This document provides an overview of techniques for securing operating systems, including file protection mechanisms, user authentication, designing trusted operating systems, and security models. It discusses basic forms of file protection like all-none and group protection and individual permissions in Unix. It covers user authentication using passwords, biometrics, and one-time passwords. It also describes how to design trusted operating systems using security policies, models, and principles like least privilege and separation of duties. Security models like Bell-LaPadula and Biba are presented for enforcing confidentiality and integrity.