public ViewingKey(UInt256 p_a_pk, ReceivingKey p_sk_enc)
 {
     a_pk   = new UInt256(p_a_pk);
     sk_enc = new ReceivingKey(p_sk_enc);
 }
 public ViewingKey()
 {
     a_pk   = new UInt256();
     sk_enc = new ReceivingKey();
 }