code: private Mac create()
          throws IOException {
          final String algo = String.format("HmacSHA%s", this.bits);
          try {
              final SecretKeySpec secret = new SecretKeySpec(
                  this.key, algo
              );
              final Mac mac = Mac.getInstance(algo);
              mac.init(secret);
              return mac;
          } catch (final NoSuchAlgorithmException | InvalidKeyException ex) {
              throw new IOException(ex);
          }
      }
docs: Create new mac based on a valid bit length from {@link Mac} class.
  
  @return The mac
  @throws IOException
  For any unexpected exceptions