code: public byte[][] keypair()
      {
          byte[][] pair = new byte[2][];
  
          byte[] publicKey = new byte[Size.PUBLICKEY.bytes()];
          byte[] secretKey = new byte[Size.SECRETKEY.bytes()];
  
          int rc = curve25519xsalsa20poly1305.crypto_box_keypair(publicKey, secretKey);
          assert (rc == 0);
  
          pair[0] = publicKey;
          pair[1] = secretKey;
  
          return pair;
      }
docs: Generates a pair of keys for use with this class.
  
  @return an array of 2 byte arrays, holding keys.
  The first element of the array is the public key,
  the second element is the private (or secret) key.