code: private Mac mac() throws IOException {
          try {
              final Mac mac = Mac.getInstance(this.alg);
              mac.init(this.key);
              return mac;
          } catch (final NoSuchAlgorithmException | InvalidKeyException err) {
              throw new IOException(err);
          }
      }
docs: Obtain MAC instance.
  @return Initialized MAC
  @throws IOException If algorithm missed or invalid key