How single sign-on works
Tags: sso, auth, ciam
Published: 2023-03-07 17:21:10

---
title: How single sign-on works
published: true
date: 2022-09-08 00:00:00 UTC
tags: sso, auth, ciam
canonical_url: https://fusionauth.io/blog/2022/09/08/how-sso-works
---

Single sign-on (SSO) is a key part of any customer identity and access management (CIAM) strategy. Why? Because your organization will almost always have more than one application for your customers. Even if you begin with only one custom application, SaaS tools such as support forums, a ticketing system, or a chat system require authentication, and you won’t want your users to have to log in to more systems than necessary.


Integrating your applications with single sign-on means that your users can access all their applications with a single set of credentials such as a username and password.

 

Let’s say you have two applications for which you want to enable single sign-on.

 
- The Pied Piper Video Chat application (PPVC)
- The Hooli Jobs application (they’re always hiring)
 

You want to let users log in to the PPVC, and then, if they visit Hooli to apply for a job, have them be automatically signed in to the Hooli Jobs application. This is single sign-on in a nutshell.

 

This pattern scales to any number of applications, and can include commercial off the shelf apps that support OIDC (OpenID Connect) or SAML (Security Assertion Markup Language). If you have a suite of applications, you can provide a seamless single sign-on experience for all your users.

 

But how does it actually work? This blog post will dig into that, but first, let’s take a step back and talk about sessions.

 
## Sessions
 

Sessions are how servers know they’ve seen a client before. They are usually implemented with cookies if the application is browser based. In the SSO scenario, the following sessions exist:

 
- FusionAuth’s session, also known as the single sign-on session
- The PPVC application’s session
- The Hooli Jobs application’s session
 

If a session doesn’t exist for a given application, or it isn’t valid, then it must be created or updated after the user has presented valid credentials and authenticated. For FusionAuth, the credentials are often a username and password. They could also be a passwordless flow or a login federated by a social provider like Google or a business directory like Azure AD.

 

For the other applications in this example, the credential, which proves authentication, is a valid FusionAuth token.

 
## Single sign-on request flow
 

Here’s the flow of an OIDC single sign-on login request. The home page of each application is unavailable to anonymous users. When a browser requests it, they are directed to the identity provider, FusionAuth, to authenticate.

 

![Single sign-on request flow during login.](https://fusionauth.io/assets/img/diagrams/blogs/sso/sso-login.svg) 
_Single sign-on request flow during login._

 

You have different sessions managed by different applications, which can have varying lifetimes and storage mechanisms.

 

(If you are a stickler, you’ll notice there is no token request from the apps to FusionAuth after the user authenticates. That is implied and required.)

 

The PPVC and Hooli Jobs applications delegate authentication to the single sign-on provider, and build their sessions on the foundation of the identity provider’s session. That session is, in turn, tied to a user authentication event.

 

That’s it. All that is required for single sign-on to work is:

 
- browsers that respect redirects
- a single source of truth for user authentication data (often called an identity provider)
- a defined protocol for an application to bounce an unauthenticated user to the source of truth
- a defined protocol for the authentication source of truth to bounce requests back to the application which needed the user to be authenticated
- sessions for every application (typically managed with cookies)
 

Let’s look at another possible implementation.

 
## SAML SSO
 

SAML is an alternative to OIDC which also allows single sign-on. It is older, more complex, but well supported across a variety of applications, especially business focused ones.

 

SAML, in contrast to the token flow of OIDC, passes around signed and/or encrypted XML documents.

 

Let’s look at a different scenario. Say you have two other applications: Zendesk and Gusto. You want to use SAML to integrate these applications with your identity provider.

 

Here’s a diagram for a SAML flow. Technically this is a service provider (SP) initiated flow where the user is trying to access an application, such as Zendesk, before they are logged in.

 

![SAML single sign-on request flow during login.](https://fusionauth.io/assets/img/diagrams/blogs/sso/saml-sso-login.svg) 
_SAML single sign-on request flow during login._

 

Why would you pick SAML over the OIDC flow mentioned above? Widespread support.

 

SAML has been around since 2005 and many many different commercial off the shelf and open source applications support it.

 

When evaluating your identity provider solution, think about what kinds of applications you want to support. Any business focused applications will typically support SAML, whereas support for OIDC is far easier to implement in your custom applications.

 

For maximum flexibility, pick an identity provider which supports both. (FusionAuth does.)

 
## Beyond the browser
 

Both SAML and OIDC are browser based. They expect functionality such as HTTP redirects, cookies and the ability to interact using URLs.

 

There are other single sign-on protocols useful for other kinds of applications. Kerberos is a common one for client server applications.

 
## Conclusion
 

Single sign-on is a key part of your application’s authentication story. It doesn’t have to be difficult, as leveraging standards like OIDC and SAML can help integrate both custom and commercial off the shelf applications with an identity provider like FusionAuth.