Security of computer networks

A few decades ago, most computers were stand-alone machines: they were able to process information using their own resources. Later computer systems were connected to each other enabling a computer system to use resources of an other computer as well. With the coupling of computer systems, security items emerge: in general the link between computers is not protected by thorough physical means. That enables an attacker to tap or modify information on the link. In this article, a model of a distributed system is given. The provided model can also form a basis for security evaluations. Threats to the process of information exchange between computer systems are indicated. In order to counter the threats envisaged, so called security services can be implemented: two communicating computer systems can use security services in order to secure their communication link. In this article, two principal implementations of security services are addressed. The qualities of the two implementations of security services are compared with each other.