One of the major reasons behind the success of Unix operating system lies in the security it offered. They act as interface between user and machine and hence are the first and most important for implementation of security in computing systems. They perform multiple activities from multiprogramming to enforce restriction. A weak operating system can provide easy to the intruders and hence security breach. They hide the secrets of computer behind them. An operating system is basically a large software offering multiple services to the user like memory protection, file protection, general object access and user authentication etc. To build confidence it must provide these consistently and effectively and hence the term trusted operating system. To design trusted o/s major considerations include (these forms remaining topics of the unit):

• Security Policies: O/S must have well defined, consistent, and implementable security rules expressed clearly and unambiguously. It must implement them effectively to meet user expectations.
• Model: To design a secure o/s we must propose a model that may meet the security requirements. It represents the policy the operating system will enforce. Designers should compare the model with the specified system requirements to ensure that the overall system functions are not compromised or degraded by the security needs.
• Design: While designing the secure o/s we consider both its intended functionality (security goals) and implementation.
• Trust: The trust in an o/s depends upon that its features/functions enforce the expected security policy and an assurance that its implementation enforces the security policy effectively).