Authentication & Authorization Implementation Patterns

Build secure, scalable authentication and authorization systems using industry-standard patterns and modern best practices.

When to Use This Skill

• Implementing user authentication systems
• Securing REST or GraphQL APIs
• Adding OAuth2/social login
• Implementing role-based access control (RBAC)
• Designing session management
• Migrating authentication systems
• Debugging auth issues
• Implementing SSO or multi-tenancy

Core Concepts

1. Authentication vs Authorization

Authentication (AuthN): Who are you?

• Verifying identity (username/password, OAuth, biometrics)
• Issuing credentials (sessions, tokens)
• Managing login/logout

Authorization (AuthZ): What can you do?

• Permission checking
• Role-based access control (RBAC)
• Resource ownership validation
• Policy enforcement

2. Authentication Strategies

Session-Based:

• Server stores session state
• Session ID in cookie
• Traditional, simple, stateful

Token-Based (JWT):

• Stateless, self-contained
• Scales horizontally
• Can store claims

OAuth2/OpenID Connect:

• Delegate authentication
• Social login (Google, GitHub)
• Enterprise SSO

JWT Authentication

Pattern 1: JWT Implementation

// JWT structure: header.payload.signature
import jwt from "jsonwebtoken";
import { Request, Response, NextFunction } from "express";

interface JWTPayload {
  userId: string;
  email: string;
  role: string;
  iat: number;
  exp: number;
}

// Generate JWT
function generateTokens(userId: string, email: string, role: string) {
  const accessToken = jwt.sign(
    { userId, email, role },
    process.env.JWT_SECRET!,
    { expiresIn: "15m" }, // Short-lived
  );

  const refreshToken = jwt.sign(
    { userId },
    process.env.JWT_REFRESH_SECRET!,
    { expiresIn: "7d" }, // Long-lived
  );

  return { accessToken, refreshToken };
}

// Verify JWT
function verifyToken(token: string): JWTPayload {
  try {
    return jwt.verify(token, process.env.JWT_SECRET!) as JWTPayload;
  } catch (error) {
    if (error instanceof jwt.TokenExpiredError) {
      throw new Error("Token expired");
    }
    if (error instanceof jwt.JsonWebTokenError) {
      throw new Error("Invalid token");
    }
    throw error;
  }
}

// Middleware
function authenticate(req: Request, res: Response, next: NextFunction) {
  const authHeader = req.headers.authorization;
  if (!authHeader?.startsWith("Bearer ")) {
    return res.status(401).json({ error: "No token provided" });
  }

  const token = authHeader.substring(7);
  try {
    const payload = verifyToken(token);
    req.user = payload; // Attach user to request
    next();
  } catch (error) {
    return res.status(401).json({ error: "Invalid token" });
  }
}

// Usage
app.get("/api/profile", authenticate, (req, res) => {
  res.json({ user: req.user });
});

Pattern 2: Refresh Token Flow

interface StoredRefreshToken {
  token: string;
  userId: string;
  expiresAt: Date;
  createdAt: Date;
}

class RefreshTokenService {
  // Store refresh token in database
  async storeRefreshToken(userId: string, refreshToken: string) {
    const expiresAt = new Date(Date.now() + 7 * 24 * 60 * 60 * 1000);
    await db.refreshTokens.create({
      token: await hash(refreshToken), // Hash before storing
      userId,
      expiresAt,
    });
  }

  // Refresh access token
  async refreshAccessToken(refreshToken: string) {
    // Verify refresh token
    let payload;
    try {
      payload = jwt.verify(refreshToken, process.env.JWT_REFRESH_SECRET!) as {
        userId: string;
      };
    } catch {
      throw new Error("Invalid refresh token");
    }

    // Check if token exists in database
    const storedToken = await db.refreshTokens.findOne({
      where: {
        token: await hash(refreshToken),
        userId: payload.userId,
        expiresAt: { $gt: new Date() },
      },
    });

    if (!storedToken) {
      throw new Error("Refresh token not found or expired");
    }

    // Get user
    const user