How does SSO work?

[metoc15411]

Explaining the system in general is easy, but explaining how SSO is implemented requires a little more information. Typically, when you log in, the service provider or domain (in this example, website.com) authenticates you on its own. Like this:

1. As a user, you are taken to a periodically appearing page on website.com that checks to see if you are already logged in. If so, SSO authentication is complete and the system will send you to where you really wanted to go (your Gmail inbox, for example).

2. If you are not already logged in, you will be presented with a login screen.

3. You enter your credentials (email address and password) into a form, website.com checks those credentials against its database, and then you are either signed in or rejected.

4. If you are logged in, website.com will issue some sort of tracker. This may be on the server or sent to you as a token.

Now, when you navigate the site, the system simply key features of office 365 database checks to see if the tracker and therefore your authentication is up to date.

If you did the same with SSO, it would look something like this:

1. As a user, you are taken to a periodically appearing page (the SSO portal) on website.com that checks to see if you are already logged in. If so, you are taken to what you really want - your Gmail inbox, for example.

2. If you're not already signed in, website.com offers options to authenticate through a third-party identity provider (Google, Amazon, Facebook, etc.). You choose your provider and then sign in with that provider, say Google.

3. Google verifies that you are, checks that website.com is the site it claims to be, then authenticates you against Google's password database and issues a token back to website.com.

4. Website.com receives a token from Google that verifies your identity. Now it links you to the rest of your user data - preferences, history, shopping cart, etc. - and you're done.

In a true single sign-on system, you will simply move from site to site with full access.
In a delegated system, Google returns both proof of identity and a set of permitted uses. Website.com may be granted access to your name and email, for example, but will not reveal your location or age. (See example below.)


Example redirect flow when using SSO from Auth0 (Source)

[yadaysrdone]

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