Secure Client Server Authentication

TL;DR

This guide shows you how to add secure authentication to your client-server application using a common pattern: username/password login with token-based verification. We’ll cover creating user accounts, hashing passwords, generating tokens, and verifying those tokens on each request.

1. User Account Creation

1. Database Setup: You’ll need a database to store user information (username, hashed password, potentially other details). Choose a suitable database like PostgreSQL, MySQL, or SQLite.
2. Registration Endpoint: Create an API endpoint for new users to register. This endpoint should:
   • Accept username and password as input.
   • Validate the inputs (e.g., check for minimum length, valid characters).
   • Hash the password using a strong hashing algorithm like bcrypt or Argon2.

     # Example using Python and bcrypt
     import bcrypt
     hashed_password = bcrypt.hashpw(password.encode('utf-8'), bcrypt.gensalt())

   • Store the username and hashed password in your database. Never store passwords in plain text!

2. Login Process

1. Login Endpoint: Create an API endpoint for users to log in.
   • Accept username and password as input.
   • Retrieve the user from the database based on the provided username.
   • Compare the provided password with the stored hashed password using the same hashing algorithm used during registration.

     # Example using Python and bcrypt
     if bcrypt.checkpw(password.encode('utf-8'), hashed_password):
         # Password matches!
     else:
         # Incorrect password

   • If the passwords match, generate a JSON Web Token (JWT).

     # Example using Python and PyJWT
     import jwt
     payload = {'username': username}
     token = jwt.encode(payload, 'your_secret_key', algorithm='HS256')

   • Return the JWT to the client.

3. Token Verification (Middleware)

1. Middleware Implementation: Implement middleware that intercepts all protected API requests.
   • Extract the token from the request headers (e.g., Authorization: Bearer <token>).
   • Verify the token’s signature using your secret key.

     # Example using Python and PyJWT
     try:
         payload = jwt.decode(token, 'your_secret_key', algorithms=['HS256'])
     except jwt.ExpiredSignatureError:
         # Token has expired
     except jwt.InvalidTokenError:
         # Invalid token

   • If the token is valid, attach the user information (from the payload) to the request object.

     request.user = payload['username']  # Attach username to request
     

   • If the token is invalid or expired, return an error response (e.g., 401 Unauthorized).

4. Protecting API Endpoints

1. Apply Middleware: Apply your authentication middleware to all API endpoints that require authentication.
2. Access User Information: Within protected endpoints, access the user information attached to the request object by the middleware (e.g., request.user).

5. Security Considerations

• Secret Key Management: Keep your secret key secure! Do not hardcode it in your application code. Use environment variables or a dedicated secrets management system.
• Token Expiration: Set appropriate token expiration times to limit the impact of compromised tokens.
• HTTPS: Always use HTTPS to encrypt communication between the client and server, protecting the token from interception.
• Input Validation: Thoroughly validate all user inputs to prevent injection attacks.
• Rate Limiting: Implement rate limiting to protect against brute-force attacks.