I'm reading about Federated Login for Google Account Users to figure out how I can have a user log in to a web application using their Google Account.
So towards the end of the process, Google returns a Google supplied identifier which is appended as openid.claimed_id. This means the web application uses this identifier to recognize the user and allow access to application features and data. My question is, is this identifier static? Can I use this identifier to repeatedly id the same user?
Yes. Consider the openid.claimed_id value to be the username. Especially with Google, but this is true for any OpenID Provider that truly implements 'directed identity', don't consider this username to be correlatible with other web sites. Any other relying party besides your own web site will get a different claimed_id value for the same Google user, by design.
Also, be sure to treat this claimed_id as case sensitive.
The specific answer to your question is found in Googles OpenID API documentation:
The Google-supplied identifier, which has no connection to the user's actual Google account name or password, is a persistent value; it remains constant even if the user changes their Google user name and/or email address. This identifier is also a "directed identity", that is, Google returns a different value to each relying party. Google uses the request parameter openid.realm to recognize the relying party, so if the third-party application decides to change this value, all user identifiers will change.
In fact, I just ran into an instance where the google claimed_id had changed for my test user. I was coming to the end of implementing OpenID into my app, and for no apparently reason the claimed_id in the response data is had changed.
I've been testing with this account for the past couple weeks, and the claimed_id was the same this entire time, as expected. Then wham, changed! I looked at the response data many times to verify, and the underlying code to retrieve the data had not changed.
I'm not sure how to handle this at the moment, but I think this is going to throw me for a loop. After initial authentication, users register to the site (as you might expect) and setup a screen name. How are we to verify it is the same user if the claimed_id had changed? We certainly can't use email address, per best practices.
EDIT
Now I have pie in my face! I missed one little detail, that turned out to be a major detail. I change my development environment and was hosting on a different v-host. This effectively change the realm, and this will change the claimed_id response according to the docs.
This was a good lesson for me, as I was about to implement OID on a subdomain in which realm was being set automatically in my code. Now I saved myself a headache down the road, because I would not have been able to use the same user database across all other sub-domains without breaking identity.
MORE INFO
Just as a side note - even if you are developing your OpenID solution for one of your subdomains, it might be prudent for you to specify realm to your top-level domain.
e.g., openid.realm = http://*.yourdomain.com
It will allow you to expand your sign-in page across all your subdomains and keep user identity across them.
(optional) Authenticated realm. Identifies the domain that the end user is being asked to trust. (Example: "http://*.myexamplesite.com") This value must be consistent with the domain defined in openid.return_to. If this parameter is not defined, Google will use the URL referenced in openid.return_to.
