TurboGears and OpenID, part 2

I wrote that I planned to return to my TurboGears version of the Picky Picky Game once I had picture-uploading working. Yesterday I finally had a spare afternoon to do some more hacking on Picky2, and got uploading of pictures working. But before that I will finish off my description of doing authentication with OpenID.

In the first episode I skipped the last step in authenticating a user: remembering they are logged in so they don't have to repeat the process for every page. In the process of adding that tiny detail I also refactored the code so that the log-in controller class (used by the CherryPy part of TurboGears to run the login page) is in authentication.py; in controllers.py you just see

class Root(controllers.Root):
    login = authentication.LoginController()
... followed by method definitions ...

The cookie is saved with this function

import sqlobject, hmac, base64, time

def setLoggedInUserByUri(uri, remember=False):
    """The user identified by this URI is logged in: record this in a cookie."""
    # 1. Ensure there is an entry in our database.
        pers = model.Person.byUri(uri)
    except sqlobject.SQLObjectNotFound:
        label = uri
        if uri.startswith('http://'):
            label = label[7:]
        pers = model.Person(uri=uri, label=label)

    # 2. Create a cookie that contains an HMAC
    # (message authentication code) for the user's log-in URI.
    s = '%d,%s' % (int(time.time() + COOKIE_TTL), uri)
    h = hmac.new(SECRET, s).digest()
    cherrypy.response.simpleCookie['you'] = base64.standard_b64encode(h) + ',' + s 
    c = cherrypy.response.simpleCookie['you'] # Get the magic morsel object
    c['path'] = '/'
    if remember:
        c['max-age'] = COOKIE_TTL

    turbogears.flash('Welcome to Picky2')

The first part adds an entry to our database if this person is new to us. Recall that the person is identified by URI they supplied to OpenID. The label field will eventually be the human-friendly name for the person; for now we fudge it by just using the URI.

The second part creates a message-authentication code (MAC) of the HMAC flavour, defined in RFC 2104. The cookie's value looks something like


The last part is the identity of the logged-in person; the second part is the time when this cookie should be considered expired, and the first part is the HMAC that makes it hard for someone else to forge a cookie: without knowing the value of SECRET, it will be infeasible to generate the correct HMAC value. The time-out value is included so that even if your cookie is stolen by some cross-site scripting naughtiness, it will not be useful for very long.

The last bit sets the cookie. The tricky part was finding documentatioin for how CherryPy handles cookies; in the end it turns out Python has a Cookie module and that CherryPy uses that.

The flip-side of all this is that when you visit a page later, it needs to know if you have logged in. To do that it uses this function to get the currently logged in user:

def getLoggedInUser():
    """Return the Person object for the logged-in user."""
    loginUri = 'login?next=%s' % util.uriParamEncode(cherrypy.request.browserUrl)
    c = cherrypy.request.simpleCookie.get('you')        
    auth = c and c.value
    if not auth:
        turbogears.flash('Please log in first')
        raise cherrypy.HTTPRedirect(loginUri)
    h, s = auth.split(',', 1)
    hh = hmac.new(SECRET, s).digest()
    if h != base64.standard_b64encode(hh):
        turbogears.flash('Cookie has expired (or had bad HMAC)')
        raise cherrypy.HTTPRedirect(loginUri)
    expires, u = s.split(',', 1)
    if int(expires) < time.time():
        turbogears.flash('Cookie has expired')
        raise cherrypy.HTTPRedirect(loginUri)

        pers = model.Person.byUri(u)
    except sqlobject.SQLObjectNotFound:
        turbogears.flash('This user has never logged in (or database broken)')
        raise cherrypy.HTTPRedirect(loginUri)

    # TODO. Refesh the cookie
    return pers

The first thing it tries is basically the last part of the set-cookie function, run backwards: it gets the cookie, tries to see whether the HMAC matches, then decomposes it in to the time-out and user-identity parts. Each time it fails to verify the cookie, it uses turbogears.flash to explain why; this is for my benefit (while debugging the application), and is probably too indiscreet for a production web server. I should probably change it to just say 'Please log in' and not offer any explanations!

Next: all about uploading pictures. This time for sure.