Added test ensuring threshold Paillier is a partially homomorphic system

paillier.go

@@ -48,6 +48,9 @@ func (pub *PublicKey) Encrypt(m *big.Int, random io.Reader) (*Cypher, error) {
 
 // Takes two cypher texts and returns a 3rd one that encode
 // the sum of the two plain texts.
+//
+// It's possible because Paillier is a homomorphic encryption scheme, where
+// E(m1) * E(m2) = E(m1 + m2)
 func (this *PublicKey) Add(cypher1, cypher2 *Cypher) *Cypher {
 	m := new(big.Int).Mul(cypher1.C, cypher2.C)
 	return &Cypher{new(big.Int).Mod(m, this.GetNSquare())}