docs: public static boolean isProperlyFormattedKey( String hexadecimalStr ) {
          if (hexadecimalStr == null) return false;
          // Length is expected to be the same as the digest ...
          final int length = hexadecimalStr.length();
          if (length != ALGORITHM.getHexadecimalStringLength()) return false;
          // The characters all must be hexadecimal digits ...
          return StringUtil.isHexString(hexadecimalStr);
      }