Authentication integration

Purpose
If a user arrives at the OpenHatch wiki, and that user is already logged into the OpenHatch website, the wiki should have a way to identify the user and log the user in with the same username. This page describes a protocol for the wiki (and other applications within the openhatch.org domain) to ask the openhatch Django code for information on the current user (specifically, username and email address), receive that information, and automatically log that user in.

This is necessary because the OpenHatch website is comprised of a few pieces. We do most of our work on the OpenHatch codebase in Django, but there's also this wiki and the bug tracker (and we're adding a forum). Users have asked for login integration.

Specific goals:


 * Users only have to authenticate once to all (web) services within openhatch.org.
 * Users can't impersonate other users.
 * Applications outside openhatch.org should not be able to use this system to gain information about users. (They might be able to use other mechanisms, but not this one.)
 * These applications don't have to be particularly securely maintained. (Specifics...?)

m_stone's notes:


 * You should plan to rotate authenticators.
 * Maybe use separate HMAC keys for separate "apps"?
 * I'm a bit nervous about having splittable cookies. Maybe add an index cookie? (fun link: canonical json)
 * I'm a bit nervous about the inter-app dataflow that these cookies represent. How are you going to avoid exposing XSS opportunities in the apps receiving data through these cookies?

Overview
Django code creates domain cookies within openhatch.org that contain the user's username and email address. The application (like the wiki) can read that information and verify it using HMAC-SHA1.

Application: redirect
If the wiki detects an OpenHatch session cookie, it redirects the user to http://openhatch.org/+create_user_data_cookie?redirect_to=http://openhatch.org/wiki/handle_login.php

Django code: create_user_data cookies
The OpenHatch site creates some cookies. All cookies contain text. We encode it as UTF-8 and then wrap that in base64.

The "message" that we HMAC is the final, base64-encoded data.


 * user_data__email_address: This contains the user's email address.
 * user_data__email_address__hmac: This contains a HMAC-SHA1 to verify the authenticity of the email address.
 * user_data__username: This contains the user's username.
 * user_data__username__hmac: This contains a HMAC-SHA1 of the username.

Application: Read cookie data, then delete cookies
Now it's time for the application (e.g., the wiki) to read the user data and log the user in.

It should delete the user_data__* cookies that it read, to avoid keeping clutter in the user's browser.

That's up to the application. It MUST verify the user_data__* using HMAC-SHA1 before trusting it, as users can tamper with this data.

The application should make its own "logged in" status expire at the end of the session.

Known issues
There are some problems with this setup. Some security issues:


 * Email addresses and usernames travel along the network unencrypted. (In practice, this is no worse than the status quo, since session IDs already do that.)
 * Not all applications treat usernames as case-sensitive. This might allow an attacker to create a new username that is treated as equivalent to a different user, effectively stealing an account.
 * We can fix this by constraining usernames on the Django side to be case-insensitively unique.
 * Right now, Django usernames seem to be case sensitive.

Some usability issues:


 * The first time the user goes to the forum, the browser will redirect a few times. That'll be weird.

Missing features:


 * If an account gets deleted within the OpenHatch Django app, we don't have a way to ask downstream applications to delete their account of the same name.