## Secure JWT Authentication Implementation

### Authentication Flow

1. User logs in with credentials
2. Server validates credentials
3. Server generates JWT token
4. Client stores token securely
5. Client sends token with each request
6. Server validates token

### Implementation

#### 1. Generate Secure JWT Tokens

\`\`\`javascript
// auth.js
const jwt = require('jsonwebtoken');
const bcrypt = require('bcrypt');

// Login endpoint
app.post('/api/auth/login', async (req, res) => {
  try {
    const { email, password } = req.body;
    
    // Validate input
    if (!email || !password) {
      return res.status(400).json({ 
        error: 'Email and password are required' 
      });
    }
    
    // Find user
    const user = await db.user.findUnique({ 
      where: { email } 
    });
    
    if (!user) {
      // Don't reveal if user exists
      return res.status(401).json({ 
        error: 'Invalid credentials' 
      });
    }
    
    // Verify password
    const validPassword = await bcrypt.compare(
      password, 
      user.passwordHash
    );
    
    if (!validPassword) {
      return res.status(401).json({ 
        error: 'Invalid credentials' 
      });
    }
    
    // Generate JWT token
    const token = jwt.sign(
      { 
        userId: user.id,
        email: user.email,
        role: user.role
      },
      process.env.JWT_SECRET,
      { 
        expiresIn: '1h',
        issuer: 'your-app',
        audience: 'your-app-users'
      }
    );
    
    // Generate refresh token
    const refreshToken = jwt.sign(
      { userId: user.id },
      process.env.JWT_REFRESH_SECRET,
      { expiresIn: '7d' }
    );
    
    // Store refresh token in database
    await db.refreshToken.create({
      data: {
        token: refreshToken,
        userId: user.id,
        expiresAt: new Date(Date.now() + 7 * 24 * 60 * 60 * 1000)
      }
    });
    
    res.json({
      token,
      refreshToken,
      expiresIn: 3600
    });
    
  } catch (error) {
    console.error('Login error:', error);
    res.status(500).json({ 
      error: 'An error occurred during login' 
    });
  }
});
\`\`\`

#### 2. Verify JWT Tokens (Middleware)

\`\`\`javascript
// middleware/auth.js
const jwt = require('jsonwebtoken');

function authenticateToken(req, res, next) {
  // Get token from header
  const authHeader = req.headers['authorization'];
  const token = authHeader && authHeader.split(' ')[1]; // Bearer TOKEN
  
  if (!token) {
    return res.status(401).json({ 
      error: 'Access token required' 
    });
  }
  
  // Verify token
  jwt.verify(
    token, 
    process.env.JWT_SECRET,
    { 
      issuer: 'your-app',
      audience: 'your-app-users'
    },
    (err, user) => {
      if (err) {
        if (err.name === 'TokenExpiredError') {
          return res.status(401).json({ 
            error: 'Token expired' 
          });
        }
        return res.status(403).json({ 
          error: 'Invalid token' 
        });