#!/usr/bin/env python
# encoding: utf-8
# Copyright (c) 2012-2016 Seafile Ltd.

import hashlib
import random
import sys
import time

# Use the system PRNG if possible
try:
    random = random.SystemRandom()
    using_sysrandom = True
except NotImplementedError:
    import warnings
    warnings.warn('A secure pseudo-random number generator is not available '
                  'on your system. Falling back to Mersenne Twister.')
    using_sysrandom = False

def get_random_string(length=12,
                      allowed_chars='abcdefghijklmnopqrstuvwxyz'
                                    'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'):
    """
    Returns a securely generated random string.

    The default length of 12 with the a-z, A-Z, 0-9 character set returns
    a 71-bit value. log_2((26+26+10)^12) =~ 71 bits
    """
    if not using_sysrandom:
        # This is ugly, and a hack, but it makes things better than
        # the alternative of predictability. This re-seeds the PRNG
        # using a value that is hard for an attacker to predict, every
        # time a random string is required. This may change the
        # properties of the chosen random sequence slightly, but this
        # is better than absolute predictability.
        random.seed(
            hashlib.sha256(
                ("%s%s%s" % (
                    random.getstate(),
                    time.time(),
                    '')).encode('utf-8')
            ).digest())
    return ''.join(random.choice(allowed_chars) for i in range(length))

if __name__ == "__main__":
    chars = 'abcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*(-_=+)'
    key = get_random_string(50, chars)
    if len(sys.argv) == 2:
        fp = open(sys.argv[1], 'w')
        fp.write("SECRET_KEY = \"%s\"\n" % key)
        fp.close()
    else:
        print key
