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Lti Launcher

An extraction layer to simplify the setup and launching of LTI tools. In the most basic form, the launcher launches the requested tool in the context of the logged-in user. The logged-in user is fetched from a OIDC server. Other contexts (like course / assignment) should be provided by the platform through a signed JWT in the query parameter of the request.

Setup flow

Administrators can log in on the admin interface of the launcher via fixed basic HTTP authentication.

In this interface, the administrators can manage and add the available tools and set up the OIDC server. The main setup of a platform will, therefore, consist of:

  • name Name of the platform
  • openid_configuration_url The well known configuration url of the OIDC server. The app will extract all relevant settings from that endpoint.
  • client_id The registered client id
  • client_secret The registered client secret
  • context_jwks_url The URL with a list of the allowed public keys for signing the context parameter(s). The canvas LMS has a nice example of such an endpoint.

For each tool, the launcher requires:

  • auth_server Select to what auth server this tool will be linked
  • name Name of the tool
  • description Description of the tool
  • icon_url URL to an icon that can be shown in the platform
  • client_id Unique identifier of the tool
  • open_id_connect_initiation_url The URL to start the OIDC flow.
  • target_link_uri The URL to perform the final launch at.

The available tools per auth server are made available through a JSON endpoint that looks like this:

https://<domain>/api/v1/auth_servers/<auth_server_uid>/tools

[
  {
    name: '<tool_name>',
    description: '<tool_description>',
    icon_url: '<tool_icon_url>',
    url: 'https://<domain>/launch/<tool_client_id>'
  },
  ...
]

The platform fetches the available tools from the endpoint and shows those links to the user. The platform can optionally add additional context by adding the context query parameter. This parameter should be a signed JWT and the payload should match the LTI 1.3 format specification. So the JWT payload could be something like this:

{
  "https://purl.imsglobal.org/spec/lti/claim/context": {
    "id": "42",
    "label": "Everything",
    "title": "Finding the Answer",
    "type": [
      "Course"
    ]
  }
}

This will result in a link on the platform like this: https://<domain>/launch/<tool_client_id>?context=xxx.xxx.xxx. If the user clicks this link, the tool will open in either the current window or a new tab (so not in an iframe) and this is where the launcher kicks in.

Launch flow

  1. The user navigates to the launcher specifying the platform, the tool, and optional additional context

    https://<domain>/launch/<tool_client_id>?context=xxx.xxx.xxx

  2. The user gets redirected to the OIDC service matching the platform

    http://<auth_service_oidc_authorization_endpoint>
  ?response_type=id_token
  &state=<jwt containing the original url from step 1>
  &client_id=<auth_service_open_id_client_id>
  &redirect_uri=https://<domain>/callback
  &scope=profile

  3. The user logs in if needed and allows the launcher to access the user's data

  4. The user gets redirected back to the launcher (specified redirect_uri)

    https://<domain>/callback
  ?id_token=<User information as specified by OIDC>
  &state=<State as provided in step 2>
  &....?

  5. The launcher generates a login_hint containing all information needed to perform the launch including user and context information. The launcher saves this login_hint in the cookie and redirects the user to the tool's open_id_connect_initiation_url

    <tool_open_id_connect_initiation_url>
  ?client_id=<tool_client_id>
  &login_hint=<login_hint>
  &iss=<issuer as defined in the secrets>
  &target_link_uri=<tool_target_link_uri>

  6. The tool generates a state, saves this in a cookie and redirects the user to the launcher auth URL (tool should know this based on the iss parameter)

    https://<domain>/auth
  ?scope=openid
  &response_type=id_token
  &client_id=<tool_client_id>
  &redirect_uri=<tool_target_link_uri>
  &login_hint=<login_hint from previous step>
  &state=<state from and for the tool>
  &response_mode=form_post
  &nonce=<nonce from tool>
  &prompt=none

  7. Launcher verifies the request

    1. Find the tool matching the client_id
    2. redirect_uri matches the tools target_link_uri
    3. login_hint from cookie matches the login_hint from the parameters
    4. nonce is not used before

    and then generates an auto-submit form to the tool's target_link_url

    <tool_target_link_url>
  ?id_token=<Signed JWT containing all launch information>
  &state=<state from tool>

Cookie considerations

Since cookies in iframes are known to give a lot of problems. This launcher does not accept being launched in an iframe. Adding support for iframes could be added later. See some thoughts in the See the IMS roundtable video.

Hosting considerations

This app has a Dockerfile file to simplify the hosting setup. The Dockerfile is intended for production and requires the following environment variables:

  • DATABASE_URL The app need access to an external postgres database. This URL should include the username and password.
  • DOMAIN The base domain of the app (for example lti-launcher.com).
  • FORCE_SSL Set to 1 if the app runs on a secured endpoint.
  • PORT Optionally change the port the container listens to (default 9393).
  • ISSUER Sets the iss key in the open id initiation (default lti_launcher)
  • ADMIN_USER Username used to login to the admin interface (default admin)
  • ADMIN_PASSWORD_FILE Location of the file with the password used to login to the admin interface. If this is not set, it will fallback to the ADMIN_PASSWORD.
  • ADMIN_PASSWORD Password used to login to the admin interface (default on development is test)

Once the app is fired up, you need to make sure to run the database migrations. So not only the first time you start the app but every time the version has changed since there could be new migrations. To run the migrations you should run bin/rake db:migrate inside the container.

Ready for use docker image

If you create a release in GitHub, the GitHub actions will build and publish that version to the GitHub docker registry. That way, you don't have to build the image yourself and you can just download it from the registry. The downside of this is that you should authorize docker with your GitHub account as described here.

Running locally with docker compose

The app also has a docker-compose file which includes the setup of the postgres database. To fire up the app, run these commands:

IMS Certification Suite

To run the tool against the certification suite, you need an IMS account and run the tool on a public url with SSL. At Drieam we use ngrok to expose our localhost:

ngrok http 9393 --region 'eu' --subdomain 'lti-launcher'
DOMAIN=lti-launcher.eu.ngrok.io FORCE_SSL=1 bin/rails s

You can start the certification suite at imsglobal.org. This are the platform configuration details:

Key Value
Testing iss Value lti_launcher
OIDC Auth URL https://lti-launcher.eu.ngrok.io/auth
Platform Well-Known/JWKS URL https://lti-launcher.eu.ngrok.io/keypairs
OAuth2 Access Token URL https://lti-launcher.eu.ngrok.io/oauth2/token
Client Id cert
Custom aud
Deployment Id a0f3de21-8f3b-48ae-a7a6-30185a91a956

You can then initialize the required launches from the admin interface.