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This chapter is from the book

This chapter is from the book

1.5 Methods of Defense

In Chapter 11, we investigate the legal and ethical restrictions on computer-based crime. But unfortunately, computer crime is certain to continue for the foreseeable future. For this reason, we must look carefully at controls for preserving confidentiality, integrity, and availability. Sometimes these controls can prevent or mitigate attacks; other, less powerful methods can only inform us that security has been compromised, by detecting a breach as it happens or after it occurs.

Harm occurs when a threat is realized against a vulnerability. To protect against harm, then, we can neutralize the threat, close the vulnerability, or both. The possibility for harm to occur is called risk. We can deal with harm in several ways. We can seek to

  • prevent it, by blocking the attack or closing the vulnerability
  • deter it, by making the attack harder but not impossible
  • deflect it, by making another target more attractive (or this one less so)
  • detect it, either as it happens or some time after the fact
  • recover from its effects

Of course, more than one of these can be done at once. So, for example, we might try to prevent intrusions. But in case we do not prevent them all, we might install a detection device to warn of an imminent attack. And we should have in place incident response procedures to help in the recovery in case an intrusion does succeed.


To consider the controls or countermeasures that attempt to prevent exploiting a computing system's vulnerabilities, we begin by thinking about traditional ways to enhance physical security. In the Middle Ages, castles and fortresses were built to protect the people and valuable property inside. The fortress might have had one or more security characteristics, including

  • a strong gate or door, to repel invaders
  • heavy walls to withstand objects thrown or projected against them
  • a surrounding moat, to control access
  • arrow slits, to let archers shoot at approaching enemies
  • crenellations to allow inhabitants to lean out from the roof and pour hot or vile liquids on attackers
  • a drawbridge to limit access to authorized people
  • gatekeepers to verify that only authorized people and goods could enter

Similarly, today we use a multipronged approach to protect our homes and offices. We may combine strong locks on the doors with a burglar alarm, reinforced windows, and even a nosy neighbor to keep an eye on our valuables. In each case, we select one or more ways to deter an intruder or attacker, and we base our selection not only on the value of what we protect but also on the effort we think an attacker or intruder will expend to get inside.

Computer security has the same characteristics. We have many controls at our disposal. Some are easier than others to use or implement. Some are cheaper than others to use or implement. And some are more difficult than others for intruders to override. Figure 1-6 illustrates how we use a combination of controls to secure our valuable resources. We use one or more controls, according to what we are protecting, how the cost of protection compares with the risk of loss, and how hard we think intruders will work to get what they want.

Figure 1-6

Figure 1-6 Multiple Controls.

In this section, we present an overview of the controls available to us. In later chapters, we examine each control in much more detail.


We noted earlier that we seek to protect hardware, software, and data. We can make it particularly hard for an intruder to find data useful if we somehow scramble the data so that interpretation is meaningless without the intruder's knowing how the scrambling was done. Indeed, the most powerful tool in providing computer security is this scrambling or encoding.

Encryption is the formal name for the scrambling process. We take data in their normal, unscrambled state, called cleartext, and transform them so that they are unintelligible to the outside observer; the transformed data are called enciphered text or ciphertext. Using encryption, security professionals can virtually nullify the value of an interception and the possibility of effective modification or fabrication. In Chapters 2 and 12 we study many ways of devising and applying these transformations.

Encryption clearly addresses the need for confidentiality of data. Additionally, it can be used to ensure integrity; data that cannot be read generally cannot easily be changed in a meaningful manner. Furthermore, as we see throughout this book, encryption is the basis of protocols that enable us to provide security while accomplishing an important system or network task. A protocol is an agreed-on sequence of actions that leads to a desired result. For example, some operating system protocols ensure availability of resources as different tasks and users request them. Thus, encryption can also be thought of as supporting availability. That is, encryption is at the heart of methods for ensuring all aspects of computer security.

Although encryption is an important tool in any computer security tool kit, we should not overrate its importance. Encryption does not solve all computer security problems, and other tools must complement its use. Furthermore, if encryption is not used properly, it may have no effect on security or could even degrade the performance of the entire system. Weak encryption can actually be worse than no encryption at all, because it gives users an unwarranted sense of protection. Therefore, we must understand those situations in which encryption is most useful as well as ways to use it effectively.

Software Controls

If encryption is the primary way of protecting valuables, programs themselves are the second facet of computer security. Programs must be secure enough to prevent outside attack. They must also be developed and maintained so that we can be confident of the programs' dependability.

Program controls include the following:

  • internal program controls: parts of the program that enforce security restrictions, such as access limitations in a database management program
  • operating system and network system controls: limitations enforced by the operating system or network to protect each user from all other users
  • independent control programs: application programs, such as password checkers, intrusion detection utilities, or virus scanners, that protect against certain types of vulnerabilities
  • development controls: quality standards under which a program is designed, coded, tested, and maintained to prevent software faults from becoming exploitable vulnerabilities

We can implement software controls by using tools and techniques such as hardware components, encryption, or information gathering. Software controls frequently affect users directly, such as when the user is interrupted and asked for a password before being given access to a program or data. For this reason, we often think of software controls when we think of how systems have been made secure in the past. Because they influence the way users interact with a computing system, software controls must be carefully designed. Ease of use and potency are often competing goals in the design of a collection of software controls.

Hardware Controls

Numerous hardware devices have been created to assist in providing computer security. These devices include a variety of means, such as

  • hardware or smart card implementations of encryption
  • locks or cables limiting access or deterring theft
  • devices to verify users' identities
  • firewalls
  • intrusion detection systems
  • circuit boards that control access to storage media

Policies and Procedures

Sometimes, we can rely on agreed-on procedures or policies among users rather than enforcing security through hardware or software means. In fact, some of the simplest controls, such as frequent changes of passwords, can be achieved at essentially no cost but with tremendous effect. Training and administration follow immediately after establishment of policies, to reinforce the importance of security policy and to ensure their proper use.

We must not forget the value of community standards and expectations when we consider how to enforce security. There are many acts that most thoughtful people would consider harmful, and we can leverage this commonality of belief in our policies. For this reason, legal and ethical controls are an important part of computer security. However, the law is slow to evolve, and the technology involving computers has emerged relatively suddenly. Although legal protection is necessary and desirable, it may not be as dependable in this area as it would be when applied to more well-understood and long-standing crimes.

Society in general and the computing community in particular have not adopted formal standards of ethical behavior. As we see in Chapter 11, some organizations have devised codes of ethics for computer professionals. However, before codes of ethics can become widely accepted and effective, the computing community and the general public must discuss and make clear what kinds of behavior are inappropriate and why.

Physical Controls

Some of the easiest, most effective, and least expensive controls are physical controls. Physical controls include locks on doors, guards at entry points, backup copies of important software and data, and physical site planning that reduces the risk of natural disasters. Often the simple physical controls are overlooked while we seek more sophisticated approaches.

Effectiveness of Controls

Merely having controls does no good unless they are used properly. Let us consider several aspects that can enhance the effectiveness of controls.

Awareness of Problem

People using controls must be convinced of the need for security. That is, people will willingly cooperate with security requirements only if they understand why security is appropriate in a given situation. However, many users are unaware of the need for security, especially in situations in which a group has recently undertaken a computing task that was previously performed with lax or no apparent security.

Likelihood of Use

Of course, no control is effective unless it is used. The lock on a computer room door does no good if people block the door open. As Sidebar 1-7 tells, some computer systems are seriously uncontrolled.

  • Principle of Effectiveness: Controls must be used—and used properly—to be effective. They must be efficient, easy to use, and appropriate.

This principle implies that computer security controls must be efficient enough, in terms of time, memory space, human activity, or other resources used, that using the control does not seriously affect the task being protected. Controls should be selective so that they do not exclude legitimate accesses.

Overlapping Controls

As we have seen with fortress or home security, several different controls may apply to address a single vulnerability. For example, we may choose to implement security for a microcomputer application by using a combination of controls on program access to the data, on physical access to the microcomputer and storage media, and even by file locking to control access to the processing programs.

Periodic Review

Few controls are permanently effective. Just when the security specialist finds a way to secure assets against certain kinds of attacks, the opposition doubles its efforts in an attempt to defeat the security mechanisms. Thus, judging the effectiveness of a control is an ongoing task. (Sidebar 1-8 reports on periodic reviews of computer security.)

Seldom, if ever, are controls perfectly effective. Controls fail, controls are incomplete, or people circumvent or misuse controls, for example. For that reason, we use overlapping controls, sometimes called a layered defense, in the expectation that one control will compensate for a failure of another. In some cases, controls do nicely complement each other. But two controls are not always better than one and, in some cases, two can even be worse than one. This brings us to another security principle.

  • Principle of Weakest Link: Security can be no stronger than its weakest link. Whether it is the power supply that powers the firewall or the operating system under the security application or the human who plans, implements, and administers controls, a failure of any control can lead to a security failure.
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