In the realm of information security, particularly for competitive exams like CISSP, understanding the core principles of Authentication, Authorization, and Accounting (AAA) is paramount. These three pillars form the bedrock of access control and security auditing, ensuring that only legitimate users can access resources and that their actions are recorded.

Authentication is the process of verifying the identity of a user, system, or entity. It answers the fundamental question: "Who are you?" This is typically achieved by presenting credentials that only the legitimate user should possess. Common authentication factors include:

Once a user's identity has been authenticated, authorization determines what actions that user is permitted to perform. It answers the question: "What are you allowed to do?" Authorization is based on policies and rules that define access rights for authenticated users. This can be granular, specifying access to specific files, applications, or system functions.

Accounting, also known as auditing or logging, is the process of recording and monitoring user activities. It answers the question: "What did you do?" This involves tracking who accessed what resources, when they accessed them, and what actions they performed. Accounting logs are crucial for security audits, incident response, and compliance.

In real-world scenarios, AAA services are often implemented by dedicated servers or systems. For example, a network device might query a RADIUS or TACACS+ server to authenticate a user attempting to log in, then check its authorization policies, and finally send accounting data back to a central logging server. Understanding how these components interact is vital for securing networks and systems.