@@ -1,4 +1,4 @@
-# cryptofun
+# cryptofun [![Go Report Card](https://goreportcard.com/badge/github.com/arnaucode/cryptofun)](https://goreportcard.com/report/github.com/arnaucode/cryptofun)
 
 Crypto algorithms from scratch. Academic purposes only.
 
@@ -23,7 +23,15 @@ - [x] Homomorphic Addition
 
 ## Shamir Secret Sharing
 https://en.wikipedia.org/wiki/Shamir%27s_Secret_Sharing
-- [x] create secret sharing from NumOfSecretsNeed, NumOfShares, RandPointP, SecretToShare
+- [x] create secret sharing from number of secrets needed, number of shares, random point p, secret to share
 - [x] Lagrange Interpolation to restore the secret from the shares
 
+## Diffie-Hellman
+https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange
+- [x] key exchange
+
 ## ECC
+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

@@ -0,0 +1,1 @@
+package dh

@@ -0,0 +1,45 @@
+package dh
+
+import (
+	"crypto/rand"
+	"math/big"
+	"testing"
+)
+
+const (
+	bits = 2048
+)
+
+func TestDiffieHellman(t *testing.T) {
+	p, err := rand.Prime(rand.Reader, bits/2)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	g, err := rand.Prime(rand.Reader, bits/2)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+
+	max, err := rand.Prime(rand.Reader, bits/2)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	a, err := rand.Int(rand.Reader, max)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	b, err := rand.Int(rand.Reader, max)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+
+	A := new(big.Int).Exp(g, a, p)
+	B := new(big.Int).Exp(g, b, p)
+
+	sA := new(big.Int).Exp(B, a, p)
+	sB := new(big.Int).Exp(A, b, p)
+
+	if sA.Int64() != sB.Int64() {
+		t.Errorf("secret not equal")
+	}
+}

@@ -0,0 +1,23 @@
+package ecc
+
+import "math/big"
+
+var (
+	bigZero   = big.NewInt(int64(0))
+	zeroPoint = Point{bigZero, bigZero}
+)
+
+type Point struct {
+	X *big.Int
+	Y *big.Int
+}
+
+func (c1 *Point) Equal(c2 Point) bool {
+	if c1.X.Int64() != c2.X.Int64() {
+		return false
+	}
+	if c1.Y.Int64() != c2.Y.Int64() {
+		return false
+	}
+	return true
+}

@@ -0,0 +1,83 @@
+package ecc
+
+import (
+	"errors"
+	"math/big"
+)
+
+type EC struct {
+	A *big.Int
+	B *big.Int
+	Q *big.Int
+}
+
+/*
+	(y^2 = x^3 + Ax + B ) mod Q
+	Q: prime number
+*/
+func NewEC(a, b, q int) (ec EC) {
+	ec.A = big.NewInt(int64(a))
+	ec.B = big.NewInt(int64(b))
+	ec.Q = big.NewInt(int64(q))
+	return ec
+}
+
+// At gets a point x in the curve
+func (ec *EC) At(x *big.Int) (Point, Point, error) {
+	if x.Cmp(ec.Q) > 0 {
+		return Point{}, Point{}, errors.New("x<ec.Q")
+	}
+	// y^2 = (x^3 + ax + b) mod q
+	// y = sqrt (x^3 + ax + b) mod q
+	// x^3
+	x3 := new(big.Int).Exp(x, big.NewInt(int64(3)), nil)
+	// a^x
+	aX := new(big.Int).Mul(ec.A, x)
+	// x^3 + a^x
+	x3aX := new(big.Int).Add(x3, aX)
+	// x^3 + a^x + b
+	x3aXb := new(big.Int).Add(x3aX, ec.B)
+	// y = sqrt (x^3 + ax + b) mod q
+	y := new(big.Int).ModSqrt(x3aXb, ec.Q)
+	return Point{x, y}, Point{x, new(big.Int).Sub(ec.Q, y)}, nil
+}
+
+// TODO add valid checker point function
+
+func (ec *EC) Neg(p Point) Point {
+	// TODO get error when point not found on the ec
+	return Point{p.X, new(big.Int).Sub(ec.Q, p.Y)}
+}
+
+// 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)")
+	}
+	if p2.Equal(zeroPoint) {
+		return p1, errors.New("p1==(0, 0)")
+	}
+	// 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)
+	// q: new point
+	var q Point
+	// s^2
+	s2 := new(big.Int).Exp(s, big.NewInt(int64(2)), nil)
+	// s^2 - p1.X
+	x2Xo := new(big.Int).Sub(s2, p1.X)
+	// s^2 - p1.X - p2.X
+	x2XoX2 := new(big.Int).Sub(x2Xo, p2.X)
+	q.X = new(big.Int).Mod(x2XoX2, ec.Q)
+
+	// p1.X - q.X
+	xoX2 := new(big.Int).Sub(p1.X, q.X)
+	// s(p1.X - q.X)
+	sXoX2 := new(big.Int).Mul(s, xoX2)
+	// s(p1.X - q.X) - p1.Y
+	sXoX2Y := new(big.Int).Sub(sXoX2, p1.Y)
+	q.Y = new(big.Int).Mod(sXoX2Y, ec.Q)
+
+	return q, nil
+}

@@ -0,0 +1,71 @@
+package ecc
+
+import (
+	"fmt"
+	"math/big"
+	"testing"
+)
+
+func TestECC(t *testing.T) {
+	ec := NewEC(0, 7, 11)
+	p1, p1_, err := ec.At(big.NewInt(int64(7)))
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	if !p1.Equal(Point{big.NewInt(int64(7)), big.NewInt(int64(3))}) {
+		t.Errorf("p1!=(7, 11)")
+	}
+	if !p1_.Equal(Point{big.NewInt(int64(7)), big.NewInt(int64(8))}) {
+		t.Errorf("p1_!=(7, 8)")
+	}
+}
+func TestNeg(t *testing.T) {
+	ec := NewEC(0, 7, 11)
+	p1, p1_, err := ec.At(big.NewInt(int64(7)))
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	p1Neg := ec.Neg(p1)
+	if !p1Neg.Equal(p1_) {
+		t.Errorf("p1Neg!=p1_")
+	}
+
+}
+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)")
+	}
+
+	// check that q exists on the elliptic curve
+	pt, pt_, err := ec.At(q.X)
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+	if !q.Equal(pt) && !q.Equal(pt_) {
+		t.Errorf("q not exist on the elliptic curve")
+	}
+
+}

@@ -10,11 +10,11 @@ const (
 	bits = 1024
 )
 
+// Create calculates the secrets to share from given parameters
 // t: number of secrets needed
 // n: number of shares
 // p: random point
 // k: secret to share
-// Create calculates the secrets to share from given parameters
 func Create(t, n, p, k *big.Int) (result [][]*big.Int, err error) {
 	if k.Cmp(p) > 0 {
 		return nil, errors.New("Error: need k<p. k: " + k.String() + ", p: " + p.String())