# -*- coding: utf-8 -*-
"""Generate a random, memorizable password: http://xkcd.com/936/

Example run:

    kragen@inexorable:~/devel/inexorable-misc$ ./mkpass.py 5 12
    Your password is "learned damage saved residential stages".
    That's equivalent to a 60-bit key.

    That password would take 2.5e+03 CPU-years to crack
    on my inexpensive Celeron E1200 from 2008,
    assuming an offline attack on a MS-Cache hash,
    which is the worst password hashing algorithm in common use,
    slightly worse than even simple MD5.

    The most common password-hashing algorithm these days is FreeBSD’s
    iterated MD5; cracking such a hash would take 5.2e+06 CPU-years.

    But a modern GPU can crack about 250 times as fast,
    so that same iterated MD5 would fall in 2e+04 GPU-years.

    That GPU costs about US$1.45 per day to run in 2011,
    so cracking the password would cost about US$3e+09.

I've started using a password generated this way in place of a 9-printable-
ASCII-character random password, which is equally strong.  Munroe's assertion
that these passwords are much easier to memorize is correct.  However, there is
still a problem: because there are many fewer bits of entropy per character
(about 1.7 instead of 6.6) there is a lot of redundancy in the password, and so
attacks such as the ssh timing-channel attack (the Song, Wagner, and Tian
Herbivore attack, which I learned about from Bram Cohen in the Bagdad Café in
the wee hours one morning, years ago) and keyboard audio recording attacks have
a much better chance of capturing enough information to make the password

My countermeasure to the Herbivore attack, which works well with 9-character
password but is extremely annoying with my new password, is to type the
password with a half-second delay between characters, so that the timing
channel does not carry much information about the actual characters used.
Additionally, the lower length of the 9-character password inherently gives the
Herbivore approach much less information to chew on.

Other possible countermeasures include using Emacs shell-mode, which prompts
you locally for the password when it recognizes a password prompt and then
sends the whole password at once, and copying and pasting the password from
somewhere else.

As you'd expect, this password also takes a little while longer to type: about
6 seconds instead of about 3 seconds.


import random, itertools, os, sys

def main(argv):
        nwords = int(argv[1])
    except IndexError:
        return usage(argv[0])

        nbits = int(argv[2])
    except IndexError:
        nbits = 11

    filename = os.path.join(os.environ['HOME'], 'devel', 'wordlist')
    wordlist = read_file(filename, nbits)
    if len(wordlist) != 2**nbits:
        sys.stderr.write("%r contains only %d words, not %d.\n" %
                         (filename, len(wordlist), 2**nbits))
        return 2

    display_password(generate_password(nwords, wordlist), nwords, nbits)
    return 0

def usage(argv0):
    p = sys.stderr.write
    p("Usage: %s nwords [nbits]\n" % argv0)
    p("Generates a password of nwords words, each with nbits bits\n")
    p("of entropy, choosing words from the first entries in\n")
    p("$HOME/devel/wordlist, which should be in the same format as\n")
    p("<http://canonical.org/~kragen/sw/wordlist>, which is a text file\n")
    p("with one word per line, preceded by its frequency, most frequent\n")
    p("words first.\n")
    p("    %s 5 12\n" % argv0)
    p("    %s 6\n" % argv0)
    return 1

def read_file(filename, nbits):
    return [line.split()[1] for line in
            itertools.islice(open(filename), 2**nbits)]

def generate_password(nwords, wordlist):
    choice = random.SystemRandom().choice
    return ' '.join(choice(wordlist) for ii in range(nwords))

def display_password(password, nwords, nbits):
    print 'Your password is "%s".' % password
    entropy = nwords * nbits
    print "That's equivalent to a %d-bit key." % entropy

    # My Celeron E1200
    # was released on January 20, 2008.  Running it in 32-bit mode,
    # john --test (<http://www.openwall.com/john/>) reports that it
    # can do 7303000 MD5 operations per second, but I’m pretty sure
    # that’s a single-core number (I don’t think John is
    # multithreaded) on a dual-core processor.
    t = years(entropy, 7303000 * 2)
    print "That password would take %.2g CPU-years to crack" % t
    print "on my inexpensive Celeron E1200 from 2008,"
    print "assuming an offline attack on a MS-Cache hash,"
    print "which is the worst password hashing algorithm in common use,"
    print "slightly worse than even simple MD5."

    t = years(entropy, 3539 * 2)
    print "The most common password-hashing algorithm these days is FreeBSD’s"
    print "iterated MD5; cracking such a hash would take %.2g CPU-years." % t

    # (As it happens, my own machines use Drepper’s SHA-2-based
    # hashing algorithm that was developed to replace the one
    # mentioned above; I am assuming that it’s at least as slow as the
    # MD5-crypt.)

    # <https://en.bitcoin.it/wiki/Mining_hardware_comparison> says a
    # Core 2 Duo U7600 can do 1.1 Mhash/s (of Bitcoin) at a 1.2GHz
    # clock with one thread.  The Celeron in my machine that I
    # benchmarked is basically a Core 2 Duo with a smaller cache, so
    # I’m going to assume that it could probably do about 1.5Mhash/s.
    # All common password-hashing algorithms (the ones mentioned
    # above, the others implemented in John, and bcrypt, but not
    # scrypt) use very little memory and, I believe, should scale on
    # GPUs comparably to the SHA-256 used in Bitcoin.

    # The same mining-hardware comparison says a Radeon 5870 card can
    # do 393.46 Mhash/s for US$350.

    print "But a modern GPU can crack about 250 times as fast,"
    print "so that same iterated MD5 would fall in %.1g GPU-years." % (t / 250)

    # Suppose we depreciate the video card by Moore’s law,
    # i.e. halving in value every 18 months.  That's a loss of about
    # 0.13% in value every day; at US$350, that’s about 44¢ per day,
    # or US$160 per GPU-year.  If someone wanted your password as
    # quickly as possible, they could distribute the cracking job
    # across a network of millions of these cards.  The cards
    # additionally use about 200 watts of power, which at 16¢/kWh
    # works out to 77¢ per day.  If we assume an additional 20%
    # overhead, that’s US$1.45/day or US$529/GPU-year.
    cost_per_day = 1.45
    cost_per_crack = cost_per_day * 365 * t
    print "That GPU costs about US$%.2f per day to run in 2011," % cost_per_day
    print "so cracking the password would cost about US$%.1g." % cost_per_crack

def years(entropy, crypts_per_second):
    return float(2**entropy) / crypts_per_second / 86400 / 365.2422

if __name__ == '__main__':
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