Merge pull request #1 from keep-network/evaluate-and-fix

Paillier scheme API - evaluation and clearing up

encoding.go

@@ -117,18 +117,13 @@ type dbPrivateKey struct {
 
 func (this *PrivateKey) GetBSON() (interface{}, error) {
 	m := make(map[string]string)
-	if this.G != nil {
-		m["g"] = fmt.Sprintf("%x", this.G)
-	}
+
 	if this.N != nil {
 		m["n"] = fmt.Sprintf("%x", this.N)
 	}
 	if this.Lambda != nil {
 		m["lambda"] = fmt.Sprintf("%x", this.Lambda)
 	}
-	if this.Mu != nil {
-		m["mu"] = fmt.Sprintf("%x", this.Mu)
-	}
 	return m, nil
 }
 
@@ -143,21 +138,13 @@ func (this *PrivateKey) SetBSON(raw bson.Raw) error {
 			return err
 		}
 	}
-	if c.G != "" {
-		this.G, err = fromHex(c.G)
-		if err != nil {
-			return err
-		}
-	}
+
 	if c.Lambda != "" {
 		this.Lambda, err = fromHex(c.Lambda)
 		if err != nil {
 			return err
 		}
 	}
-	if c.Mu != "" {
-		this.Mu, err = fromHex(c.Mu)
-	}
 
 	return err
 }

encoding_test.go

@@ -118,6 +118,6 @@ func TestPartialDecryptionZKPBSONification(t *testing.T) {
 }
 
 func TestThresholdKeyBSON(t *testing.T) {
-	key := &ThresholdKey{PublicKey{b(9), b(8), nil}, 7, 6, b(3), []*big.Int{b(2), b(34)}}
+	key := &ThresholdKey{PublicKey{b(9)}, 7, 6, b(3), []*big.Int{b(2), b(34)}, b(8)}
 	AssertBSONIsGood(key, new(ThresholdKey), t)
 }

paillier.go

@@ -7,29 +7,28 @@ import (
 )
 
 type PublicKey struct {
-	N, G *big.Int // usually G is set to N+1
-	n2   *big.Int // the cache value of N^2
+	N *big.Int
 }
 
 func (this *PublicKey) GetBSON() (interface{}, error) {
 	m := make(map[string]string)
-	m["g"] = fmt.Sprintf("%x", this.G)
 	m["n"] = fmt.Sprintf("%x", this.N)
 	return m, nil
 }
 
 func (this *PublicKey) GetNSquare() *big.Int {
-	if this.n2 != nil {
-		return this.n2
-	}
-	this.n2 = new(big.Int).Mul(this.N, this.N)
-	return this.n2
+	return new(big.Int).Mul(this.N, this.N)
 }
 
-// Encode a plain text in a cypher one.  The plain text must be smaller that
-// N and bigger or equal than zero.  random is usually rand.Reader from the
+// Encode a plaintext in a cypher one. The plain text must be smaller that
+// N and bigger or equal than zero. random is usually rand.Reader from the
 // package crypto/rand.
 //
+// m - plaintext to encrypt
+// E(m, r) = [(1 + N) r^N] mod N^2
+//
+// See [KL 08] construction 11.32, page 414.
+//
 // Returns an error if an error has be returned by io.Reader.
 func (pub *PublicKey) Encrypt(m *big.Int, random io.Reader) (*Cypher, error) {
 	r, err := GetRandomNumberInMultiplicativeGroup(pub.N, random)
@@ -38,7 +37,8 @@ func (pub *PublicKey) Encrypt(m *big.Int, random io.Reader) (*Cypher, error) {
 	}
 	nSquare := pub.GetNSquare()
 
-	gm := new(big.Int).Exp(pub.G, m, nSquare)
+	g := new(big.Int).Add(pub.N, big.NewInt(1))
+	gm := new(big.Int).Exp(g, m, nSquare)
 	rn := new(big.Int).Exp(r, pub.N, nSquare)
 	return &Cypher{new(big.Int).Mod(new(big.Int).Mul(rn, gm), nSquare)}, nil
 }
@@ -52,20 +52,21 @@ func (this *PublicKey) Add(cypher1, cypher2 *Cypher) *Cypher {
 
 type PrivateKey struct {
 	PublicKey
-	Lambda, Mu *big.Int
-}
-
-func (this *PrivateKey) String() string {
-	ret := fmt.Sprintf("g     :  %x", this.G)
-	ret += fmt.Sprintf("n     :  %x", this.N)
-	ret += fmt.Sprintf("lambda:  %x", this.Lambda)
-	ret += fmt.Sprintf("mu    :  %x", this.Mu)
-	return ret
+	Lambda *big.Int
 }
 
+// Decodes ciphertext into a plaintext message.
+//
+// c - cyphertext to decrypt
+// N, lambda - key attributes
+//
+// D(c) = [ ((c^lambda) mod N^2) - 1) / N ] lambda^-1 mod N
+//
+// See [KL 08] construction 11.32, page 414.
 func (priv *PrivateKey) Decrypt(cypher *Cypher) (msg *big.Int) {
+	mu := new(big.Int).ModInverse(priv.Lambda, priv.N)
 	tmp := new(big.Int).Exp(cypher.C, priv.Lambda, priv.GetNSquare())
-	msg = new(big.Int).Mod(new(big.Int).Mul(L(tmp, priv.N), priv.Mu), priv.N)
+	msg = new(big.Int).Mod(new(big.Int).Mul(L(tmp, priv.N), mu), priv.N)
 	return
 }
 
@@ -82,35 +83,38 @@ func L(u, n *big.Int) *big.Int {
 	return new(big.Int).Div(t, n)
 }
 
-func LCM(x, y *big.Int) *big.Int {
-	return new(big.Int).Mul(new(big.Int).Div(x, new(big.Int).GCD(nil, nil, x, y)), y)
-}
-
 func minusOne(x *big.Int) *big.Int {
 	return new(big.Int).Add(x, big.NewInt(-1))
 }
 
-func computeMu(g, lambda, n *big.Int) *big.Int {
-	n2 := new(big.Int).Mul(n, n)
-	u := new(big.Int).Exp(g, lambda, n2)
-	return new(big.Int).ModInverse(L(u, n), n)
-}
-
-func computeLamda(p, q *big.Int) *big.Int {
-	return LCM(minusOne(p), minusOne(q))
+func computePhi(p, q *big.Int) *big.Int {
+	return new(big.Int).Mul(minusOne(p), minusOne(q))
 }
 
+// CreatePrivateKey generates a Paillier private key accepting two large prime
+// numbers of equal length or other such that gcd(pq, (p-1)(q-1)) = 1.
+//
+// Algorithm is based on approach described in [KL 08], construction 11.32,
+// page 414 which is compatible with one described in [DJN 10], section 3.2
+// except that instead of generating Lambda private key component from LCM
+// of p and q we use Euler's totient function as suggested in [KL 08].
+//
+//     [KL 08]:  Jonathan Katz, Yehuda Lindell, (2008)
+//               Introduction to Modern Cryptography: Principles and Protocols,
+//               Chapman & Hall/CRC
+//
+//     [DJN 10]: Ivan Damgard, Mads Jurik, Jesper Buus Nielsen, (2010)
+//               A Generalization of Paillier’s Public-Key System
+//               with Applications to Electronic Voting
+//               Aarhus University, Dept. of Computer Science, BRICS
 func CreatePrivateKey(p, q *big.Int) *PrivateKey {
 	n := new(big.Int).Mul(p, q)
-	lambda := new(big.Int).Mul(minusOne(p), minusOne(q))
-	g := new(big.Int).Add(n, big.NewInt(1))
-	mu := new(big.Int).ModInverse(lambda, n)
+	lambda := computePhi(p, q)
+
 	return &PrivateKey{
 		PublicKey: PublicKey{
 			N: n,
-			G: g,
 		},
 		Lambda: lambda,
-		Mu:     mu,
 	}
 }

paillier_test.go

@@ -7,35 +7,42 @@ import (
 	"testing"
 )
 
-func TestLCM(t *testing.T) {
-	a := big.NewInt(2 * 3 * 3 * 3 * 5 * 5)
-	b := big.NewInt(3 * 3 * 5 * 5 * 57 * 11)
-	exp := big.NewInt(3 * 3 * 5 * 5)
-	if reflect.DeepEqual(exp, LCM(a, b)) {
-		t.Fail()
-	}
-}
-
-func TestL(t *testing.T) {
+func TestComputeL(t *testing.T) {
 	u := big.NewInt(21)
 	n := big.NewInt(3)
-	exp := big.NewInt(6)
-	if !reflect.DeepEqual(exp, L(u, n)) {
-		t.Error("L function is not good")
+
+	expected := big.NewInt(6)
+	actual := L(u, n)
+
+	if !reflect.DeepEqual(expected, actual) {
+		t.Errorf("Unexpected L function result [%v]", actual)
 	}
 }
 
-func TestComputeMu(t *testing.T) {
-	p := big.NewInt(13)
-	q := big.NewInt(11)
+func TestComputePhi(t *testing.T) {
+	a := big.NewInt(5)
+	b := big.NewInt(7)
 
-	lambda := computeLamda(p, q)
-	g := big.NewInt(5000)
-	n := new(big.Int).Mul(p, q)
+	expected := big.NewInt(24)
+	actual := computePhi(a, b)
 
-	exp := big.NewInt(3)
-	if !reflect.DeepEqual(computeMu(g, lambda, n), exp) {
-		t.Error("lambda is not well computed")
+	if !reflect.DeepEqual(expected, actual) {
+		t.Errorf("Unexpected phi value [%v]", actual)
+	}
+}
+
+func TestCreatePrivateKey(t *testing.T) {
+	p := big.NewInt(463)
+	q := big.NewInt(631)
+
+	privateKey := CreatePrivateKey(p, q)
+
+	if !reflect.DeepEqual(privateKey.N, big.NewInt(292153)) {
+		t.Errorf("Unexpected N PublicKey value [%v]", privateKey.N)
+	}
+
+	if !reflect.DeepEqual(privateKey.Lambda, big.NewInt(291060)) {
+		t.Errorf("Unexpected Lambda Public key value [%v]", privateKey.Lambda)
 	}
 }
 
@@ -52,10 +59,9 @@ func TestEncryptDecryptSmall(t *testing.T) {
 		}
 		returnedValue := privateKey.Decrypt(cypher)
 		if !reflect.DeepEqual(inicialValue, returnedValue) {
-			t.Error("wrond decryption ", returnedValue, " is not ", inicialValue)
+			t.Error("wrong decryption ", returnedValue, " is not ", inicialValue)
 		}
 	}
-
 }
 
 func TestAddCypher(t *testing.T) {

thresholdkey.go

@@ -14,6 +14,7 @@ type ThresholdKey struct {
 	Threshold                      int
 	V                              *big.Int
 	Vi                             []*big.Int
+	G                              *big.Int // usually G is set to N+1
 }
 
 // returns the value of (4*delta**2)** -1  mod n