Ruby gem for ALTCHA

ALTCHA is a protocol designed for safeguarding against spam and abuse by utilizing a proof-of-work mechanism. This protocol comprises both a client-facing widget and a server-side verification process.

altcha-rails is a Ruby gem that provides a simple way to integrate ALTCHA into your Ruby on Rails application.

The main functionality of the gem is to generate a challenge and verify the response from the client. This is done in the library code. An initializer and a controller is installed in the host application to handle the challenge generation and verification.

Add this line to your application's Gemfile:

gem 'altcha-rails'

Then execute bundle install to install the gem for your application.

Next, run the generator to install the initializer and the controller:

$ rails generate altcha:install
      create  app/models/altcha_solution.rb
      create  app/controllers/altcha_controller.rb
      create  config/initializers/altcha.rb
       route  get '/altcha', to: 'altcha#new'
      create  db/migrate/20240211145410_create_altcha_solutions.rb

This will create an initializer file at config/initializers/altcha.rb and a controller at app/controllers/altcha_controller.rb as well as a route in config/routes.rb and a model at app/models/altcha-solutions.rb (see below).

You will also have to run 'rails db:migrate` to apply pending changes to the database.

The initializer file config/initializers/altcha.rb contains the following configuration options:

Altcha.setup do |config|
  config.algorithm = 'SHA-256'
  config.num_range = (50_000..500_000)
  config.timeout = 5.minutes
  config.hmac_key = 'change-me'
end

The algorithm option specifies the hashing algorithm to use and must currently be set to SHA-256. It is crucial change the hmac_key to a random value. This key is used to sign the challenge and the response, so it must be treated as a secret within your application. The num_range option specifies the range of numbers to use in the challenge and determines the difficulty of the proof-of-work. For an explanation of the timeout option see below.

The current time of the server is included in the salt of the challenge. When the client responds, it has to send the same salt back, so the server can determine when the challenge was issued. The timeout option in the initializer file specifies the time that a challenge is valid. If the response is received after the timeout, the verification will fail.

As users might complete the captcha before filling out a complex form, the timeout should be set to a reasonable value.

To also guard against replay attacks within the configured timeout period, the gem uses a model named AltchaSolution to store completed responses. A unique constraint is added to the database to prevent the same response from being stored.

As these stored solutions are useless after the timeout period, the AltchaSolution.cleanup convenience function should be called regularly to purge outdates soltutions from the database.

You need to include the ALTCHA javascript widget in your application's asset pipeline. This is not done by the gem at this point. Read up on the ALTCHA documentation for more information.

Then add the following code to the form you want to protect:

<altcha-widget challengeurl="<%= altcha_url() %>"></altcha-widget>

Once the user clicks the checkbox, the widget will send a request to the server to get a new challenge. When the user-side code inside the widget found the solution to the challenge, the spinner will stop and a hidden input field with the name altcha will be created in the form to convey the solution as base64 encoded JSON dictionary.

In the controller that handles the form submission, you can verify the response with the following code:

def create
  @model = Model.create(model_params)

  unless AltchaSolution.verify_and_save(params.permit(:altcha)[:altcha])
    flash.now[:alert] = 'ALTCHA verification failed.'
    render :new, status: :unprocessable_entity
    return
  end

  # ...
end

The verify_and_save method will return true if the response is valid and has not been used before.

Bug reports and pull requests are welcome.

The gem is available as open source under the terms of the MIT License.