Below is a practical, end-to-end guide to designing and implementing single sign‑on (SSO) across multiple applications. I’ll cover architecture options, recommended protocols, key components, typical flows, security considerations, deployment checklist, and suggested tools/implementations.

High-level goal

• Users authenticate once with a trusted identity provider (IdP) and then access multiple applications (service providers / relying parties) without re‑entering credentials, while preserving strong security, centralized control, and good user experience.

Choose the right protocol (modern recommendation)

• OpenID Connect (OIDC) on top of OAuth 2.0 — best for modern web and mobile apps (REST/APIs, JSON tokens). Supports federated SSO, tokens, refresh, logout, and modern security practices.
• SAML 2.0 — widely used in enterprise and legacy web SSO (XML-based). Good for many enterprise SaaS integrations.
• Kerberos — used for internal network SSO (Windows/Active Directory environments).
• CAS, WS-Fed, or proprietary systems — less common; use when required.

Recommended architecture (central IdP / federated)

• Central Identity Provider (IdP): Auth server that authenticates users and issues identity tokens / assertions (e.g., OIDC ID token, OAuth access token, SAML assertion).
• Service Providers (SPs) / Relying Parties (RPs): Applications that rely on the IdP for authentication and accept tokens/assertions.
• Optionally: Identity federation (connect external IdPs), user directory (LDAP/Active Directory), user provisioning (SCIM), and API Gateway for token validation.

Core components and responsibilities

• IdP (Auth server): login UI, MFA, session management, token issuance, logout, audit logs, management UI for apps/clients.
• Client (Application): handles redirect to IdP, receives token/assertion, validates tokens, enforces authorization.
• Token validation layer: JWT signature verification, token introspection (for opaque tokens), or SAML signature/conditions check.
• User directory and profile store: source of truth (LDAP, AD, database). Optionally synchronized via SCIM.
• Consent/Authorization server (if needed): manage scopes/consent and fine-grained permissions.
• Audit & monitoring: login attempts, token issuance, refresh, failed logins, suspicious activity.

Typical SSO flows (OIDC examples)

1. Web app (authorization code flow with PKCE for public clients):
   • User accesses App A → App A redirects to IdP authorization endpoint with client_id, redirect_uri, scope=openid profile, state, nonce, (code_challenge if PKCE).
   • User authenticates at IdP (password, SSO, MFA).
   • IdP redirects back to App A with authorization code and state.
   • App A exchanges code for tokens (ID token, access token, refresh token) on token endpoint (server-to-server).
   • App A creates application session (e.g., cookie) and grants access.
2. When user opens App B:
   • App B redirects to IdP; IdP sees active IdP session and immediately returns code/token → user SSO without re-entering credentials.
3. Token refresh:
   • Use refresh tokens (if allowed) or silent renew via iframe / refresh grant to maintain session without re-login.
4. Logout:
   • Single Logout (SLO) — call IdP logout endpoint and optionally trigger RP logout endpoints (front-channel/back-channel) to clear sessions across apps.

Key implementation steps

1. Design identity model:

   • Decide primary identifier (email, username, sub/UUID).
   • Map attributes/claims required by apps (roles, groups, tenant id).
   • Plan account linking if users exist in multiple identity stores.

2. Select or deploy an IdP:

   • Options: managed IdP (Okta, Auth0, Azure AD, Google Identity), self-hosted (Keycloak, FusionAuth, Gluu), or built-in corporate IdP (AD FS).
   • Confirm support for OIDC and SAML for compatibility needs.

3. Register applications (clients) with the IdP:

   • For each app define client_id, client_secret (or use PKCE for public clients), redirect URIs, allowed scopes, grant types, and token lifetimes.

4. Implement client (app) integration:

   • Use OIDC/OAuth client libraries for your platform (Node, Java, .NET, Python, Ruby, iOS, Android).
   • Implement proper state and nonce handling to prevent CSRF and replay attacks.
   • Validate ID token signature, issuer (iss), audience (aud), nonce, and expiry (exp).
   • Validate access tokens as needed (JWT signature or token introspection).

5. Session & cookie strategy:

   • Use short-lived tokens for access; use refresh tokens or silent re-auth for user sessions.
   • For web apps, maintain local session cookie and map token expiry to cookie.
   • Protect cookies with Secure, HttpOnly, SameSite attributes.

6. Single Logout (SLO) and session synchronization:

   • Implement logout at IdP and configure apps to use OIDC logout endpoints.
   • For multi-tab/single sign out, use front-channel mechanisms (post-logout redirect) or back-channel notifications if supported.

7. Federation & external IdPs:

   • If you must support login via enterprise IdPs, configure IdP as a federation gateway to accept SAML or OIDC IdPs and map external attributes.

8. Provisioning & lifecycle:

   • Implement SCIM or IdP-provisioning for automating user creation/update/termination across apps.
   • Ensure deprovisioning flows immediately revoke access (invalidate sessions/tokens).

9. MFA & adaptive authentication:

   • Enforce MFA at IdP level; optionally require based on risk, IP, device, or app sensitivity.

10. Auditing, logging, and monitoring:

• Centralize authentication logs, token issuance, failed attempts, and admin changes.
• Monitor suspicious patterns and integrate with SIEM.

Security best practices

• Use HTTPS/TLS everywhere.
• Use OIDC Authorization Code Flow with PKCE for public clients; confidential clients use secure server-side code exchanges.
• Keep access tokens short-lived (minutes to hours). Use refresh tokens carefully and rotate them.
• Validate token signatures and claims strictly (iss, aud, exp, nbf, nonce).
• Implement token revocation and rotation.
• Enforce strong MFA for sensitive apps and admin consoles.
• Limit scope and privileges in tokens (least privilege).
• Protect client secrets; prefer asymmetric client authentication where supported.
• Harden the IdP (rate limiting, account lockouts, monitoring).
• For SAML, validate XML signatures and replay conditions.

Operational concerns

• High availability and scaling for IdP; failure of IdP impacts all apps.
• Caching of validation keys (e.g., JWKS) with rotate/refresh logic.
• Backwards compatibility when rotating signing keys (support key rollover).
• Plan maintenance windows and how apps will behave if IdP is down.
• Provide fallback auth or emergency access for admins.

Logout and session termination details

• End-user flow: redirect to IdP logout endpoint; IdP clears IdP session and optionally redirects to post-logout URI.
• App logout: clear local session and optionally call IdP end session endpoint with id_token_hint and post_logout_redirect_uri.
• For complete SLO: use front-channel (redirects) or back-channel notifications; many deployments rely on clearing IdP session + short app sessions for simplicity.

Testing and rollout strategy

• Start with a staging IdP and integrate a single low-risk app.
• Test authentication, token validation, refresh, logout, MFA, and error cases.
• Test interoperability with SAML apps if needed.
• Gradually onboard additional apps, and then enable federated login if required.
• Provide user documentation and support for first sign-in flows and device trust.

Common pitfalls

• Not validating tokens properly (skipping signature or claim checks).
• Long-lived access tokens without refresh or revocation.
• No central provisioning/deprovisioning — orphaned accounts remain.
• Assuming IdP downtime is harmless — design HA and fallback access.
• Overly broad scopes in tokens (exposing unnecessary data).

Suggested tools & libraries (examples)

• Self-hosted IdPs: Keycloak, FusionAuth, Gluu.
• Managed: Okta, Auth0, Azure AD, Google Workspace Identity.
• Client libraries: oidc-client-js (web SPA), AppAuth (iOS/Android), Microsoft.Identity.Web (.NET), passport-openidconnect / openid-client (Node), Authlib (Python).
• Provisioning: SCIM servers/clients for automated user lifecycle.
• SSO gateways: API Gateway or reverse proxy that validates tokens for microservices.

Minimal example flow for a web app using OIDC Authorization Code:

1. User → App redirects: GET /authorize?response_type=code&client_id=APP&redirect_uri=https://app/cb&scope=openid%20profile&state=xyz&nonce=abc&code_challenge=...
2. User authenticates at IdP (MFA if required).
3. IdP → redirect to app with code: https://app/cb?code=AUTHCODE&state=xyz
4. App posts to token endpoint with code (and code_verifier) for tokens.
5. App validates ID token (signature, iss, aud, nonce, exp) and creates session.

Checklist before go-live

• IdP chosen and HA planned.
• All apps registered and client metadata recorded.
• Token lifetimes and refresh policy defined.
• MFA and password policies configured.
• SCIM provisioning/integration tested (if used).
• Logging, monitoring, and alerting in place.
• Security review and pen test for auth flows.
• Rollback/maintenance plan for IdP outages.
• User-facing documentation and support ready.

If you want, I can:

• Recommend a specific stack based on your tech stack (languages, web vs mobile, on‑prem vs cloud).
• Provide example code snippets for a particular platform (Node, Java, .NET, Python, iOS/Android).
• Draft a minimal config for Keycloak/Okta/Auth0 and a sample app flow.

Which platform or IdP are you using (or considering)? I can give concrete sample configs and code.