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Single Sign-On lets a user authenticate once with a central identity provider and then access many applications without logging in again. OpenID Connect is the modern foundation for web and mobile SSO.

The magic lives at the OpenID Provider (OP). When the user logs in, the OP establishes its own session (often a cookie). Each app relies on that central session rather than maintaining its own credentials.

App A redirects the user to the OP's /authorize endpoint. The user enters credentials, the OP sets its session cookie, and returns an authorization code to App A, which exchanges it for tokens.

Later the user opens App B, which also redirects to /authorize. Because the OP session cookie already exists, the OP recognizes the user and returns a code without prompting for credentials again. That is the SSO experience.

To check silently whether a session exists, apps can use prompt=none. The OP either returns a code immediately or an error like login_required if no session is present.

Conversely, sensitive operations can require a fresh login with prompt=login or a max_age constraint, overriding the SSO session for that request.

The OP itself may federate to upstream providers (corporate IdP, Google, social logins). To the apps it still looks like one OP, but the OP brokers authentication to the chosen source. This centralizes policy and simplifies clients.

SSO needs Single Logout too: when the user signs out of one app, related sessions should end. OIDC offers front-channel and back-channel logout to notify participating apps and clear the central session.

Balance convenience and security: short access tokens with refresh tokens for ongoing access, and an OP session lifetime that matches your risk tolerance. Long SSO sessions are convenient but widen the impact of a compromised device.

On mobile, SSO uses the system browser (ASWebAuthenticationSession / Custom Tabs) so the OP cookie is shared across apps. Embedded WebViews break SSO and are discouraged for security and usability reasons.

SSO improves UX, centralizes MFA and auditing, and reduces password fatigue. The trade-off: the OP becomes a high-value target, so it must be hardened, monitored, and protected with strong authentication.

Test your SSO understanding.

Single Sign-On centralizes authentication at the OpenID Provider.

• The OP session lets subsequent apps log in silently.
• prompt=none checks for a session; prompt=login/max_age force re-auth.
• Single Logout coordinates ending sessions across apps.
• Use the system browser, not embedded WebViews, for mobile SSO.