copied the project from https://bitbucket.org/ustraca/crypto

elgamal.go

@@ -0,0 +1,133 @@
+package elgamal
+
+import (
+	"crypto/rand"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"math/big"
+
+	"gopkg.in/mgo.v2/bson"
+)
+
+// PublicKey represents an ElGamal public key.
+type PublicKey struct {
+	G, P, Y *big.Int
+}
+
+func (pub *PublicKey) Encrypt(msg []byte, random io.Reader) (cypher *Cypher, err error) {
+	m := new(big.Int).SetBytes(msg)
+
+	k, err := rand.Int(random, pub.P)
+	if err != nil {
+		return
+	}
+
+	c1 := new(big.Int).Exp(pub.G, k, pub.P)
+	s := new(big.Int).Exp(pub.Y, k, pub.P)
+
+	c2 := s.Mul(s, m)
+	c2.Mod(c2, pub.P)
+	cypher = &Cypher{c1, c2, pub.P}
+	return
+}
+
+func (this *PublicKey) GetBSON() (interface{}, error) {
+	return bson.D{
+		{"g", fmt.Sprintf("%x", this.G)},
+		{"p", fmt.Sprintf("%x", this.P)},
+		{"y", fmt.Sprintf("%x", this.Y)},
+	}, nil
+}
+
+// PrivateKey represents an ElGamal private key.
+type PrivateKey struct {
+	PublicKey
+	X *big.Int
+}
+
+func (this *PrivateKey) GetBSON() (interface{}, error) {
+	return bson.D{
+		{"x", fmt.Sprintf("%x", this.X)},
+		{"g", fmt.Sprintf("%x", this.G)},
+		{"p", fmt.Sprintf("%x", this.P)},
+		{"y", fmt.Sprintf("%x", this.Y)},
+	}, nil
+}
+
+func (priv *PrivateKey) Decrypt(cypher *Cypher) (msg []byte) {
+	s := new(big.Int).Exp(cypher.C1, priv.X, priv.P)
+	s.ModInverse(s, priv.P)
+	s.Mul(s, cypher.C2)
+	s.Mod(s, priv.P)
+	em := s.Bytes()
+	return em
+}
+
+type Cypher struct {
+	C1, C2, P *big.Int
+}
+
+func (this *Cypher) GetBSON() (interface{}, error) {
+	return bson.D{
+		{"c1", fmt.Sprintf("%x", this.C1)},
+		{"c2", fmt.Sprintf("%x", this.C2)},
+		{"p", fmt.Sprintf("%x", this.P)},
+	}, nil
+}
+
+func (this *Cypher) Mul(cypher1, cypher2 *Cypher) *Cypher {
+	this.C1 = new(big.Int).Mod(new(big.Int).Mul(cypher1.C1, cypher2.C1), cypher1.P)
+	this.C2 = new(big.Int).Mod(new(big.Int).Mul(cypher1.C2, cypher2.C2), cypher1.P)
+	this.P = cypher1.P
+	return this
+}
+
+func (this *Cypher) ToJSON() map[string]string {
+	return map[string]string{
+		"C1": fmt.Sprintf("%x", this.C1),
+		"C2": fmt.Sprintf("%x", this.C2),
+		"P":  fmt.Sprintf("%x", this.P),
+	}
+}
+
+func (this *Cypher) MarshalJSON() ([]byte, error) {
+	return json.Marshal(this.ToJSON())
+}
+
+func (this *Cypher) UnmarshalJSON(bytes []byte) error {
+	m := make(map[string]string)
+	err := json.Unmarshal(bytes, &m)
+	if err != nil {
+		return err
+	}
+	this.FromJSON(m)
+	return nil
+}
+
+func (this *Cypher) FromJSON(json map[string]string) (*Cypher, error) {
+	var err error
+	this.C1, err = fromHex(json["C1"])
+	if err != nil {
+		return nil, err
+	}
+	this.C2, err = fromHex(json["C2"])
+	if err != nil {
+		return nil, err
+	}
+	this.P, err = fromHex(json["P"])
+	if err != nil {
+		return nil, err
+	}
+	return this, err
+}
+
+func fromHex(hex string) (*big.Int, error) {
+	n, err := new(big.Int).SetString(hex, 16)
+	if !err {
+		msg := fmt.Sprintf("Cannot convert %s to int as hexadecimal", hex)
+		return nil, errors.New(msg)
+	}
+	return n, nil
+}