Devise-Two-Factor Authentication

By Tinfoil Security (acq. Synopsys 2020). Interested in working with us? We're hiring!

Devise-Two-Factor is a minimalist extension to Devise which offers support for two-factor authentication, through the TOTP scheme. It:

• Allows you to incorporate two-factor authentication into your existing models
• Is opinionated about security, so you don't have to be
• Integrates easily with two-factor applications like Google Authenticator and Authy
• Is extensible, and includes two-factor backup codes as an example of how plugins can be structured

Contributing

We welcome pull requests, bug reports, and other contributions. We're especially looking for help getting this gem fully compatible with Rails 5+ and squashing any deprecation messages.

Example App

An example Rails 4 application is provided in the demo directory. It showcases a minimal example of Devise-Two-Factor in action, and can act as a reference for integrating the gem into your own application.

For the demo app to work, create an encryption key and store it as an environment variable. One way to do this is to create a file named local_env.yml in the application root. Set the value of ENCRYPTION_KEY in the YML file. That value will be loaded into the application environment by application.rb.

Getting Started

Devise-Two-Factor doesn't require much to get started, but there are a few prerequisites before you can start using it in your application.

First, you'll need a Rails application setup with Devise. Visit the Devise homepage for instructions.

You can add Devise-Two-Factor to your Gemfile with:

gem 'devise-two-factor'

Next, since Devise-Two-Factor encrypts its secrets before storing them in the database, you'll need to generate an encryption key, and store it in an environment variable of your choice. Set the encryption key in the model that uses Devise:

  devise :two_factor_authenticatable,
         :otp_secret_encryption_key => ENV['YOUR_ENCRYPTION_KEY_HERE']

Finally, you can automate all of the required setup by simply running:

rails generate devise_two_factor MODEL ENVIRONMENT_VARIABLE

Where MODEL is the name of the model you wish to add two-factor functionality to (for example user), and ENVIRONMENT_VARIABLE is the name of the variable you're storing your encryption key in.

This generator will add a few columns to the specified model:

• encrypted_otp_secret
• encrypted_otp_secret_iv
• encrypted_otp_secret_salt
• consumed_timestep
• otp_required_for_login

Remember to apply the new migration.

bundle exec rake db:migrate

It also adds the :two_factor_authenticatable directive to your model, and sets up your encryption key. If present, it will remove :database_authenticatable from the model, as the two strategies are incompatible. Lastly, the generator will add a Warden config block to your Devise initializer, which enables the strategies required for two-factor authentication.

If you're running Rails 3, or do not have strong parameters enabled, the generator will also setup the required mass-assignment security options in your model.

If you're running Rails 4, you'll also need to whitelist :otp_attempt as a permitted parameter in Devise :sign_in controller. You can do this by adding the following to your application_controller.rb:

before_action :configure_permitted_parameters, if: :devise_controller?

...

protected

def configure_permitted_parameters
  devise_parameter_sanitizer.for(:sign_in) << :otp_attempt
end

If you're running Devise 4.0.0 or above, you'll want to use .permit instead:

before_action :configure_permitted_parameters, if: :devise_controller?

...

protected

def configure_permitted_parameters
  devise_parameter_sanitizer.permit(:sign_in, keys: [:otp_attempt])
end

After running the generator, verify that :database_authenticatable is not being loaded by your model. The generator will try to remove it, but if you have a non-standard Devise setup, this step may fail. Loading both :database_authenticatable and :two_factor_authenticatable in a model will allow users to bypass two-factor authenticatable due to the way Warden handles cascading strategies.

Designing Your Workflow

Devise-Two-Factor only worries about the backend, leaving the details of the integration up to you. This means that you're responsible for building the UI that drives the gem. While there is an example Rails application included in the gem, it is important to remember that this gem is intentionally very open-ended, and you should build a user experience which fits your individual application.

There are two key workflows you'll have to think about:

1. Logging in with two-factor authentication
2. Enabling two-factor authentication for a given user

We chose to keep things as simple as possible, and our implementation can be found by registering at Tinfoil Security, and enabling two-factor authentication from the security settings page.

Logging In

Logging in with two-factor authentication works extremely similarly to regular database authentication in Devise. The TwoFactorAuthenticatable strategy accepts three parameters:

1. email
2. password
3. otp_attempt (Their one-time password for this session)

These parameters can be submitted to the standard Devise login route, and the strategy will handle the authentication of the user for you.

Disabling Automatic Login After Password Resets

If you use the Devise recoverable strategy, the default behavior after a password reset is to automatically authenticate the user and log them in. This is obviously a problem if a user has two-factor authentication enabled, as resetting the password would get around the two-factor requirement.

Because of this, you need to set sign_in_after_reset_password to false (either globally in your Devise initializer or via devise_for).

Enabling Two-Factor Authentication

Enabling two-factor authentication for a user is easy. For example, if my user model were named User, I could do the following:

current_user.otp_required_for_login = true
current_user.otp_secret = User.generate_otp_secret
current_user.save!

Before you can do this however, you need to decide how you're going to transmit two-factor tokens to a user. Common strategies include sending an SMS, or using a mobile application such as Google Authenticator.

At Tinfoil Security, we opted to use the excellent rqrcode-rails3 gem to generate a QR-code representing the user's secret key, which can then be scanned by any mobile two-factor authentication client.

If you decide to do this you'll need to generate a URI to act as the source for the QR code. This can be done using the User#otp_provisioning_uri method.

issuer = 'Your App'
label = "#{issuer}:#{current_user.email}"

current_user.otp_provisioning_uri(label, issuer: issuer)

# > "otpauth://totp/Your%20App:[email protected]?secret=[otp_secret]&issuer=Your+App"

If you instead to decide to send the one-time password to the user directly, such as via SMS, you'll need a mechanism for generating the one-time password on the server:

current_user.current_otp

The generated code will be valid for the duration specified by otp_allowed_drift. This value can be modified by adding a config in config/initializers/devise.rb.

Devise.otp_allowed_drift = 240 # value in seconds
Devise.setup do |config|
...
end

However you decide to handle enrollment, there are a few important considerations to be made:

• Whether you'll force the use of two-factor authentication, and if so, how you'll migrate existing users to system, and what your on-boarding experience will look like
• If you authenticate using SMS, you'll want to verify the user's ownership of the phone, in much the same way you're probably verifying their email address
• How you'll handle device revocation in the event that a user loses access to their device, or that device is rendered temporarily unavailable (This gem includes TwoFactorBackupable as an example extension meant to solve this problem)

It sounds like a lot of work, but most of these problems have been very elegantly solved by other people. We recommend taking a look at the excellent workflows used by Heroku and Google for inspiration.

Filtering sensitive parameters from the logs

To prevent two-factor authentication codes from leaking if your application logs get breached, you'll want to filter sensitive parameters from the Rails logs. Add the following to config/initializers/filter_parameter_logging.rb:

Rails.application.config.filter_parameters += [:otp_attempt]

Backup Codes

Devise-Two-Factor is designed with extensibility in mind. One such extension, TwoFactorBackupable, is included and serves as a good example of how to extend this gem. This plugin allows you to add the ability to generate single-use backup codes for a user, which they may use to bypass two-factor authentication, in the event that they lose access to their device.

To install it, you need to add the :two_factor_backupable directive to your model.

devise :two_factor_backupable

You'll also be required to enable the :two_factor_backupable strategy, by adding the following line to your Warden config in your Devise initializer, substituting :user for the name of your Devise scope.

manager.default_strategies(:scope => :user).unshift :two_factor_backupable

The final installation step is dependent on your version of Rails. If you're not running Rails 4, skip to the next section. Otherwise, create the following migration:

class AddDeviseTwoFactorBackupableToUsers < ActiveRecord::Migration
  def change
    # Change type from :string to :text if using MySQL database
    add_column :users, :otp_backup_codes, :string, array: true
  end
end

You can then generate backup codes for a user:

codes = current_user.generate_otp_backup_codes!
current_user.save!
# Display codes to the user somehow!

The backup codes are stored in the database as bcrypt hashes, so be sure to display them to the user at this point. If all went well, the user should be able to login using each of the generated codes in place of their two-factor token. Each code is single-use, and generating a new set of backup codes for that user will invalidate all of the old ones.

You can customize the length of each code, and the number of codes generated by passing the options into :two_factor_backupable in the Devise directive:

devise :two_factor_backupable, otp_backup_code_length:     32,
                               otp_number_of_backup_codes: 10

Help! I'm not using Rails 4.0!

Don't worry! TwoFactorBackupable stores the backup codes as an array of strings in the database. In Rails 4.0 this is supported natively, but in earlier versions you can use a gem to emulate this behavior: we recommend activerecord-postgres-array.

You'll then simply have to create a migration to add an array named otp_backup_codes to your model. If you use the above gem, this migration might look like:

class AddTwoFactorBackupCodesToUsers < ActiveRecord::Migration
  def change
    # Change type from :string_array to :text_array if using MySQL database
    add_column :users, :otp_backup_codes, :string_array
  end
end

Now just continue with the setup in the previous section, skipping the generator step.

Testing

Devise-Two-Factor includes shared-examples for both TwoFactorAuthenticatable and TwoFactorBackupable. Adding the following two lines to the specs for your two-factor enabled models will allow you to test your models for two-factor functionality:

require 'devise_two_factor/spec_helpers'

it_behaves_like "two_factor_authenticatable"
it_behaves_like "two_factor_backupable"

Troubleshooting

If you are using Rails 4.x and Ruby >= 2.7, you may get an error like

An error occurred while loading ./spec/devise/models/two_factor_authenticatable_spec.rb.
Failure/Error: require 'devise'

NoMethodError:
  undefined method `new' for BigDecimal:Class

see https://github.com/ruby/bigdecimal#which-version-should-you-select and ruby/bigdecimal#127 for more details, but you should be able to solve this by explicitly requiring an older version of bigdecimal in your gemfile like

gem "bigdecimal", "~> 1.4"