return p1, nil
 	}
-	// slope
-	numerator := new(big.Int).Sub(p1.Y, p2.Y)
-	denominator := new(big.Int).Sub(p1.X, p2.X)
-	s := new(big.Int).Div(numerator, denominator)
+
+	var numerator, denominator, sRaw, s *big.Int
+	if bytes.Equal(p1.X.Bytes(), p2.X.Bytes()) && (!bytes.Equal(p1.Y.Bytes(), p2.Y.Bytes()) || bytes.Equal(p1.Y.Bytes(), bigZero.Bytes())) {
+		return zeroPoint, nil
+	} else if bytes.Equal(p1.X.Bytes(), p2.X.Bytes()) {
+		// use tangent as slope
+		// x^2
+		x2 := new(big.Int).Mul(p1.X, p1.X)
+		// 3 * x^2
+		x23 := new(big.Int).Mul(big.NewInt(int64(3)), x2)
+		// 3 * x^2 + a
+		numerator = new(big.Int).Add(x23, ec.A)
+		// 2 * y
+		denominator = new(big.Int).Mul(big.NewInt(int64(2)), p1.Y)
+		// (3 * x^2 + a) / (2 * y) mod ec.Q
+		denInv := new(big.Int).ModInverse(denominator, ec.Q)
+		sRaw = new(big.Int).Mul(numerator, denInv)
+		s = new(big.Int).Mod(sRaw, ec.Q)
+	} else {
+		// slope
+		// y0-y1
+		numerator = new(big.Int).Sub(p1.Y, p2.Y)
+		// x0-x1
+		denominator = new(big.Int).Sub(p1.X, p2.X)
+		// (y0-y1) / (x0-x1) mod ec.Q
+		denInv := new(big.Int).ModInverse(denominator, ec.Q)
+		sRaw = new(big.Int).Mul(numerator, denInv)
+		s = new(big.Int).Mod(sRaw, ec.Q)
+	}
+
 	// q: new point
 	var q Point
 	// s^2
@@ -77,7 +101,20 @@ func (ec *EC) Add(p1, p2 Point) (Point, error) {
 	sXoX2 := new(big.Int).Mul(s, xoX2)
 	// s(p1.X - q.X) - p1.Y
 	sXoX2Y := new(big.Int).Sub(sXoX2, p1.Y)
+	// q.Y = (s(p1.X - q.X) - p1.Y) mod ec.Q
 	q.Y = new(big.Int).Mod(sXoX2Y, ec.Q)
 
 	return q, nil
 }
+
+// Mul multiplies a point n times on the elliptic curve
+func (ec *EC) Mul(p Point, n int) (Point, error) {
+	var err error
+	for i := 0; i < n; i++ {
+		p, err = ec.Add(p, p)
+		if err != nil {
+			return zeroPoint, err
+		}
+	}
+	return p, nil
+}
diff --git a/ecc/ecc_test.go b/ecc/ecc_test.go
index 1234cb8..ced18cf 100644
--- a/ecc/ecc_test.go
+++ b/ecc/ecc_test.go
@@ -1,7 +1,6 @@
 package ecc
 
 import (
-	"fmt"
 	"math/big"
 	"testing"
 )
@@ -32,29 +31,19 @@ func TestNeg(t *testing.T) {
 
 }
 func TestAdd(t *testing.T) {
-	fmt.Println("y^2 = x^3 + 7")
-	fmt.Print("ec: ")
 	ec := NewEC(0, 7, 11)
-	fmt.Println(ec)
 	p1, _, err := ec.At(big.NewInt(int64(7)))
 	if err != nil {
 		t.Errorf(err.Error())
 	}
-	fmt.Print("p1: ")
-	fmt.Println(p1)
 	p2, _, err := ec.At(big.NewInt(int64(6)))
 	if err != nil {
 		t.Errorf(err.Error())
 	}
-	fmt.Print("p2: ")
-	fmt.Println(p2)
-
 	q, err := ec.Add(p1, p2)
 	if err != nil {
 		t.Errorf(err.Error())
 	}
-	fmt.Print("q: ")
-	fmt.Println(q)
 	if !q.Equal(Point{big.NewInt(int64(2)), big.NewInt(int64(9))}) {
 		t.Errorf("q!=(2, 9)")
 	}
@@ -69,3 +58,73 @@ func TestAdd(t *testing.T) {
 	}
 
 }
+
+func TestAddSamePoint(t *testing.T) {
+	ec := NewEC(0, 7, 11)
+	p1, p1_, err := ec.At(big.NewInt(int64(4)))
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+
+	q, err := ec.Add(p1, p1)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	if !q.Equal(Point{big.NewInt(int64(6)), big.NewInt(int64(6))}) {
+		t.Errorf("q!=(6, 6)")
+	}
+
+	q_, err := ec.Add(p1_, p1_)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	if !q_.Equal(Point{big.NewInt(int64(6)), big.NewInt(int64(5))}) {
+		t.Errorf("q_!=(6, 5)")
+	}
+
+}
+
+func TestMulEqualSelfAdd(t *testing.T) {
+	ec := NewEC(0, 7, 11)
+	p1, _, err := ec.At(big.NewInt(int64(4)))
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	p1p1, err := ec.Add(p1, p1)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	q, err := ec.Mul(p1, 1)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	if !q.Equal(p1p1) {
+		t.Errorf("q!=p1*p1")
+	}
+}
+func TestMul(t *testing.T) {
+	ec := NewEC(0, 7, 29)
+	p1 := Point{big.NewInt(int64(4)), big.NewInt(int64(19))}
+	q3, err := ec.Mul(p1, 3)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	if !q3.Equal(Point{big.NewInt(int64(19)), big.NewInt(int64(15))}) {
+		t.Errorf("q3!=(19, 15)")
+	}
+	q7, err := ec.Mul(p1, 7)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	if !q7.Equal(Point{big.NewInt(int64(19)), big.NewInt(int64(15))}) {
+		t.Errorf("q7!=(19, 15)")
+	}
+
+	q8, err := ec.Mul(p1, 8)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	if !q8.Equal(Point{big.NewInt(int64(4)), big.NewInt(int64(19))}) {
+		t.Errorf("q8!=(4, 19)")
+	}
+}