I have always felt that in identity community we spend most of our time discussing identity providers and their concerns (such as token format, protocol etc.) and do not spend enough time on and attention to relying parties. At eBay we play both roles i.e. we are both an identity provider (we provide sellers' identity and attributes to 3rd party developers) and relying party (accepting identities provisioned outside eBay marketplaces as eBay users). I can say that building a architecturally sound relying party is as challenging as building an identity provider.
In this post I simply want to enumerate the major use cases that any major relying party (that is anyone that for example plans to accept Facebook Connect) has to account for. I used the qualifier "major" to denote that these use cases are important if a given relying party has millions of uses and many services and applications (much like eBay does).
The list of use case are:
1-Sign-In and out
2-Connect and Reception
4-Profile Access Extension
10-Capturing Alternative/Second Secret
and here is the descriptions:
1-Sign-In and out: This includes changes to your standard sign-in page to make it a "federated sign-in" page. The challenge here is mostly user experience i.e. how to design the UI correctly to achieve two goals:
- Not confuse existing users who will not sign up with external IDP
- Communicate to the user of external IDP what they need to do (without creating the NASCAR problem)
There are also techniques for detecting the IDPs that user may have an account with and to show a "smart" list of IDPs
2-Connect and Reception: Once users clicks on "Connect" button (if you are using connect style IDPs such as FB) or entered her OpenId URI (although this is unlikely to be adopted by users), the user is send to IDPs to sign-in and given consent to his/her information to be send to you site - let's refer to this process as Connect - then user is sent back to your site to a page/application that we refer to as "Reception". This is the processes that greets the users for the first time and provision an account in RP for him/her. I use the word "reception" to make it distinct from "registration" which is when provision is done based on data collected by RP itself. The reception process is significant b/c it covers the gap between data received from IDP and what is needed for a user to be provisioned, also it assigns the roles for new user. These roles are typically minimal since data coming from external IDPs are normally not trusted or verified. Also during reception token received from external IDP together with associated meta data is stored in a central location accessible to different functional units (application) of RP
3-Link/Unlink: Another use case (often part of reception) is to detect whether the user connecting to RP is one who already has an account. The detection can be done based on mapping the data received from IDPs to existing account, the simplest form is to check whether email addressed returned by IDP already exists.Once an account is detected, user has to prove s/he actually owns it (normally by providing password) and the accounts are link. Since architecture hygiene calls for symmetric operation, you should also allow for unlinking of accounts.
4-Profile Access Extension: RP obtain a token during reception (such as OAuth token that comes with FB Connect), this token stores a set of access permissions to user resources (perhaps hosted by IDP). Any large RP has a set of applications that will use this token (for example MyeBay application as well as eBay Search Application) it is likely that one of these applications requires more information/access privileges that user originally consented to, in these case RPs should provide a central capabilities that conduct the process of requesting, receiving extended permissions from user and updating token meta information associated with user
5-Role Elevation: The first time users connect to RP they are granted a certain role (roles), normally this is a basic role since data provided by most IDPs are not reliable (eBay as an IDP does provides verified reliable data), at some point during the user's life cycle, users needs to perform an action that requires higher role assignment, in this cases RPs should provide capabilities to assign users higher role, this normally requires users to enter more information or go thru verification. This processes produce more attributes that will become part of users profile at RP.
6-Recovery: Every RP always has to establish a method for externally provisioned identities to authenticate WITHOUT the presence of external IDP. What does this mean? suppose you accept FB Connect and FB is down for 6 hours (an event that recently happened), further imagine that you operate a site that every minute of users not being able to login means financial loss. What do you do in this scenario? You may say, this is easy, ask users to enter a password during the first time reception, but wouldn't this defeat the whole purpose (or a big part of it) of users not having to remember many passwords?
7-Disconnect: All RPs must provide the capability for a user to "disconnect" i.e. close the account that was created based on an identity provided by external IDP. I personally believe that user owns his/her data and if user wants to disconnect and remove all of his/her activities from the record. s/he should be able to (to the extent that is legal)
8-Force Authentication: This is actually a capability of IDP, but RPs need to use this when they require user to be authenticated regardless of session's authentication state as seen by IDP. For certain operation RPs require a fresh session (or session that started in the past N minutes), in this cases RPs should request a forced authentication (I am using SAML terminology here) from IDP.