What are you building?
│
├─ Traditional web application (server-rendered)?
│  └─ Session-based authentication
│     ├─ Server stores session data (Redis/DB)
│     ├─ Session ID in httpOnly cookie
│     └─ Best for: monoliths, SSR apps, admin panels
│
├─ API consumed by multiple clients?
│  └─ JWT (JSON Web Tokens)
│     ├─ Stateless, self-contained tokens
│     ├─ Access token (short-lived) + refresh token (long-lived)
│     └─ Best for: microservices, mobile apps, SPAs via BFF
│
├─ Service-to-service communication?
│  └─ API keys or Client Credentials (OAuth2)
│     ├─ API keys: simple, scoped, rotatable
│     ├─ Client Credentials: OAuth2 standard, token-based
│     └─ Best for: internal services, third-party integrations
│
├─ Third-party login (Google, GitHub, etc.)?
│  └─ OAuth2 / OpenID Connect
│     ├─ Authorization Code + PKCE for web/mobile
│     ├─ Delegate identity to trusted providers
│     └─ Best for: consumer apps, social login
│
└─ Passwordless authentication?
   └─ Passkeys (WebAuthn) or Magic Links
      ├─ Passkeys: phishing-resistant, biometric/hardware
      ├─ Magic links: email-based, time-limited
      └─ Best for: high-security, modern UX

Header.Payload.Signature

Header:  { "alg": "RS256", "typ": "JWT" }
Payload: { "iss": "auth.example.com", "sub": "user_123", ... }
Signature: RSASHA256(base64(header) + "." + base64(payload), privateKey)

┌──────────┐                    ┌──────────┐
│  Client   │─── login ────────>│  Auth    │
│           │<── access (15m) ──│  Server  │
│           │<── refresh (7d) ──│          │
│           │                   └──────────┘
│           │─── API call ─────>┌──────────┐
│           │    (access token) │ Resource │
│           │<── response ──────│  Server  │
│           │                   └──────────┘
│           │─── access expired │          │
│           │─── refresh ──────>│  Auth    │
│           │<── new access ────│  Server  │
│           │<── new refresh ───│  (rotate)│
└──────────┘                    └──────────┘

What type of client?
│
├─ Web app with backend (Next.js, Rails, Django)?
│  └─ Authorization Code + PKCE
│     ├─ Redirect user to authorization server
│     ├─ Receive code at callback URL
│     ├─ Exchange code for tokens server-side
│     └─ PKCE prevents code interception attacks
│
├─ SPA (React, Vue) without backend?
│  └─ Authorization Code + PKCE (via BFF)
│     ├─ Use a Backend-for-Frontend to handle tokens
│     ├─ Never store tokens in browser-accessible storage
│     └─ BFF proxies API calls with token attached
│
├─ Mobile app (iOS, Android)?
│  └─ Authorization Code + PKCE
│     ├─ Use custom URI scheme or universal links for redirect
│     ├─ PKCE is mandatory (public client)
│     └─ Store tokens in secure enclave/keystore
│
├─ Server-to-server (no user)?
│  └─ Client Credentials
│     ├─ Authenticate with client_id + client_secret
│     ├─ No user context, service-level access
│     └─ Token cached until expiry
│
├─ CLI tool or smart TV?
│  └─ Device Code
│     ├─ Display code and URL to user
│     ├─ User authenticates on another device
│     ├─ CLI/TV polls for completion
│     └─ Good UX for input-constrained devices
│
└─ Microservice acting on behalf of a user?
   └─ Token Exchange (RFC 8693)
      ├─ Exchange user's token for a scoped downstream token
      ├─ Maintains user context across services
      └─ Use `act` claim for delegation chain

How complex are your access control needs?
│
├─ Simple: just "can user X do action Y"?
│  └─ Permission-based (direct)
│     ├─ user_permissions table
│     ├─ Simple to implement, hard to scale
│     └─ Good for: small apps, prototypes
│
├─ Users grouped into roles with fixed permissions?
│  └─ RBAC (Role-Based Access Control)
│     ├─ Roles: admin, editor, viewer
│     ├─ Each role has a set of permissions
│     ├─ Users assigned one or more roles
│     └─ Good for: most apps, admin panels, team tools
│
├─ Decisions depend on attributes (time, location, resource owner)?
│  └─ ABAC (Attribute-Based Access Control)
│     ├─ Policies evaluate subject + resource + environment attributes
│     ├─ "Allow if user.department == resource.department AND time < 17:00"
│     ├─ Flexible but complex
│     └─ Good for: enterprise, compliance-heavy, context-dependent access
│
└─ Access based on relationships (owner, parent, shared with)?
   └─ ReBAC (Relationship-Based Access Control)
      ├─ Google Zanzibar model
      ├─ Tuples: user:alice#viewer@document:report
      ├─ Supports inheritance: folder viewer → document viewer
      ├─ Tools: OpenFGA, SpiceDB, Ory Keto
      └─ Good for: file sharing, nested resources, social features

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