Do not cache N^2 value in PublicKey

Let's keep this structure clean. Computing N^2 is just a really small piece of encryption/decryption work and if we decide we need to tweak performance, we will need a different approach anyway.

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), b(8)}, 7, 6, b(3), []*big.Int{b(2), b(34)}}
 	AssertBSONIsGood(key, new(ThresholdKey), t)
 }

paillier.go

@@ -8,7 +8,6 @@ import (
 
 type PublicKey struct {
 	N, G *big.Int // usually G is set to N+1
-	n2   *big.Int // the cache value of N^2
 }
 
 func (this *PublicKey) GetBSON() (interface{}, error) {
@@ -19,11 +18,7 @@ func (this *PublicKey) GetBSON() (interface{}, error) {
 }
 
 func (this *PublicKey) GetNSquare() *big.Int {
-	if this.n2 != nil {
+	return new(big.Int).Mul(this.N, this.N)
-		return this.n2
-	}
-	this.n2 = new(big.Int).Mul(this.N, this.N)
-	return this.n2
 }
 
 // Encode a plaintext in a cypher one. The plain text must be smaller that