Ruby + Rails support for the FIDO U2F security key standard
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What's the status of this plugin?

I created this gem in December of 2015 when the U2F standard was first released, and it was just too early. Nobody understood what two-factor authentication was, why they needed it, or why they should use hardware instead of horrifically insecure SMS. As a solo developer, I couldn't afford to do the market education to turn this into a viable project.

Now, in mid-2018, I've seen increasing interest in U2F and am considering updating this plugin for Rails 5. It would only take two sites willing to buy an annual to make it worth my time. Please email me if you want to secure your users or would like to chat about anything related to the project.


You know your users reuse passwords. Their account on your site is only as safe as the least-secure site they've ever reused their password on... so, not very safe at all.

Two factor authentication ("2FA") is a step up for your site's security. Users provide their password and then connect a USB security key. Stolen passwords are no longer a disaster.

Google started an industry alliance called FIDO to create a very smart standard called Universal Second Factor ("U2F").

  • GMail, PayPal, and already support it
  • Security keys are cheap ($6-18 USD) and available from many vendors
  • Users can use a single security key on any number of sites
  • ...without installing drivers or typing random numbers
  • ...and reusing a key doesn't let sites track the user betwen them
  • Sites don't have to authenticate any external service
  • ...or pay a licensing fee to FIDO or a patent holder

U2F is young and growing. Google Chrome and Mozilla Firefox support it fully out-of-the-box. Microsoft has joined the Board of Directors for FIDO Alliance, so perhaps Internet Explorer and Edge will support it soon. Safari supports it via an extension and it's impossible to predict if Apple will add official support.

The TwoFactorAuth gem drops into your site to add support for U2F. You don't have to learn the crypto behind it, you just integrate it into your user interface. This should take an hour or two.


The steps below assume you're using TwoFactorAuth with Ruby on Rails. I will write non-Rails instructions later.

  1. Add it to your Gemfile and run bundle install:

    gem "two_factor_auth"
  2. Show server-side errors (like a failed authentication) by displaying flash :alert messages, usually in app/views/layouts/application.html.erb. See the Rails Guide for more information.

  3. Show client-side errors (like using the wrong device) from the security key by including a div with the class twofactorauth-status. This div must exist and be visible even without any flash alerts/errors from the server.

    If your flash[:alert] div is always present, you can combine these two steps:

    <div class="alert twofactorauth-status"><%= flash[:alert] %></div>
  4. Generate the initializer and run the migration:

    rails generate two_factor_auth:install
    rake db:migrate
    rake db:migrate RAILS_ENV=test

    Edit config/initializers/two_factor_auth.rb to include your app's domain name. For security reasons, U2F won't authenticate against fake top-level domains like ".dev", so you may need to add an alias to /etc/hosts and use that when developing locally:
  5. Test starting your server with bundle exec rails server. TwoFactorAuth has a lot of assertions for subtle U2F requirements around domains names, so it may stop the Rails server from starting and tell you to make a URL HTTPS, etc.

  6. When you want to require registration or authentication in your controllers, call a filter:

    before_action :two_factor_auth_registration
    # or:
    before_action :two_factor_auth_authentication

    If you only want to require users authenticate if they've registered a key:

    before_action :two_factor_auth_authentication, if: :user_two-factor_auth_registered?
  7. TwoFactorAuth ships with a RegistrationsController and AuthenticationsController, use rake routes to see their routes.

  8. Override after_two_factor_auth_registration_path_for(resource) and after_two_factor_auth_authentication_path_for(resource) in your specific controller or ApplicationController to specify where to send users after the successfully register or authenticate.

  9. If you roll your own Registrations/Authentications controller, remember to never ever show the error messages from the Verifiers in production. The messages are useful for debugging TwoFactorAuth, your app, or your JavaScript, but they would also be very useful to an attacker trying to bypass or break your two-factor authentication.

Is it any good?


Integrating TwoFactorAuth is an hour or two, mostly front-end coding. You get well-tested code to keep your users safer.

Implementing it yourself is... frustrating. U2F is well-designed and robust, but the specification is 34,000 words of dense, fairly confusing documentation. For example, it took me five hours to track down a parameter that had to be formatted a single character differently from every example. This was never explained, but a link in a single footnote led to another spec that defined it implicitly after about 4,500 words. And that's before integrating with finicky, magical code like Rails and Devise. Save yourself four weeks of senior dev time and use TwoFactorAuth.


The test/dummy directory has a Rails app, you can run it with:

$ bundle
$ bundle exec rake db:setup
$ bundle exec rails server

Use bundle exec rake to run the library's test suite.

See TODO for what's planned and might affect your code. I'd love to hear what you're thinking of writing by email (run git log --format="format:%an <%ae>"), on IRC as pushcx on Freenode, or on Twitter as @pushcx.

Fork the repo, make your change in a branch, don't forget to run the tests, and send a pull request. Because of the dual-licensing (explained below), you'll need to sign a Contrubutor License Agreement. I'll contact you with that.


TwoFactorAuth is available at no charge under the Affero General Public License v3 (note that section 13 will apply to the codebase you use this in), see LICENSE for details.

If you don't want to publish your app's code, visit the gem's homepage for a commercial license that includes support and, if you want, installation.

If you would like to have a licensing flamewar, please contact me on Slashdot in 1998, as that is the only time and place I have had the interest in one. Even RMS approves of selling exceptions to the GPL.