def _random_string():
rng = random.SystemRandom()<EOL>token_value = u'<STR_LIT>'.join(rng.choice(CHARSET) for _ in range(CHAR_GROUPS * CHARS_PER_GROUP))<EOL>if CHARS_PER_GROUP > <NUM_LIT:1>:<EOL><INDENT>return GROUP_SEP.join(map(u'<STR_LIT>'.join, zip(*[iter(token_value)] * CHARS_PER_GROUP)))<EOL><DEDENT>else:<EOL><INDENT>return token_value<EOL><DEDENT>
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