1. Introduction

This tutorial will guide you on how to secure APIs in your web application. We will be covering the best practices for API security, which include authentication, authorization, and encryption. After completing this tutorial, you will have a good understanding of:

• How to use authentication and authorization to secure your APIs
• How to encrypt your data for an extra layer of security
• The basic principles of API security

Prerequisites:
Basic knowledge of web development and APIs is required. Familiarity with a programming language like JavaScript and a web framework like Express.js will be beneficial.

2. Step-by-Step Guide

Authentication

Authentication is about verifying the identity of the user. One common method for API authentication is the use of JSON Web Tokens (JWT). A JWT consists of three parts: a header, a payload, and a signature.

Authorization

After authentication, we need to determine what resources the authenticated user can access. This is called authorization.

Encryption

Encryption is the process of encoding information in such a way that only authorized parties can access it. HTTPS should be used to encrypt the data between the client and the server.

3. Code Examples

Implementing JWT Authentication

const express = require('express');
const jwt = require('jsonwebtoken');
const app = express();

app.get('/api', (req, res) => {
  res.json({
    message: 'Welcome to the API'
  });
});

app.post('/api/posts', verifyToken, (req, res) => {  
  jwt.verify(req.token, 'secretkey', (err, authData) => {
    if(err) {
      res.sendStatus(403);
    } else {
      res.json({
        message: 'Post created...',
        authData
      });
    }
  });
});

app.post('/api/login', (req, res) => {
  // Mock user
  const user = {
    id: 1, 
    username: 'brad',
    email: 'brad@gmail.com'
  }

  jwt.sign({user}, 'secretkey', (err, token) => {
    res.json({
      token
    });
  });
});

// FORMAT OF TOKEN
// Authorization: Bearer <access_token>

// Verify Token
function verifyToken(req, res, next) {
  // Get auth header value
  const bearerHeader = req.headers['authorization'];
  // Check if bearer is undefined
  if(typeof bearerHeader !== 'undefined') {
    // Split at the space
    const bearer = bearerHeader.split(' ');
    // Get token from array
    const bearerToken = bearer[1];
    // Set the token
    req.token = bearerToken;
    // Next middleware
    next();
  } else {
    // Forbidden
    res.sendStatus(403);
  }

}

app.listen(5000, () => console.log('Server started on port 5000'));

4. Summary

In this tutorial, we've covered the basic principles of API security, including authentication, authorization, and encryption. We've also seen how to implement these concepts in JavaScript using Express.js and JWT.

5. Practice Exercises

1. Implement an API that uses both JWT authentication and authorization in Express.js.
2. Add HTTPS encryption to your API.

Solutions:
1. The above code example is a good starting point for this exercise. You just need to add your own routes and authorization checks.
2. To add HTTPS encryption, you'll need a SSL certificate. You can get a free one from Let's Encrypt. After you have your certificate, you can use the https module in Node.js to create a secure server.

Tips for further practice:
- Try implementing other forms of authentication, like OAuth.
- Learn about and implement rate limiting to protect your API from brute force attacks.
- Learn about and implement input validation to protect your API from invalid or malicious input.