docs: public static String hmacSha256(String input, String secretKey) {
          if (input == null) {
              throw new NullPointerException("input");
          } else if (secretKey == null) {
              throw new NullPointerException("secretKey");
          }
          return hmacSha256(input.getBytes(CHARSET_UTF8), secretKey.getBytes(CHARSET_UTF8));
      }