Every reference to passive entities or change of authorizations must go through the reference monitor. In a reference monitor all active entities such as people or computer processes reference to passive entities such as documents or segments of memory using a set of correct access authorizations. The reference monitor provides an underlying security theory for conceptualizing the idea of protection.
In the security kernel approach, a very important concept is the reference monitor which is an abstract notation adopted from the models of Butler Lampson. For example, users select quite often easily guessed passwords or write them down which makes the intruder work relatively easy. Additionally, they cannot protect system from authorized, careless users. They are as efficient as the chosen policy which is discussed. The security kernels cannot guarantee full protection. These are discussed in detail later on this chapter. However, the little difference is that security kernels involve an isolated portion of a system architecture for security functions but in TCBs, security functions may be spread throughout various portions of a system.
The difference is small and often security kernels and TCBs are viewed as synonymous. A TCB is defined as the totality of hardware and software protection mechanisms responsible for enforcing the security policy of a given system. The security kernel is defined as an isolated portion of a computer system that is designed to enforce the security policy of the system. The security kernels can be divided into two categories: actual security kernels and trusted computing bases (TCB). Also, security kernels are able to co-operate over networks which is very important nowadays. They are especially useful in organisations where different users are trusted on different levels. The security kernel is a methodology that provides the functionality of the operating system and good internal security in multiuser systems. This paper introduces concepts of the security kernels as well as two examples of them: Kernelized Security Operating System and Honeywell Secure Communications Processor.
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