private static function _tagToASN1Class(int $tag): string
    {
        if (!array_key_exists($tag, self::MAP_TAG_TO_CLASS)) {
            throw new \UnexpectedValueException(
                "Type " . Element::tagToName($tag) .
                     " is not valid DirectoryString.");
        }
        return self::MAP_TAG_TO_CLASS[$tag];
    }