Create a full-stack web application using React and Express that allows users to sign up, log in, and post messages on a public message board. The application should store user information and messages in a MySQL database, and implement proper authentication and authorization mechanisms to protect user data.
To create a full-stack web application using React and Express that allows users to sign up, log in, and post messages on a public message board, follow these steps:

1. Set up the project structure
Create a new folder for your project and navigate into it. Then, set up two separate folders: one for the client-side (React) and another for the server-side (Express).

```
mkdir message-board-app
cd message-board-app
mkdir client
mkdir server
```

2. Create the server-side with Express

Navigate to the `server` folder and initialize a new Node.js project.

```
cd server
npm init -y
```

Install necessary dependencies.

```
npm install express mysql bcryptjs jsonwebtoken cors dotenv
```

Create an `.env` file to store environment variables like database credentials and JWT secret key.

```
touch .env
```

Add the following content to the `.env` file:

```
DB_HOST=localhost
DB_USER=root
DB_PASSWORD=your_password
DB_NAME=message_board_db
JWT_SECRET=your_jwt_secret_key
```

Create a `server.js` file and set up a basic Express server.

```javascript
const express = require('express');
const app = express();
const port = 3001;

app.use(express.json());

app.get('/', (req, res) => {
  res.send('Hello World!');
});

app.listen(port, () => {
  console.log(`Server is running at http://localhost:${port}`);
});
```

3. Set up MySQL database

Install MySQL if you haven't already, then create a new database and table for storing user information and messages.

```sql
CREATE DATABASE message_board_db;
USE message_board_db;

CREATE TABLE users (
  id INT AUTO_INCREMENT PRIMARY KEY,
  username VARCHAR(255) NOT NULL UNIQUE,
  email VARCHAR(255) NOT NULL UNIQUE,
  password VARCHAR(255) NOT NULL
);

CREATE TABLE messages (
  id INT AUTO_INCREMENT PRIMARY KEY,
  user_id INT NOT NULL,
  content TEXT NOT NULL,
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  FOREIGN KEY (user_id) REFERENCES users(id)
);
```

4. Create API routes for authentication and message posting

In the `server` folder, create a new folder called `routes`, then create two files: `auth.js` and `messages.js`.

Inside `auth.js`, set up routes for user registration and login.

```javascript
const express = require('express');
const router = express.Router();
const bcrypt = require('bcryptjs');
const jwt = require('jsonwebtoken');
const dotenv = require('dotenv');

// Load environment variables from .env file
dotenv.config();

// Your database connection logic here

router.post('/register', async (req, res) => {
  // Registration logic here
});

router.post('/login', async (req, res) => {
  // Login logic here
});

module.exports = router;
```

Inside `messages.js`, set up routes for fetching all messages and posting a new message.

```javascript
const express = require('express');
const router = express.Router();
const jwt = require('jsonwebtoken');
const dotenv = require('dotenv');

// Load environment variables from .env file
dotenv.config();

// Your database connection logic here

router.get('/', async (req, res) => {
  // Fetch all messages logic here
});

router.post('/', async (req, res) => {
  // Post a new message logic here
});

module.exports = router;
```

5. Create the client-side with React

Navigate to the `client` folder and create a new React app using `create-react-app`.

```
cd ../client
npx create-react-app .
```

Install necessary dependencies.

```
npm install axios react-router-dom
```

6. Build the React components

Create the necessary components for your app, such as `App`, `Navbar`, `Login`, `Register`, and `MessageBoard`. Use React Router to handle navigation between different pages.

7. Connect the client-side with the server-side

Use Axios to make API calls from your React components to your Express server endpoints.

8. Run both the client and server concurrently

Install `concurrently` in your `server` folder.

```
npm install concurrently
```

Modify the `scripts` section of your `package.json` file in the `server` folder:

```json
"scripts": {
  "start": "node server.js",
  "client": "cd ../client && npm start",
  "dev": "concurrently \"npm start\" \"npm run client\""
}
```

Now you can run both the client and server at the same time by running `npm run dev` in the `server` folder.

That's it! You now have a full-stack web application using React and Express that allows users to sign up, log in, and post messages on a public message board. Make sure to properly implement authentication and authorization mechanisms to protect user data.