Implement a basic but effective security layer using static API keys sent in request headers.
API Key authentication is one of the simplest and most common methods for securing an API. The basic principle is that the server issues a unique, secret string of characters (the API key) to each authorized client. The client must then include this key in every request it makes to protected endpoints. The server, upon receiving a request, checks for the presence and validity of the API key. If the key is valid, the request is processed; otherwise, it's rejected, typically with a `401 Unauthorized` or `403 Forbidden` status code. The key is usually sent in a custom HTTP header, such as `X-API-Key` or `Authorization: ApiKey <key>`. Sending the key in a header is generally preferred over sending it in the URL as a query parameter, as URLs are often logged, potentially exposing the key. While simple to implement, API key authentication has its limitations. The keys are often long-lived and, if compromised, can grant an attacker full access until the key is manually revoked. It's primarily an authentication mechanism (identifying the calling application), not an authorization mechanism (determining what the application is allowed to do). Despite these drawbacks, it's an excellent choice for public APIs with usage tiers, internal microservice communication, or any scenario where a simple, shared secret is sufficient to grant access.