I know it’s kind of stupid, but it is what I need now. So I made it.

1. Add the following code to your Gemfile

   gem 'idp_captcha', github: 'niandalu/idp_captcha'
       

2. Mount it in config/route.rb

   mount IdpCaptcha::Engine => '/idp_captcha'
       

1. Request SERVER/idp_captcha/new to get :code
2. Display the image by assigning src to SERVER/idp_captcha/:code
3. To validate the user input captcha, DELETE SERVER/idp_captcha/:code with { captcha: <USER INPUT> }