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zxcvbn, named after a crappy password, is a JavaScript password strength estimation library. Use it to implement a custom strength bar on a signup form near you!

zxcvbn attempts to give sound password advice through pattern matching and conservative entropy calculations. It finds 10k common passwords, common American names and surnames, common English words, and common patterns like dates, repeats (aaa), sequences (abcd), and QWERTY patterns.

For full motivation, see:

http://tech.dropbox.com/?p=165

Installation

<script type="text/javascript" src="zxcvbn-async.js">
</script>

is the best way to add zxcvbn to your site. Host zxcvbn.js and zxcvbn-async.js somewhere on your web server, and make the hardcoded path inside zxcvbn-async.js point to zxcvbn.js. A relative path works well.

zxcvbn-async.js is a tiny 350 bytes. On window.load, after your page loads and renders, it'll fetch zxcvbn.js, which is more like 700k (330k gzipped), most of which is a series of dictionaries.

I haven't found 700k to be too large -- especially because a password isn't the first thing a user typically enters on a registration form.

zxcvbn.js can also be included directly:

<script type="text/javascript" src="zxcvbn.js">
</script>

But this isn't recommended, as the 700k download will block your initial page load.

zxcvbn adds a single function to the global namespace:

zxcvbn(password, user_inputs)

It takes one required argument, a password, and returns a result object. The result includes a few properties:

result.entropy            # bits

result.crack_time         # estimation of actual crack time, in seconds.

result.crack_time_display # same crack time, as a friendlier string:
                          # "instant", "6 minutes", "centuries", etc.

result.score              # [0,1,2,3,4] if crack time is less than
                          # [10**2, 10**4, 10**6, 10**8, Infinity].
                          # (useful for implementing a strength bar.)

result.match_sequence     # the list of patterns that zxcvbn based the
                          # entropy calculation on.

result.calc_time          # how long it took to calculate an answer,
                          # in milliseconds. usually only a few ms.

The optional user_inputs argument is an array of strings that zxcvbn will add to its internal dictionary. This can be whatever list of strings you like, but is meant for user inputs from other fields of the form, like name and email. That way a password that includes the user's personal info can be heavily penalized. This list is also good for site-specific vocabulary.

When zxcvbn loads (after the async script fetch is complete), it'll check if a function named zxcvbn_load_hook is defined, and run it with no arguments if so. Most sites shouldn't need this.

Development

Bug reports and pull requests welcome!

zxcvbn is written in CoffeeScript and Python. zxcvbn.js is built with compile_and_minify.sh, which compiles CoffeeScript into JavaScript, then JavaScript into efficient, minified JavaScript.

For development, include these scripts instead of zxcvbn.js:

<script type="text/javascript" src="adjacency_graphs.js">
</script>
<script type="text/javascript" src="frequency_lists.js">
</script>
<script type="text/javascript" src="matching.js">
</script>
<script type="text/javascript" src="scoring.js">
</script>
<script type="text/javascript" src="init.js">
</script>

Data lives in the first two scripts. These get produced by:

scripts/build_keyboard_adjacency_graph.py
scripts/build_frequency_lists.py

matching.coffee, scoring.coffee, and init.coffee make up the rest of the library.

init.js needs to come last, otherwise script order doesn't matter.

I recommend setting up coffee-mode in emacs, or whatever equivalent, so that CoffeeScript compiles to js on save. Otherwise you'll need to repetitively run compile_and_minify.js

Acknowledgments

Dropbox, thank you in so many ways, but in particular, for supporting independent projects both inside and outside of hackweek.

Many thanks to Mark Burnett for releasing his 10k top passwords list:

http://xato.net/passwords/more-top-worst-passwords

and for his 2006 book, "Perfect Passwords: Selection, Protection, Authentication"

Huge thanks to Wiktionary contributors for building a frequency list of English as used in television and movies: http://en.wiktionary.org/wiki/Wiktionary:Frequency_lists

Researchers encourage others to model their strength meters after this one: http://www.concordia.ca/cunews/main/stories/2015/03/25/does-your-password-pass-muster.html

Last but not least, big thanks to xkcd :) https://xkcd.com/936/