Design an Identity Management System (OAuth/SSO)

To design a robust SSO service, I would start by defining the core actors: the User, the Client Application, and the Identity Provider (IdP). For a scenario involving public clients like SPAs, I'd implement the Authorization Code Flow with PKCE to prevent authorization code interception. The IdP would handle authentication and issue short-lived Access Tokens via JWT and Refresh Tokens stored securely in HttpOnly cookies. Regarding token management, Access Tokens should have an expiration of 15 minutes to limit exposure, while Refresh Tokens rotate upon use to detect theft. We must validate signatures using asymmetric keys, allowing resource servers to verify tokens without calling the IdP, ensuring scalability. For OpenID Connect, we add the ID Token to confirm user identity. Security is paramount. We must enforce HTTPS everywhere, implement strict CORS policies, and protect against CSRF using same-site cookies or anti-CSRF tokens. At Microsoft, where enterprise security is non-negotiable, we would also integrate Conditional Access policies to block logins from suspicious locations or devices. Finally, the system should be stateless regarding session data, relying on database sharding for high availability across global regions.