Let to pass an arbitrary number of cyphers to PublicKey.Add

`PublicKey.Add` accepts now an arbitrary number of `*Cypher`s.
Previously, it was possible to add only two `*Cypher*` instances and it
was quite unwieldy for a lot of use cases.

The current implementation is backward compatible. It's still possible
to pass just two `*Cypher` arguments, so every project referencing the
library will observe no compilation issues.

paillier_test.go

@@ -64,13 +64,17 @@ func TestEncryptDecryptSmall(t *testing.T) {
 	}
 }
 
-func TestAddCypher(t *testing.T) {
-	privateKey := CreatePrivateKey(big.NewInt(13), big.NewInt(11))
-	cypher1, _ := privateKey.Encrypt(big.NewInt(12), rand.Reader)
-	cypher2, _ := privateKey.Encrypt(big.NewInt(13), rand.Reader)
-	cypher3 := privateKey.Add(cypher1, cypher2)
-	m := privateKey.Decrypt(cypher3)
-	if !reflect.DeepEqual(m, big.NewInt(25)) {
-		t.Error(m)
+func TestAddCyphers(t *testing.T) {
+	privateKey := CreatePrivateKey(big.NewInt(17), big.NewInt(13))
+
+	cypher1, _ := privateKey.Encrypt(big.NewInt(5), rand.Reader)
+	cypher2, _ := privateKey.Encrypt(big.NewInt(6), rand.Reader)
+	cypher3, _ := privateKey.Encrypt(big.NewInt(7), rand.Reader)
+	cypher4, _ := privateKey.Encrypt(big.NewInt(8), rand.Reader)
+	cypher5 := privateKey.Add(cypher1, cypher2, cypher3, cypher4)
+
+	m := privateKey.Decrypt(cypher5)
+	if !reflect.DeepEqual(m, big.NewInt(26)) {
+		t.Errorf("Unexpected decrypted value [%v]", m)
 	}
 }