auth-security

# Auth Security

## Core Principles

- **OAuth 2.1** — Follow RFC 9700 (January 2025)
- **PKCE Required** — All clients must use PKCE
- **Short-lived Tokens** — Access tokens expire in 5-15 minutes
- **Token Rotation** — Refresh tokens are single-use
- **HttpOnly Storage** — Browser tokens in HttpOnly cookies
- **Explicit Algorithm** — Never trust JWT header algorithm
- **No backwards compatibility** — Delete deprecated auth flows

---

## OAuth 2.1 Key Changes

### Deprecated Flows (DO NOT USE)

| Flow | Status | Replacement |
|------|--------|-------------|
| Implicit Grant | Removed | Authorization Code + PKCE |
| Password Grant | Removed | Authorization Code + PKCE |
| Auth Code without PKCE | Removed | Must use PKCE |

### Required: Authorization Code + PKCE

```typescript
import crypto from 'crypto';

// 1. Generate code verifier (43-128 chars)
function generateCodeVerifier(): string {
  return crypto.randomBytes(32).toString('base64url');
}

// 2. Generate code challenge
function generateCodeChallenge(verifier: string): string {
  return crypto
    .createHash('sha256')
    .update(verifier)
    .digest('base64url');
}

// 3. Authorization request
const verifier = generateCodeVerifier();
const challenge = generateCodeChallenge(verifier);

const authUrl = new URL('https://auth.example.com/authorize');
authUrl.searchParams.set('response_type', 'code');
authUrl.searchParams.set('client_id', CLIENT_ID);
authUrl.searchParams.set('redirect_uri', REDIRECT_URI);
authUrl.searchParams.set('code_challenge', challenge);
authUrl.searchParams.set('code_challenge_method', 'S256');
authUrl.searchParams.set('scope', 'openid profile email');
authUrl.searchParams.set('state', generateState());

// 4. Token exchange (after redirect)
const tokenResponse = await fetch('https://auth.example.com/token', {
  method: 'POST',
  headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
  body: new URLSearchParams({
    grant_type: 'authorization_code',
    code: authorizationCode,
    redirect_uri: REDIRECT_URI,
    client_id: CLIENT_ID,
    code_verifier: verifier, // Prove we initiated the request
  }),
});
```

---

## JWT Best Practices

### Algorithm Selection (2025)

| Priority | Algorithm | Notes |
|----------|-----------|-------|
| 1 | EdDSA (Ed25519) | Most secure, quantum-resistant properties |
| 2 | ES256 (ECDSA P-256) | Widely supported, compact signatures |
| 3 | PS256 (RSA-PSS) | More secure than RS256 |
| 4 | RS256 (RSA PKCS#1) | Best compatibility |

```typescript
// Recommended: ES256
import { SignJWT, jwtVerify } from 'jose';

const privateKey = await importPKCS8(PRIVATE_KEY_PEM, 'ES256');
const publicKey = await importSPKI(PUBLIC_KEY_PEM, 'ES256');

// Sign
const token = await new SignJWT({ sub: userId, scope: 'read write' })
  .setProtectedHeader({ alg: 'ES256', typ: 'JWT', kid: keyId })
  .setIssuer('https://auth.example.com')
  .setAudience('https://api.example.com')
  .setExpirationTime('15m')
  .setIssuedAt()
  .setJti(crypto.randomUUID())
  .sign(privateKey);
```

### Token Structure

```typescript
interface AccessTokenPayload {
  // Standard claims
  iss: string;  // Issuer
  sub: string;  // Subject (user ID)
  aud: string;  // Audience
  exp: number;  // Expiration (Unix timestamp)
  iat: number;  // Issued at
  jti: string;  // JWT ID (unique identifier)

  // Custom claims
  scope: string;      // Permissions
  email?: string;     // User email
  roles?: string[];   // User roles
}
```

### Verification (Critical)

```typescript
import { jwtVerify, errors } from 'jose';

async function verifyAccessToken(token: string): Promise<AccessTokenPayload> {
  try {
    const { payload } = await jwtVerify(token, publicKey, {
      // CRITICAL: Explicitly specify allowed algorithms
      algorithms: ['ES256'],

      // Validate standard claims
      issuer: 'https://auth.example.com',
      audience: 'https://api.example.com',

      // Clock tolerance for sync issues
      clockTolerance: 30,
    });

    // Additional validation
    if (!payload.scope?.includes('read')) {
      throw new Error('Insufficient scope');
    }

    return payload as AccessTokenPayload;
  } catch (err) {
    if (err instanceof errors.JWTExpired) {
      throw new AuthError('Token expired', 'TOKEN_EXPIRED');
    }
    if (err instanceof errors.JWTClaimValidationFailed) {
      throw new AuthError('Invalid token claims', 'INVALID_CLAIMS');
    }
    throw new AuthError('Invalid token', 'INVALID_TOKEN');
  }
}
```

---

## Token Storage

### Web Applications

```typescript
// Set token in HttpOnly cookie (server-side)
function setAuthCookie(res: Response, token: string) {
  res.cookie('access_token', token, {
    httpOnly: true,     // Not accessible via JavaScript
    secure: true,       // HTTPS only
    sameSite: 'strict', // CSRF protection
    maxAge: 15 * 60 * 1000, // 15 minutes
    path: '/api',       // Only sent to API routes
  });
}

// Refresh token (longer-lived)
function setRefreshCookie(res: Response, token: string) {
  res.cookie('refresh_token', token, {
    httpOnly: true,
    secure: true,
    sameSite: 'strict',
    maxAge: 7 * 24 * 60 * 60 * 1000, // 7 days
    path: '/api/auth/refresh',  // Only for refresh endpoint
  });
}
```

### Single Page Applications (SPA)

```typescript
// Store in memory (NOT localStorage/sessionStorage)
class TokenManager {
  private accessToken: string | null = null;

  setToken(token: string) {
    this.accessToken = token;
  }

  getToken(): string | null {
    return this.accessToken;
  }

  clearToken() {
    this.accessToken = null;
  }
}

// Use with Refresh Token Rotation
// Refresh token in HttpOnly cookie
// Access token in memory
```

### Storage Comparison

| Storage | XSS Safe | CSRF Safe | Persistence |
|---------|----------|-----------|-------------|
| HttpOnly Cookie | Yes | Needs SameSite | Yes |
| Memory | Yes | Yes | No (lost on reload) |
| localStorage | No | Yes | Yes |
| sessionStorage | No | Yes | Tab only |

---

## Refresh Token Ro