Gem to authenticate with Instructure Canvas via OAuth2

OmniAuth Canvas grew out of the need to simplify the process of setting up LTI and connecting a user account on to Instructure Canvas on various projects built by Atomic Jolt.

Atomic Jolt has created an LTI started application that demonstrates how to use the OAuth gem: LTI Starter App

Contact Instructure or your Canvas administrator to get an OAuth key and secret. By default omniauth-canvas will attempt to authenticate with http://canvas.instructure.com.

NOTE: you will need to set env['rack.session']['oauth_site'] to the current Canvas instance that you wish to OAuth with. By default this is https://canvas.instructure.com

-- OR --

to dynamically set the canvas site url do one of the following.

use OmniAuth::Builder do
  provider :canvas, 'canvas_key', 'canvas_secret', :setup => lambda{|env|
    request = Rack::Request.new(env)
    env['omniauth.strategy'].options[:client_options].site = env['rack.session']['oauth_site']
  }
end

config.omniauth :canvas, 'canvas_key', 'canvas_secret', :setup => lambda{|env|
  request = Rack::Request.new(env)
  env['omniauth.strategy'].options[:client_options].site = env['rack.session']['oauth_site']
}

In this setup, you do not have to set env['rack.session']['oauth_site']

Rails.application.config.middleware.use OmniAuth::Builder do
  provider :canvas, APP_CONFIG['canvas_client_id'], APP_CONFIG['canvas_client_secret'],
  {
    :client_options => {
      :site => APP_CONFIG['canvas_host']
    }
  }
end

If you are running your own installation of canvas, you can generate a client ID and secret in the Site Admin account of your Canvas install. There will be a "Developer Keys" tab on the left navigation sidebar. For more information, consult the Canvas OAuth Documentation

In most cases your application will need to restore state after handling the OAuth process with Canvas. Since many applications that integrate with Canvas will be launched via the LTI protocol inside of an iframe sessions may not be available. To restore application state the omniauth-canvas gem uses the "state" parameter provided by the LTI proctocol. You will need to add the following code to your application to take advantage of this functionality:

Add the following initializer in config/initializers/omniauth.rb:

OmniAuth.config.before_request_phase do |env|
  request = Rack::Request.new(env)
  state = "#{SecureRandom.hex(24)}#{DateTime.now.to_i}"
  OauthState.create!(state: state, payload: request.params.to_json)
  env["omniauth.strategy"].options[:authorize_params].state = state

  # Bye default omniauth will store all params in the session. The code above
  # stores the values in the database so we remove the values from the session
  # since the amount of data in the original params object will overflow the
  # allowed cookie size
  env["rack.session"].delete("omniauth.params")
end

Add the following middleware to lib/middlware/oauth_state_middleware.rb:

class OauthStateMiddleware
  def initialize(app)
    @app = app
  end

  def call(env)
    request = Rack::Request.new(env)
    if request.params["state"] && request.params["code"]
      if oauth_state = OauthState.find_by(state: request.params["state"])
        # Restore the param from before the OAuth dance
        state_params = JSON.parse(oauth_state.payload) || {}
        state_params.each do |key, value|
          request.update_param(key, value)
        end
        application_instance = ApplicationInstance.find_by(lti_key: state_params["oauth_consumer_key"])
        env["canvas.url"] = application_instance.lti_consumer_uri
        oauth_state.destroy
      else
        raise OauthStateMiddlewareException, "Invalid state in OAuth callback"
      end
    end
    @app.call(env)
  end
end

class OauthStateMiddlewareException < RuntimeError
end

This middleware relies upon two models - OauthState and ApplicationInstance. OauthState is used to store relevant state before sending the user to Canvas to finish the OAuth. ApplicationInstance is model used in Atomic Jolt projects that is used to store the Canvas Url so that it can be reset in the environment. You don't need to implement the same model, but you will need to store the user's Canvas URL somewhere before sending the user to OAuth with Canvas. Change the following lines in the above code to recover the Canvas URL from where ever it is stored:

application_instance = ApplicationInstance.find_by(lti_key: state_params["oauth_consumer_key"])
env["canvas.url"] = application_instance.lti_consumer_uri

The OauthState model looks like this:

class OauthState < ActiveRecord::Base
  validates :state, presence: true, uniqueness: true
end

With the following schema:

create_table "oauth_states", force: :cascade do |t|
  t.string   "state"
  t.text     "payload"
  t.datetime "created_at", null: false
  t.datetime "updated_at", null: false
  t.index ["state"], name: "index_oauth_states_on_state", using: :btree
end

Last, enable the middleware by adding the following to config/application.rb:

# Middleware that can restore state after an OAuth request
config.middleware.insert_before 0, "OauthStateMiddleware"

Copyright (C) 2012-2017 by Justin Ball and Atomic Jolt.

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