OAuth2 Protocol

The mobile app uses the standard OAuth 2.0 protocol for authenticating users to the open edX LMS. The OAuth2 protocol supports authenticating the "client" and/or the "resource owner" by verifying "shared credentials" that were exchanged out-of-band with the "authorization server". A successful authentication transaction results in a valid "access_token" that the client uses for accessing resources on the "resource server".

In the mobile context:

the client is the mobile device
the resource owner is the user using the mobile device
the resource server is LMS or any service hosted on the edX platform that exports a public API to be called by the mobile app
the shared credential is either
- the user's "password" (if the user has an edX password), or
- the user's "social auth ID" (for 3rd party authentication)
the authorization server is either
- LMS (if authenticating with the user's password), or
- a 3rd party server (if authenticating with a 3rd party social auth)

Mobile API Authentication Classes

All APIs called by the edX mobile app are authenticated using the OAuth2AuthenticationAllowInactiveUser authentication class. It verifies the caller has a valid OAuth2 token and bypasses verification of their email address. For a streamlined on-boarding experience, the mobile app supports ongoing usage of its features without ever requiring the user to verify their email address.

Additionally, while most edX views support session authentication, some of those APIs are decorated with the SessionAuthenticationAllowInactiveUser authentication class, which also bypasses email verification. For example, the enrollment API doesn't require the user to be "active" (i.e., doesn't need a verified email address).

OAuth2 Access Tokens

The mobile app obtains an edX-issued access token in either of the following ways:

AccessToken View: username/email + password combo with grant_type=password
- curl -X POST -d "client_id=INSERT_CLIENT_ID&grant_type=password&username=INSERT_USERNAME&password=INSERT_PASSWORD" http://localhost:8000/oauth2/access_token/
AccessTokenExchangeView: 3rd party (social-auth) OAuth 2.0 access token -> 1st party (open-edx) OAuth 2.0 access token

- curl -X POST -d "client_id=INSERT_CLIENT_ID&access_token=INSERT_THIRD_PARTY_ACCESS_TOKEN" http://localhost:8000/oauth2/exchange_access_token/INSERT_BACKEND
- For now, the supported backends are "facebook" and "google-oauth2"

Example Response

The response from either of the above endpoints would provide the edX access_token as follows:

{"access_token": "5e0a0cb315e66aa96bab910faa8c70ee0ca91236", "token_type": "Bearer", "expires_in": 2591999, "scope": ""}

Authorization Bearer

Once an access token is obtained, it can be used to authenticate the user in any API call that supports the OAuth2AuthenticationAllowInactiveUser authentication class. The access token is passed in the Bearer field of the Authorization HTTP header, as follows:

curl -H "Authorization: Bearer INSERT_EDX_ACCESS_TOKEN" http://localhost:8000/api/mobile/v0.5/..

Expiration

OAuth2 -> Session Cookie

Additionally, the mobile app can exchange an access token for a session cookie to be used in a WebView:

LoginWithAccessTokenView: 1st party (open-edx) OAuth 2.0 access token -> session cookie
- Returns a 204 (no content), but with the user's session cookies in the response.

OAuth2 Client Type, Client ID, and Client Secret

OAuth defines two client types, based on their ability to
   authenticate securely with the authorization server (i.e., ability to
   maintain the confidentiality of their client credentials):

   confidential
      Clients capable of maintaining the confidentiality of their
      credentials (e.g., client implemented on a secure server with
      restricted access to the client credentials), or capable of secure
      client authentication using other means.

   public
      Clients incapable of maintaining the confidentiality of their
      credentials (e.g., clients executing on the device used by the
      resource owner, such as an installed native application or a web
      browser-based application), and incapable of secure client
      authentication via any other means.


   native application
      A native application is a public client installed and executed on
      the device used by the resource owner.  Protocol data and
      credentials are accessible to the resource owner.  It is assumed
      that any client authentication credentials included in the
      application can be extracted.  On the other hand, dynamically
      issued credentials such as access tokens or refresh tokens can
      receive an acceptable level of protection.  At a minimum, these
      credentials are protected from hostile servers with which the
      application may interact.  On some platforms, these credentials
      might be protected from other applications residing on the same
      device.

OAuth2 Refresh Tokens

*Update:*

The default django-oauth2 implementation does *not* support refresh tokens for public clients for the password grant type:
* [AccessToken.password|https://github.com/caffeinehit/django-oauth2-provider/blob/6b5bc0d3ad706d2aaa47fa476f38406cddd01236/provider/views.py#L541]
* [AccessToken.access_token_response|https://github.com/caffeinehit/django-oauth2-provider/blob/6b5bc0d3ad706d2aaa47fa476f38406cddd01236/provider/views.py#L482]

*However*, the [RFC|http://tools.ietf.org/html/rfc6749#section-10.4] *does* allow it.
Here is the relevant quote from the spec:

+10.4. Refresh Tokens+

*Authorization servers MAY issue refresh tokens to* web application
clients and *native application clients*.
...
*When client authentication is not possible*, the authorization server
SHOULD deploy other means to detect refresh token abuse.
...
*For example, *the authorization server could employ refresh token
rotation in which a *new refresh token is issued with every access
token refresh response*. The previous refresh token is invalidated
but retained by the authorization server. If a refresh token is
compromised and subsequently used by both the attacker and the
legitimate client, one of them will present an invalidated refresh
token, which will inform the authorization server of the breach.
...

Determine

What is the recommended expiration time for (1) refresh tokens and (2) oAuth tokens for mobile devices?
What does our server-side oAuth library support for refreshing tokens? What additional code is required to support it?
What is the recommended path for current app instances in the market and for future versions of the app?

WIP: OAuth2 and Mobile