How Does SSO Work? Your Questions Answered

How Does SSO Work?

SSO functions through a trust relationship between service providers (SP) and an identity provider (IdP). The authentication process follows these steps:

Step 1: User Initiates Login

The user tries to access a website or application that requires authentication.

Step 2: Redirection to Identity Provider (IdP)

The application redirects the user to an Identity Provider (IdP) for authentication.
The IdP is a centralized system that verifies the user’s identity (e.g., Google, Microsoft Azure AD, Okta).

Step 3: User Authenticates

The user enters login credentials (username/password, biometrics, or multi-factor authentication).
The IdP verifies the credentials against its database.

Step 4: Token Generation

Upon successful authentication, the IdP generates a secure authentication token (e.g., SAML, OAuth, OpenID Connect).
This token contains user identity information and is digitally signed to prevent tampering.

Step 5: Token Sent to Service Provider (SP)

The IdP sends the token back to the original application (SP).
The application validates the token to ensure it is genuine and hasn’t been altered.

Step 6: Access Granted

The user is granted access to the requested application without needing to enter login details again.

Step 7: Seamless Access to Other Connected Applications

Since the user is already authenticated with the IdP, they can access other connected applications without logging in again.

SSO Protocols & Standards

SSO is implemented using various authentication protocols that define how user authentication is performed and how credentials are shared between applications.

A. SAML (Security Assertion Markup Language)

Uses XML-based authentication tokens.
Commonly used for enterprise applications and cloud-based services (e.g., Salesforce, Microsoft 365).
Provides secure authentication via encrypted SAML assertions.

B. OAuth 2.0

Used for API and mobile authentication.
Instead of credentials, it uses access tokens.
Allows third-party apps to request limited access to user data (e.g., signing into a website using Google or Facebook).

C. OpenID Connect (OIDC)

An identity layer built on OAuth 2.0.
Provides authentication and user identity information.
Used for Single Sign-On with apps like Google Sign-In.

Key Concepts of SSO

Think of SSO like this: Imagine you have a master key that unlocks many different doors. That master key is your single login, and the doors are all the different apps and websites you use.

Here’s how it works:

The Master Key Holder (Identity Provider or IdP):

This is the place that verifies who you are. It’s like the security guard at the front desk who checks your ID. They’re in charge of knowing who’s allowed in.
Examples: Think of companies like Okta, Microsoft (with Azure AD), Google (with Google Workspace), or Auth0. They’re specialists in managing logins.

The Doors (Service Provider or SP):

These are the apps or websites you want to use. They don’t want to check your ID themselves; they trust the security guard (the IdP) to do it.

Examples: This could be anything from your company’s Salesforce system, your team’s Slack workspace, or even a custom website your company made.

The Agreement (Trust Relationship):

The security guard (IdP) and the door owners (SP) have a prior agreement. They’ve decided on a secret code (using protocols like SAML, OAuth, or OpenID Connect) that they’ll use to communicate.
Essentially, the doors trust the master key holders validation of who you are.
This agreement means the doors will believe the security guard when they say, “Yes, this person is allowed in.”

Example Workflow: Seamless Access with Single Sign-On

Initial Access Attempt: A user opens their web browser and attempts to access “Application A,” a service that requires authentication.
Redirection to the Identity Provider (IdP): Application A recognizes that the user is not currently authenticated. Instead of presenting a login form itself, it redirects the user’s browser to the designated Identity Provider (IdP), in this case, “Okta.” This redirection includes information that tells Okta where to send the user back after authentication.
User Authentication at the IdP: The user is presented with Okta’s login interface. They enter their credentials (username and password, potentially with Multi-Factor Authentication). Okta verifies these credentials against its user database.
Token Issuance and Delivery: Upon successful authentication, Okta generates a secure authentication token (e.g., a SAML assertion or a JWT). This token acts as a digital “pass” confirming the user’s identity. Okta then redirects the user’s browser back to Application A, embedding the authentication token within the redirect.
Application A Token Validation and Access Grant: Application A receives the token from Okta. It validates the token’s digital signature and verifies its validity. If the token is valid, Application A extracts the user’s identity information and grants the user access to the requested resources. Application A may also establish a local session to avoid repeated IdP checks until that session expires.
Subsequent Access to Application B: Later, the same user attempts to access “Application B,” another service that also utilizes Okta as its IdP.
IdP Session Recognition and Seamless Access: Application B, like Application A, redirects the user to Okta for authentication. However, Okta recognizes that the user already has an active session from the previous login to Application A. Therefore, Okta bypasses the login prompt and immediately sends an authentication token to Application B.
Application B Token Validation and Access Grant: Application B validates the token from Okta and grants the user immediate access, without requiring the user to re-enter their credentials.

Differences Between SSO & Related Authentication Methods

Feature	SSO	Password-Based Authentication	Multi-Factor Authentication (MFA)
Number of Logins Required	One login for all apps	Separate login for each app	One login but requires extra authentication steps
Security Level	High (if MFA is used)	Lower	Very High
User Convenience	Very High	Low	Moderate
Risk of Phishing	Lower	High	Lower (if phishing-resistant MFA is used)

Benefits of SSO

Improved User Experience: Users only need to remember one set of credentials.
Enhanced Security: Reduces the risk of weak or reused passwords.
Centralized Management: Admins can manage user access from a single system.
Reduced IT Costs: Fewer password reset requests and streamlined access management.

Challenges & Considerations of SSO

A. Single Point of Failure (SPoF)

If the SSO provider is down, users may be locked out of all connected applications.
Solution: Use backup authentication methods or federated authentication.

B. Security Risks

If an attacker gains access to an SSO account, they can access all linked applications.
Solution: Enforce strong authentication (e.g., Multi-Factor Authentication).