launch.rb

class GradeCam::Launch

  attr_reader :assessment_session, :user, :url

  def initialize(assessment_session, user, url)
    @assessment_session, @user, @url = assessment_session, user, url
  end

  def params
    @params ||= { context_id: assessment_session.classroom.id,
      custom_resource_assignment_id: assessment_session.id,
      launch_presentation_document_target: "frame",
      launch_presentation_return_url: url,
      lti_message_type: "basic-lti-launch-request",
      lti_version: "LTI-1p0",
      oauth_callback: "about:blank",
      oauth_consumer_key: ENV['GRADECAM_OAUTH_KEY'],
      oauth_nonce: oauth_nonce,
      oauth_signature_method: "HMAC-SHA1",
      oauth_timestamp: Time.now.getutc.to_i.to_s,
      oauth_version: "1.0",
      resource_link_id: "0",
      roles: "Instructor",
      user_id: user.id,
      lis_person_name_given: user.first_name,
      lis_person_name_family: user.last_name,
      custom_lis_person_contact_email_primary: user.email
    }
  end

  def launch_fields
    params.merge(oauth_signature: oauth_signature)
  end

  def oauth_nonce(size=7)
    Base64.encode64(OpenSSL::Random.random_bytes(size)).gsub(/\W/, '')
  end

  def url_encode(string)
    CGI::escape(string)
  end

  def oauth_signature
    encoded_params = params.sort.collect{ |k, v| url_encode("#{k}=#{v}") }.join('%26')
    base_string = "POST&#{url_encode(ENV['GRADECAM_LAUNCH_ENDPOINT'])}&#{encoded_params}"
    sign("#{ENV['GRADECAM_OAUTH_SECRET']}&#{ENV['GRADECAM_OAUTH_KEY']}", base_string)
  end

  def sign(key, base_string)
    digest = OpenSSL::Digest.new('sha1')
    hmac = OpenSSL::HMAC.digest(digest, key, base_string)
    Base64.encode64(hmac).chomp.gsub(/\n/, '')
  end

end