This post is an in-depth breakdown of Authentication (Who are you?) and Authorization (What are you allowed to do?). In the early days, identity was based on simple trust. In modern web applications, we rely on complex and secure systems. As a backend engineer, it is essential to understand the key differences between stateful (Sessions) and stateless (JWTs) authentication, when to use API keys, and exactly how “Sign in with Google” (OAuth 2.0 / OIDC) works behind the scenes. The post also covers practical security risks such as how hackers exploit timing attacks and detailed error messages, along with ways to keep your systems secure. Key Concepts Breakdown 1. Authentication vs. Authorization Simple Explanation: Authentication (AuthN) is the process of verifying identity (like showing your ID card). Authorization (AuthZ) is the process of checking permissions (like whether that ID card allows you to enter the server room). Why It Matters: Mixing up the two leads to insecure systems.…