# Authentication Strategy

## Decision Tree: Authentication Strategy

```
How should we implement user authentication?
  ├─ JWT (JSON Web Tokens) [weight: 8] [priority: high] [risk: low] [recommended]
  │   ├─ Token Storage
  │   │   ├─ HTTP-only Cookies → Most secure, prevents XSS
  │   │   ├─ LocalStorage → Easy but vulnerable to XSS
  │   │   └─ SessionStorage → Better than localStorage, cleared on tab close
  │   ├─ Token Strategy
  │   │   ├─ Access + Refresh Tokens [recommended] → Industry standard, best security
  │   │   │   ├─ Refresh token rotation → Maximum security
  │   │   │   └─ Fixed refresh tokens → Simpler implementation
  │   │   └─ Single Access Token → Simple but less secure
  │   └─ Implementation
  │       ├─ Use jsonwebtoken library → Most popular, well-maintained
  │       ├─ Custom implementation → Full control but risky
  │       └─ Auth0/Firebase → Managed solution, quick setup
  │
  ├─ OAuth 2.0 / OpenID Connect [weight: 7] [priority: medium] [risk: medium]
  │   ├─ Provider Choice
  │   │   ├─ Social Providers (Google, GitHub, Facebook) → Quick user onboarding
  │   │   ├─ Enterprise SSO (SAML, Azure AD) → Corporate environments
  │   │   └─ Custom OAuth Server → Full control, complex setup
  │   ├─ Implementation
  │   │   ├─ Passport.js → Node.js standard, many strategies
  │   │   ├─ NextAuth.js → Modern, great for Next.js apps
  │   │   └─ Custom OAuth client → Maximum flexibility
  │   └─ Choose if: Third-party authentication needed
  │
  ├─ Session-based Authentication [weight: 5] [priority: low] [risk: low]
  │   ├─ Session Storage
  │   │   ├─ Server Memory → Simple but not scalable
  │   │   ├─ Redis → Fast, scalable, good for distributed systems
  │   │   └─ Database → Persistent but slower
  │   ├─ Cookie Configuration
  │   │   ├─ Secure + HTTP-only + SameSite [recommended] → Maximum security
  │   │   └─ Basic cookies → Less secure, simpler
  │   └─ Choose if: Traditional web app with server-side rendering
  │
  ├─ API Keys [weight: 4] [priority: low] [risk: high]
  │   ├─ Key Types
  │   │   ├─ Single permanent key → Simple but risky
  │   │   ├─ Rotating keys → Better security
  │   │   └─ Scoped keys → Different permissions per key
  │   ├─ Storage
  │   │   ├─ Environment variables [recommended] → Secure server-side
  │   │   └─ Database with encryption → Centralized management
  │   └─ Choose if: Service-to-service authentication only
  │
  └─ Passwordless Authentication [weight: 6] [priority: medium] [risk: medium]
      ├─ Magic Links (Email) → User-friendly, no password management
      ├─ SMS OTP → Fast but costs money
      ├─ Authenticator Apps (TOTP) → Most secure, offline capable
      └─ Choose if: Modern UX, reduce password-related support
```

## Context

This decision tree helps you choose the right authentication strategy for your application based on:
- Security requirements
- User experience expectations
- Technical constraints
- Team expertise
- Budget and resources

## Recommendations

**JWT is recommended** for most modern web applications because:
- Stateless authentication (scales horizontally)
- Works well with microservices and APIs
- Mobile-friendly
- Industry standard with good library support

**Use OAuth/OpenID Connect** if:
- You want social login
- Enterprise SSO is required
- You want to delegate authentication to experts

**Use Session-based** if:
- You have a traditional server-rendered app
- You prefer server-side security control
- Your team is familiar with sessions

**Avoid API Keys** for user authentication - they're better suited for machine-to-machine communication.

## Implementation Checklist

After making your decision:

- [ ] Set up secure token/session storage
- [ ] Implement password hashing (bcrypt/argon2)
- [ ] Add rate limiting to prevent brute force
- [ ] Set up HTTPS in production
- [ ] Implement password reset flow
- [ ] Add account lockout after failed attempts
- [ ] Configure CORS properly
- [ ] Add logging for security events
- [ ] Implement MFA (optional but recommended)
- [ ] Document authentication flow for team