Merge pull request #151 from binance-chain/paillier-check

Check paillier ciphertext

crypto/paillier/paillier.go

@@ -52,7 +52,8 @@ type (
 )
 
 var (
-	ErrMessageTooLong = fmt.Errorf("the message is too large or < 0")
+	ErrMessageTooLong   = fmt.Errorf("the message is too large or < 0")
+	ErrMessageMalFormed = fmt.Errorf("the message is mal-formed")
 
 	zero = big.NewInt(0)
 	one  = big.NewInt(1)
@@ -173,6 +174,10 @@ func (privateKey *PrivateKey) Decrypt(c *big.Int) (m *big.Int, err error) {
 	if c.Cmp(zero) == -1 || c.Cmp(N2) != -1 { // c < 0 || c >= N2 ?
 		return nil, ErrMessageTooLong
 	}
+	cg := new(big.Int).GCD(nil, nil, c, N2)
+	if cg.Cmp(one) == 1 {
+		return nil, ErrMessageMalFormed
+	}
 	// 1. L(u) = (c^LambdaN-1 mod N2) / N
 	Lc := L(new(big.Int).Exp(c, privateKey.LambdaN, N2), privateKey.N)
 	// 2. L(u) = (Gamma^LambdaN-1 mod N2) / N

crypto/paillier/paillier_test.go

@@ -64,6 +64,10 @@ func TestEncryptDecrypt(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, 0, exp.Cmp(ret),
 		"wrong decryption ", ret, " is not ", exp)
+
+	cypher = new(big.Int).Set(privateKey.N)
+	_, err = privateKey.Decrypt(cypher)
+	assert.Error(t, err)
 }
 
 func TestHomoMul(t *testing.T) {