require 'active_support/core_ext/class/attribute'

module ActionController #:nodoc:
  class InvalidAuthenticityToken < ActionControllerError #:nodoc:
  end

  # Controller actions are protected from Cross-Site Request Forgery (CSRF) attacks
  # by including a token in the rendered html for your application. This token is
  # stored as a random string in the session, to which an attacker does not have
  # access. When a request reaches your application, \Rails then verifies the received
  # token with the token in the session. Only HTML and javascript requests are checked,
  # so this will not protect your XML API (presumably you'll have a different
  # authentication scheme there anyway). Also, GET requests are not protected as these
  # should be idempotent.
  #
  # CSRF protection is turned on with the <tt>protect_from_forgery</tt> method,
  # which will check the token and raise an ActionController::InvalidAuthenticityToken
  # if it doesn't match what was expected. A call to this method is generated for new
  # \Rails applications by default. You can customize the error message by editing
  # public/422.html. 
  #
  # The token parameter is named <tt>authenticity_token</tt> by default. The name and
  # value of this token must be added to every layout that renders forms by including
  # <tt>csrf_meta_tag</tt> in the html +head+.
  #
  # Learn more about CSRF attacks and securing your application in the
  # {Ruby on Rails Security Guide}[http://guides.rubyonrails.org/security.html].
  module RequestForgeryProtection
    extend ActiveSupport::Concern

    include AbstractController::Helpers
    include AbstractController::Callbacks

    included do
      # Sets the token parameter name for RequestForgery. Calling +protect_from_forgery+
      # sets it to <tt>:authenticity_token</tt> by default.
      config_accessor :request_forgery_protection_token
      self.request_forgery_protection_token ||= :authenticity_token

      # Controls whether request forgery protection is turned on or not. Turned off by default only in test mode.
      config_accessor :allow_forgery_protection
      self.allow_forgery_protection = true if allow_forgery_protection.nil?

      helper_method :form_authenticity_token
      helper_method :protect_against_forgery?
    end

    module ClassMethods
      # Turn on request forgery protection. Bear in mind that only non-GET, HTML/JavaScript requests are checked.
      #
      # Example:
      #
      #   class FooController < ApplicationController
      #     protect_from_forgery :except => :index
      #
      #     # you can disable csrf protection on controller-by-controller basis:
      #     skip_before_filter :verify_authenticity_token
      #   end
      #
      # Valid Options:
      #
      # * <tt>:only/:except</tt> - Passed to the <tt>before_filter</tt> call.  Set which actions are verified.
      def protect_from_forgery(options = {})
        self.request_forgery_protection_token ||= :authenticity_token
        before_filter :verify_authenticity_token, options
      end
    end

    protected

      def protect_from_forgery(options = {})
        self.request_forgery_protection_token ||= :authenticity_token
        before_filter :verify_authenticity_token, options
      end

      # The actual before_filter that is used.  Modify this to change how you handle unverified requests.
      def verify_authenticity_token
        verified_request? || raise(ActionController::InvalidAuthenticityToken)
      end

      # Returns true or false if a request is verified.  Checks:
      #
      # * is the format restricted?  By default, only HTML requests are checked.
      # * is it a GET request?  Gets should be safe and idempotent
      # * Does the form_authenticity_token match the given token value from the params?
      def verified_request?
        !protect_against_forgery? || request.forgery_whitelisted? ||
          form_authenticity_token == params[request_forgery_protection_token]
      end

      # Sets the token value for the current session.
      def form_authenticity_token
        session[:_csrf_token] ||= ActiveSupport::SecureRandom.base64(32)
      end

      # The form's authenticity parameter. Override to provide your own.
      def form_authenticity_param
        params[request_forgery_protection_token]
      end

      def protect_against_forgery?
        allow_forgery_protection
      end
  end
end