ims-lti

This is a nodejs library used to help create Tool Providers for the IMS LTI standard. Tool Consumer implmentation is left as an excersise to the reader :P

Install

npm install ims-lti --save

To require the library into your project

require 'ims-lti'

Supported LTI Versions

• 1.0 - Implementation Guide
• 1.1 - Implementation Guide
• 1.1.1 - Implementation Guide

Usage

The LTI standard won't be covered here, but it would be good to familiarize yourself with the specs. LTI documentation

This library doesn't help you manage or distribute the consumer keys and secrets. The POST parameters will contain the oauth_consumer_key and your application should use that to look up the consumer secret from your own datastore.

This library offers a few interfaces to use for managing the OAuth nonces to make sure the same nonce isn't used twice with the same timestamp. Read the LTI documentation on OAuth. They will be covered below.

Setting up a Tool Provider (TP)

As a TP your app will receive a POST request with LTI launch data that will be signed with OAuth using a key/secret that both the TP and Tool Consumer (TC) share. This is all covered in the LTI security model

Once you find the oauth_consumer_secret based on the oauth_consumer_key in the POST request, you can initialize a Provider object with them and a few other optional parameters:

lti = require 'ims-lti'
 
provider = new lti.Provider consumer_key, consumer_secret, [nonce_store=MemoryStore], [signature_method=HMAC_SHA1]

Once the provider has been initialized, a reqest object can be validated against it. During validation, OAuth signatures are checked against the passed consumer_secret and signautre_method ( HMAC_SHA1 assumed ). isValid returns true if the request is an lti request and is properly signed.

provider.valid_request req, (err, isValid) ->
  # isValid = Boolean | always false if err 
  # err = Error object with method descibing error if err, null if no error 

After validating the reqest, the provider object both stores the requests parameters (excluding oauth parameters) and provides convinience accessors to common onces including provider.student, provider.ta, provider.username, and more. All request data can be accessed through provider.body in an effort to namespace the values.

Currently there is not an emplementation for posting back to the Tool Consumer, although there is a boolean accessor provider.outcome_service that will return true if the TC will accept a POSTback.

Nonce Stores

ims-lti does not standardize the way in which the OAuth nonce/timestamp is to be stored. Since it is a crutial part of OAuth security, this library implements an Interface to allow the user to implement their own nonce-stores.

Nonce Interface

All custom Nonce stores should extend the NonceStore class and implment isNew and setUsed

class NonceStore
  isNew:   (nonce,timestamp,callback)=>
    # Sets any new nonce to used 
  setUsed: (nonce,timestamp,callback)=>

Two nonce stores have been implemented for convinience.

MemoryNonceStore

The default nonce store (if none is specified) is the Memory Nonce Store. This store simply keeps an array of nocne/timestamp keys. Timestamps must be valid within a 5 minute grace period.

RedisNonceStore

A superior nonce store is the RedisNonceStore. This store requires a secondary input into the constructor, a redis-client. The redis client is used to store the nonce keys and set them to expire within a set amount of time (default 5 minutes). A RedisNonceStore is initialized like:

RedisNonceStore = require '../lib/redis-nonce-store'
client          = require('redis').createClient()
store           = new RedisNonceStore('consumer_key', client)
 
provider = new lti.Provider consumer_key, consumer_secret, store

Outcomes Extension

The outcomes feature is part of the LTI 1.1 specification and is new to ims-lti 1.0. All of the behind-the-scenes work necessary to get the ball rolling with it is already implemented for you, all you need to do is submit grades.

provider = new lti.Provider consumer_key, consumer_secret
 
provider.valid_request req, (err, is_valid) ->
  # Check if the request is valid and if the outcomes service exists. 
  if (!is_valid || !provider.outcome_service) return false
 
  # Check if the outcome service supports the result data extension using the 
  # text format. Available formats include text and url. 
  console.log provider.outcome_service.supports_result_data('text')
 
  # Replace accepts a value between 0 and 1. 
  provider.outcome_service.send_replace_result .5, (err, result) ->
    console.log result # True or false 
 
  provider.outcome_service.send_read_result (err, result) ->
    console.log result # Value of the result already submitted from this embed 
 
  provider.outcome_service.send_delete_result (err, result) ->
    console.log result # True or false 
 
  provider.outcome_service.send_replace_result_with_text .5, 'Hello, world!', (err, result) ->
    console.log result # True or false 
 
  provider.outcome_service.send_replace_result_with_url .5, 'https://google.com', (err, result) ->
    console.log result # True or false 

Content Extension

The content extension is an extension supported by most LMS platforms. It provides LTI providers a way to send content back to the LMS in the form of urls, images, files, oembeds, iframes, and even lti launch urls.

provider = new lti.Provider consumer_key, consumer_secret
 
provider.valid_request req, (err, is_valid) ->
  #check if the request is valid and if the content extension is loaded. 
  if (!is_valid || !provider.ext_content) return false
 
  provider.ext_content.has_return_type 'file' # Does the consumer support files 
  provider.ext_content.has_file_extension 'jpg' # Does the consumer support jpg 
 
  # All send requests take a response object as the first parameter. How the 
  # response object is manipulated can be overrided by replacing 
  # lti.Extensions.Content.redirector with your own function that accepts two 
  # parameters, the response object and the url to redirect to. 
  provider.ext_content.send_file res, file_url, text, content_mime_type
 
  provider.ext_content.send_iframe res, iframe_url, title_attribute, width, height
 
  provider.ext_content.send_image_url res, image_url, text, width, height
 
  provider.ext_content.send_lti_launch_url res, launch_url, title_attribute, text
 
  provider.ext_content.send_oembed res, oembed_url, endpoint
 
  provider.ext_content.send_url res, hyperlink_url, text, title_attribute, target_attribute

Running Tests

To run the test suite first installing the dependencies:

npm install
make test