Generate random string for encryption usage

randomstr.py

#!/usr/bin/env python3

import sys

HQ_RND_LEN = 64
LOWER_ALPHA = 'abcdefghijklmnopqrstuvwxyz'
UPPER_ALPHA = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
ALPHA = LOWER_ALPHA + UPPER_ALPHA

def help(argv):
    help_msg = '''\
Usage: {script} [options] length

  Generate random string from character pool

  Options:
    --help, -h          Show this message
    --stdin             Character pool from stdin

  Options for character pool:
    --number, -n        Use numbers
    --alpha, -a         Use upper and lower alpha
    --upper, -u         Use upper alpha
    --lower, -l         Use lower alpha
    --underscore, -U    Use underscore
'''.format(script = argv[0])
    print(help_msg)

def main(argv):
    length = -1
    seed = ''

    for arg in argv[1:]:
        if arg == '--stdin':
            seed += sys.stdin.read()
        elif arg == '--number' or arg == '-n':
            seed += ''.join(map(str, range(10)))
        elif arg == '--alpha' or arg == '-a':
            seed += ALPHA
        elif arg == '--upper' or arg == '-u':
            seed += UPPER_ALPHA
        elif arg == '--lower' or arg == '-l':
            seed += LOWER_ALPHA
        elif arg == '--underscore' or arg == '-U':
            seed += '_'
        elif arg == '--help' or arg == '-h':
            help(argv)
            exit()
        else:
            try:
                length = int(arg)
            except ValueError:
                pass

    if len(seed) == 0:
        seed = ''.join(map(chr, range(ord(' '), ord('~')) ))

    if length == -1:
        sys.stderr.write('You must specify a length\n')
        exit(1)

    hq_rnd_len = HQ_RND_LEN + length // 16
    random = open('/dev/random', 'rb')
    hq_rnd = bytearray(random.read(hq_rnd_len))
    random.close()

    urandom = open('/dev/urandom', 'rb')
    rnd = bytearray(urandom.read(length))
    urandom.close()

    for i in range(length):
        I = i % hq_rnd_len
        rnd[i] ^= hq_rnd[I]
        hq_rnd[I] = (rnd[i] + 17) & 0xFF

    buffer = ''
    N = len(seed)
    for n in rnd:
        sys.stdout.write(seed[n % N])

if __name__ == '__main__':
    main(sys.argv)