Simple authentication wrapper for an Google App Engine app
===========================================================

Supported specs:
  - OAuth 2.0
  - OAuth 1.0(a)
  - OpenID

Supported providers out of the box:
  - Google (OAuth 2.0)
  - Facebook (OAuth 2.0)
  - Windows Live (OAuth 2.0)
  - Twitter (OAuth 1.0a)
  - LinkedIn (OAuth 1.0a)
  - OpenID, using App Engine users module API

Dependencies:
  - python-oauth2. This is actually a library implementing OAuth 1.0 spec.
  - httplib2 (as a dependency of python-oauth2)
  - lxml (e.g. LinkedIn user profile data parsing)

Getting Started
================

1. Install the library on your local Mac/PC with one of:
  a. "easy_install -U simpleauth"
  b. "pip install simpleauth"
  c. clone the source repo, e.g. "git clone https://code.google.com/p/gae-simpleauth/"

2. Place the subdir called "simpleauth" into your app root.

3. You'll also need to get python-oauth2 (pip install oauth2) 
   and httplib2 (http://code.google.com/p/httplib2/)

4. Create a request handler by subclassing SimpleAuthHandler, e.g.

   class AuthHandler(SomeBaseRequestHandler, SimpleAuthHandler):
     """Authentication handler for all kinds of auth."""

     def _on_signin(self, data, auth_info, provider):
       """Callback whenever a new or existing user is logging in.
       data is a user info dictionary.
       auth_info contains access token or oauth token and secret.
       
       See what's in it with logging.info(data, auth_info)
       """
       
       auth_id = '%s:%s' % (provider, data['id'])
       
       # 1. check whether user exist, e.g.
       #    User.get_by_auth_id(auth_id)
       #
       # 2. create a new user if it doesn't
       #    User(**data).put()
       #
       # 3. sign in the user
       #    self.session['_user_id'] = auth_id
       #
       # 4. redirect somewhere, e.g. self.redirect('/profile')
       #
       # See more on how to work the above steps here:
       # http://webapp-improved.appspot.com/api/webapp2_extras/auth.html
       # http://code.google.com/p/webapp-improved/issues/detail?id=20
       

     def logout(self):
       self.auth.unset_session()
       self.redirect('/')

     def _callback_uri_for(self, provider):
       return self.uri_for('auth_callback', provider=provider, _full=True)

     def _get_consumer_info_for(self, provider):
       """Should return a tuple (key, secret) for auth init requests.
       For OAuth 2.0 you should also return a scope, e.g.
       ('my app id', 'my app secret', 'email,user_about_me')
       
       The scope depends solely on the provider.
       See example/secrets.py.template
       """
       return secrets.AUTH_CONFIG[provider]

Note that SimpleAuthHandler isn't a real request handler. It's up to you.
For instance, SomeBaseRequestHandler could be webapp2.RequestHandler.

5. Add routing so that '/auth/PROVIDER', '/auth/PROVIDER/callback' and '/logout' requests
   go to your AuthHandler. 
   
   For instance, in webapp2 you could do:
   
   # Map URLs to handlers
   routes = [
     Route('/auth/<provider>',
       handler='handlers.AuthHandler:_simple_auth', name='auth_login'),
     Route('/auth/<provider>/callback', 
       handler='handlers.AuthHandler:_auth_callback', name='auth_callback'),
     Route('/logout',
       handler='handlers.AuthHandler:logout', name='logout')
   ]

6. That's it. See a sample app in the example dir. 
   To run the example app, copy example/secrets.py.template into example/secrets.py
   and start the app locally by executing run.sh

OAuth scopes, keys and secrets
===============================

This section is just a bunch of links to the docs on authentication with
various providers.

== Google
Docs:
https://developers.google.com/accounts/docs/OAuth2WebServer

Where to get client/secret:
http://code.google.com/apis/console

Multiple scopes should be space-separated, e.g.
"https://www.googleapis.com/auth/userinfo.profile https://www.googleapis.com/auth/userinfo.email"

== Facebook
Docs:
https://developers.facebook.com/docs/authentication/server-side/

Where to get client/secret:
https://developers.facebook.com/apps

Multiple Scopes should be comma-separated, e.g. "user_about_me,email".
Full list of scopes:
http://developers.facebook.com/docs/authentication/permissions/

== Windows Live
Docs:
http://msdn.microsoft.com/en-us/library/live/hh243649.aspx

Where to get client/secret:
https://manage.dev.live.com/AddApplication.aspx

Scopes are space-separated, e.g. "wl.signin wl.basic".
Full list of scopes:
http://msdn.microsoft.com/en-us/library/live/hh243646.aspx

CSRF protection
================

You can optionally enable cross-site-request-forgery protection for OAuth 2.0:

   class AuthHandler(webapp2.RequestHandler, SimpleAuthHandler):

     # enabled CSRF state token for OAuth 2.0
     OAUTH2_CSRF_STATE = True

     # ...
     # rest of the stuff from step 4 of the above.

This will use the optional OAuth 2.0 'state' param to guard against CSRFs by
setting a user session token during Authorization step and comparing it 
against 'state' parameter on callback.

For this to work your handler has to have a session dict-like object on the 
instance. Here's an example using webapp2_extras session:

   import webapp2
   from webapp2_extras import sessions

   class AuthHandler(webapp2.RequestHandler, SimpleAuthHandler):
     # enabled CSRF state token for OAuth 2.0
     OAUTH2_CSRF_STATE = True

     @webapp2.cached_property
     def session(self):
       """Returns a session using the default cookie key"""
       return self.session_store.get_session()

     def dispatch(self):
       # Get a session store for this request.
       self.session_store = sessions.get_store(request=self.request)
       try:
         # Dispatch the request.
         webapp2.RequestHandler.dispatch(self)
       finally:
         # Save all sessions.
         self.session_store.save_sessions(self.response)

     # ...
     # rest of the stuff from step 4 of the above.

This simple implementation assumes it is safe to use user sessions.
If, however, user's session can be hijacked, the authentication flow could
probably be bypassed anyway and this CSRF protection becomes the least 
of the problems.

Alternative implementation could involve HMAC digest. If anything serious 
pops up (e.g. see this SO question: http://goo.gl/3hiOv) please submit 
a bug on the issue tracker.

Catching errors
================

There are a couple ways to catch authentication errors if you don't want your
app to display a "Server Error" message when something goes wrong during
an auth flow.

You can use webapp2's built-in functionality and define
handle_exception(self, exception, debug) instance method on the handler
that processes authentication requests or on a base handler if you have one.
Here's a simple example:

   class AuthHandler(webapp2.RequestHandler, SimpleAuthHandler):
     # _on_signin() and other stuff
     # ...

     def handle_exception(self, exception, debug):
       # Log the error
       logging.error(exception)
       # Do something based on the exception: notify users, etc.
       self.response.write(exception)
       self.response.set_status(500)

You can also define global (app-wise) error handlers using app.error_handlers
dict (where app is a webapp2.WSGIApplication instance). It's all described
in details on this page:
http://webapp-improved.appspot.com/guide/exceptions.html

Another solution is, if you're using webapp2's dispatch method like in the
CSRF snippet above, you could do something like this:

   from simpleauth import Error as AuthError

   def dispatch(self):
     try:
       # Dispatch the request.
       webapp2.RequestHandler.dispatch(self)
     except AuthError as e:
       # Do something based on the error: notify users, etc.
       logging.error(e)
       self.redirect('/')

Alternatively, you can also use App Engine built-in functionality and define
error handlers in app.yaml. Docs on this can be found here:
https://developers.google.com/appengine/docs/python/config/appconfig
(see Custom Error Responses section).

Lastly, if nothing from the above works for you, override _simple_auth()
and/or _auth_callback() methods, e.g.

   from simpleauth import SimpleAuthHandler
   from simpleauth import Error as AuthError

   class AuthHandler(webapp2.RequestHandler, SimpleAuthHandler):
     def _simple_auth(self, provider=None):
       try:
         super(AuthHandler, self)._simple_auth(provider)
       except AuthError as e:
         # Do something based on the error: notify users, etc.
         logging.error(e)
         self.redirect('/')

CHANGELOG
================

v0.1.3 - 2012-09-19
  * CSRF protection for OAuth 2.0
    http://code.google.com/p/gae-simpleauth/issues/detail?id=1
  * Custom exceptions
    http://code.google.com/p/gae-simpleauth/issues/detail?id=2
  * Example app improvements, including:
    - CSRF guard
    - show exception messages for demo purposes
    - prettier output of session, profile data and auth_info dictionaries
    - x1ddos#4
    - x1ddos#5
  * More useful info in README