There are four major classes of attack on a system by intruders: interception, fabrication, modification, and interruption [2]. A fifth class of attacks Repudiation is an attack against the accountability of information. It is an attack from within the system by either the source entity or the destination entity. Each of these classes of attack can addressed with a security mechanism (Table 7.1). Together, the security mechanisms form a cryptosystem.

Under normal circumstances, information is sent from the source to the destination (Figure 7.1). When an attack occurs it can come in the forms listed in Table 7.1 and discussed in the following subsections.

Figure 7.1: Normal flow.

Interception

Interception is a passive attack on confidentiality in which an intruding entity is able to read the information that is sent from the source entity to the destination entity (Figure 7.2). Sniffing is an example of an interception attack.

Figure 7.2: Interception in a network.

The intruder attempts to learn or make use of information from the system but does not affect system resources. The identity of the source entity can be intercepted and later used in a masquerade attack, or the intruder may be interested in releasing message contents such as authentication information, passwords, credit card numbers, intellectual property, or other sensitive information. The...