def _random_string():
    """
    Generate a globally unique serial / product code of the form ``u'YRAC-EL4X-FQQE-AW4T-WNUV-VN6T'``.
    The generated value is cryptographically strong and has (at least) 114 bits of entropy.

    :return: new random string key
    """
    rng = random.SystemRandom()
    token_value = u''.join(rng.choice(CHARSET) for _ in range(CHAR_GROUPS * CHARS_PER_GROUP))
    if CHARS_PER_GROUP > 1:
        return GROUP_SEP.join(map(u''.join, zip(*[iter(token_value)] * CHARS_PER_GROUP)))
    else:
        return token_value
Generate a globally unique serial / product code of the form ``u'YRAC-EL4X-FQQE-AW4T-WNUV-VN6T'``.
    The generated value is cryptographically strong and has (at least) 114 bits of entropy.

    :return: new random string key