Auth Patterns

Core Philosophy

1. Authentication is not authorization — Solve them separately. Authentication proves identity; authorization enforces what that identity may do.
2. Default deny — All resources are denied unless explicitly permitted. Never default allow.
3. Shortest lifetime possible — Access tokens: 5-15 minutes. Sessions: idle + absolute timeout. Backup codes: one use. Shorter lifetime = smaller breach window.
4. Validate every input, every time — JWT signature, expiry, issuer, audience, nonce. Skipping one check is the vulnerability.
5. Store nothing sensitive client-side — Tokens in HttpOnly cookies, secrets in secret managers, TOTP seeds encrypted at rest.

Quick-Start: Common Problems

"Which auth approach should I use?"

App Type	Recommended
Server-rendered web app	Server-side sessions + HttpOnly cookie
SPA / mobile calling your own API	Auth Code + PKCE → short-lived JWT in HttpOnly cookie
Microservices (your API → your API)	Client Credentials → short-lived JWT
Third-party delegated access	Auth Code + PKCE with consent screen
"Sign in with Google/GitHub"	OIDC → Auth Code + PKCE
→ See references/oauth2-flows.md and references/jwt-internals.md

"My JWT implementation feels wrong"

1. Are you pinning the algorithm in code (not trusting the token header)? → Verify
2. Are you validating iss, aud, exp, and signature? → All four required
3. Where is the token stored? localStorage = wrong → move to HttpOnly cookie
4. Access token lifespan > 15 minutes? → Shorten it → See references/jwt-internals.md

"I need to add MFA"

1. Pick primary method: TOTP (practical baseline) or WebAuthn/passkeys (phishing-resistant)
2. Always generate backup codes at enrollment
3. Implement step-up auth for sensitive operations (password change, payments) → See references/mfa-implementation.md

"How do I model permissions?"

1. Start with RBAC — users → roles → permissions
2. Hitting role explosion? Move resource-specific access to ABAC
3. Sharing / collaboration model? Consider ReBAC (Zanzibar) → See references/rbac-abac.md

"Social login edge cases are biting me"

• Use sub (not email) as the stable user identifier per provider
• Always check email_verified: true before trusting email
• Validate nonce, iss, aud, and signature on ID tokens → See references/oidc-social-login.md

Decision Trees

Token vs Session

Signal	Use
Traditional server-rendered app	Server-side sessions
Immediate revocation required	Server-side sessions (or opaque tokens + introspection)
Multiple independent services verify token	JWT (asymmetric: RS256 or EdDSA)
Single service	JWT (symmetric: HS256) or sessions
"Stateless" is a hard requirement	JWT with short expiry + refresh rotation

JWT Signing Algorithm

Situation	Algorithm
Multiple services verify	RS256 or EdDSA (Ed25519)
Single service, simple	HS256 with 256-bit random secret
New system, modern stack	EdDSA (fastest, most secure)
Widest library compatibility needed	RS256
Never	none, MD5, SHA-1

Authorization Model

Signal	Model
Permissions map to job functions	RBAC
Access depends on resource/context attributes	ABAC
Sharing and ownership relationships drive access	ReBAC
Multi-tenant SaaS	RBAC + tenant-scoped namespacing
Role count > user count	Refactor to ABAC or hierarchy

MFA Method

Context	Method
Privileged accounts, enterprise	Hardware key (FIDO2 roaming)
Consumer apps, best UX	Passkeys (synced FIDO2)
Practical baseline anywhere	TOTP authenticator app
Absolute last resort only	SMS OTP

Reference Files

File	Contents
references/jwt-internals.md	JWT structure, signing algorithms (RS256/HS256/EdDSA), validation steps, attack vectors (none alg, alg confusion, kid injection), access/refresh token patterns, revocation strategies
references/oauth2-flows.md	All grant types (Auth Code, PKCE, Client Credentials, Device Code), token endpoint, scope design, refresh token rotation, deprecated implicit grant
references/session-management.md	Server-side session storage (Redis vs DB), cookie security attributes (HttpOnly, Secure, SameSite), session ID generation, session fixation, concurrent sessions, expiry strategies
references/rbac-abac.md	RBAC levels (0-3), role explosion prevention, ABAC vs RBAC selection, ReBAC/Zanzibar model, multi-tenant authorization, permission modeling, enforcement patterns
references/oidc-social-login.md	OIDC on OAuth2, ID token vs access token, nonce, standard scopes, provider patterns, user mapping, account linking, edge cases, JIT provisioning
references/mfa-implementation.md	TOTP algorithm and storage, WebAuthn/passkeys registration and authentication flows, SMS weaknesses, backup codes, step-up authentication, recovery flows
references/auth-security.md	Token storage (HttpOnly cookies vs localStorage), XSS defense (CSP, SRI), CSRF prevention (SameSite, synchronizer tokens), rate limiting, credential stuffing, secure headers, password hashing