Explain the different methods for handling user authentication and authorization in a WordPress plugin, ensuring both security and usability.

Handling user authentication and authorization in a WordPress plugin requires careful consideration of both security and usability. Authentication verifies who a user is, while authorization determines what a user is allowed to do. Implementing these securely is essential to protect sensitive data and prevent unauthorized access. WordPress has built in mechanisms and APIs to help manage these, however, it also requires developers to adhere to best practices to prevent vulnerabilities.

1. WordPress Authentication Methods:
WordPress has a built-in authentication system that is primarily based on cookies. When a user logs in successfully, WordPress sets a cookie in the user's browser that contains information to verify the user's session. For plugins that need to perform actions on the front end, these cookies are sufficient. However, for back-end or API related functionality, other authentication methods can be more appropriate. Here are some common methods:

WordPress Cookies:
WordPress manages sessions using cookies. These cookies contain encrypted user data. For typical plugins that work within the WordPress environment, this is usually sufficient. For example, if you have a plugin that displays user-specific information, you can use the WordPress cookies to determine the currently logged in user and display relevant content using functions like `wp_get_current_user()` or `is_user_logged_in()`.

Example:
```php
if ( is_user_logged_in() ) {
$current_user = wp_get_current_user();
echo 'Hello, ' . $current_user->user_login . '!';
} else {
echo 'You are not logged in.';
}
```
This snippet checks if the user is logged in and then shows user-specific content.

Nonce Verification:
WordPress nonces (number used once) are security tokens that verify that a request comes from the intended user within the current WordPress session. Nonces should be used in forms or any actions that require user input, to prevent CSRF (Cross-Site Request Forgery) attacks. WordPress provides functions like `wp_create_nonce()` to generate nonces, `wp_verify_nonce()` to verify them, and `check_admin_referer()` or `check_ajax_referer()` for specific use cases. This is important for every action that you perform using a form that the user submits.

Example:
```php
// Create nonce when form is displayed
$nonce = wp_create_nonce( 'my_form_action' );
echo '<form method="post">';
echo '<input type="hidden" name="my_nonce" value="' . esc_attr( $nonce ) . '" />';
echo '<input type="submit" value="Submit" />';
echo '</form>';

// Verify nonce on form submission
if ( isset($_POST['my_nonce'] ) && wp_verify_nonce( $_POST['my_nonce'], 'my_form_action' ) ) {
// Process the form data
} else {
// Handle invalid nonce
}
```

API Key Authentication:
If your plugin interacts with a third-party API, you might require users to generate and enter an API key within your plugin settings. This ensures the user has access to the external service. Store the API key securely using the WordPress options API. Validate the API key when communicating with the third-party service. Do not use client side code to use the API keys as they will be visible to users. Instead use the server side for all requests using the API keys.

JWT Authentication:
JSON Web Tokens are used for stateless authentication, especially useful for REST API requests and headless WordPress setups. The user authentication generates a JWT token, which is then included with each API request to authenticate the user. Libraries like `php-jwt` can be used to generate and verify JWT tokens. The JWT is usually stored in the local storage or a session and then is passed with each request in the authorization header.

2. WordPress Authorization Techniques:
After authenticating the user, authorization ensures that they only perform allowed actions. WordPress provides several mechanisms for this:

User Roles and Capabilities:
WordPress uses user roles (like administrator, editor, author) and capabilities to define what actions each user is allowed to perform. WordPress provides functions like `current_user_can()` to check a user's capabilities before performing an action. Define custom capabilities for your plugin using the `add_cap()` function. Then assign these capabilities to appropriate roles, such as administrator.

Example:
```php
if ( current_user_can( 'manage_my_plugin_settings' ) ) {
// Allow admin to edit settings
} else {
// Access denied
}
```
This prevents users without the capability to access specific features.

REST API Permissions:
For plugin features that interact with the WordPress REST API, you can use the `register_rest_route` and `permission_callback` to define specific permissions. The permission callback function will determine if a user has permission to perform the action on that particular endpoint. You can check capabilities, nonce tokens, and other authentication information in your permission callback function.

Example:
```php
register_rest_route( 'myplugin/v1', '/posts', array(
'methods' => 'POST',
'callback' => 'my_plugin_create_post',
'permission_callback' => function () {
return current_user_can( 'edit_posts' );
}
));
```
This example restricts access to the `/posts` POST endpoint to only users with the ‘edit_posts’ capability.

Sanitizing and Validating User Input:
Always sanitize and validate user input to prevent security vulnerabilities like SQL injection and XSS. WordPress provides sanitization functions like `sanitize_text_field()`, `esc_html()`, `esc_attr()` and many more. Proper input sanitization and output escaping is an important part of security. Always sanitize incoming data and escape output data before presenting it to users.

3. Best Practices:
Use HTTPS: Ensure that your WordPress site uses HTTPS to encrypt data transmitted between the user's browser and the server. This protects user credentials and other sensitive data.
Avoid Storing Sensitive Data: If you have to store sensitive information encrypt the data before storing it and always use the WordPress options API.
Secure API Calls: When communicating with third-party APIs, never include the API keys in client-side code. Always perform API calls through your server-side code.

By carefully implementing these authentication and authorization methods, while also adhering to best practices you can create secure and user-friendly plugins that protect sensitive data and offer appropriate access controls.