Fundamental security design principles Comparison

Primary Task Response: , write a 150-300-word response to the following:

After reading chapter 1, compare and contrast two fundamental security design principles. Analyze how these principles impact an organization’s security posture. 

  • You must use at least one scholarly resource. 
  • Every discussion posting must be properly APA formatted.

The attached doc is chapter 1 text. 

No Plagiarism.





Computer and Network Security Concepts

1.1 Computer Security Concepts

A Definition of Computer Security Examples The Challenges of Computer Security

1.2 The OSI Security Architecture

1.3 Security Attacks

Passive Attacks Active Attacks

1.4 Security Services

Authentication Access Control Data Confidentiality Data Integrity Nonrepudiation Availability Service

1.5 Security Mechanisms

1.6 Fundamental Security Design Principles

1.7 Attack Surfaces and Attack Trees

Attack Surfaces Attack Trees

1.8 A Model for Network Security

1.9 Standards

1.10 Key Terms, Review Questions, and Problems



This book focuses on two broad areas: cryptographic algorithms and protocols, which have a broad range of applications; and network and Internet security, which rely heavily on cryptographic techniques.

Cryptographic algorithms and protocols can be grouped into four main areas:

■ Symmetric encryption: Used to conceal the contents of blocks or streams of data of any size, including messages, files, encryption keys, and passwords.

■ Asymmetric encryption: Used to conceal small blocks of data, such as encryp- tion keys and hash function values, which are used in digital signatures.

■ Data integrity algorithms: Used to protect blocks of data, such as messages, from alteration.

■ Authentication protocols: These are schemes based on the use of crypto- graphic algorithms designed to authenticate the identity of entities.

The field of network and Internet security consists of measures to deter, prevent, detect, and correct security violations that involve the transmission of information. That is a broad statement that covers a host of possibilities. To give you a feel for the areas covered in this book, consider the following examples of security violations:

1. User A transmits a file to user B. The file contains sensitive information (e.g., payroll records) that is to be protected from disclosure. User C, who is not authorized to read the file, is able to monitor the transmission and capture a copy of the file during its transmission.

2. A network manager, D, transmits a message to a computer, E, under its man- agement. The message instructs computer E to update an authorization file to include the identities of a number of new users who are to be given access to that computer. User F intercepts the message, alters its contents to add or delete entries, and then forwards the message to computer E, which accepts the mes- sage as coming from manager D and updates its authorization file accordingly.


After studying this chapter, you should be able to:

◆ Describe the key security requirements of confidentiality, integrity, and availability.

◆ Describe the X.800 security architecture for OSI.

◆ Discuss the types of security threats and attacks that must be dealt with and give examples of the types of threats and attacks that apply to differ- ent categories of computer and network assets.

◆ Explain the fundamental security design principles.

◆ Discuss the use of attack surfaces and attack trees.

◆ List and briefly describe key organizations involved in cryptography standards.


3. Rather than intercept a message, user F constructs its own message with the desired entries and transmits that message to computer E as if it had come from manager D. Computer E accepts the message as coming from manager D and updates its authorization file accordingly.

4. An employee is fired without warning. The personnel manager sends a mes- sage to a server system to invalidate the employee’s account. When the invali- dation is accomplished, the server is to post a notice to the employee’s file as confirmation of the action. The employee is able to intercept the message and delay it long enough to make a final access to the server to retrieve sensitive information. The message is then forwarded, the action taken, and the confir- mation posted. The employee’s action may go unnoticed for some consider- able time.

5. A message is sent from a customer to a stockbroker with instructions for vari- ous transactions. Subsequently, the investments lose value and the customer denies sending the message.

Although this list by no means exhausts the possible types of network security viola- tions, it illustrates the range of concerns of network security.


A Definition of Computer Security

The NIST Computer Security Handbook [NIST95] defines the term computer secu- rity as follows:

Computer Security: The protection afforded to an automated information system in order to attain the applicable objectives of preserving the integrity, availability, and confidentiality of information system resources (includes hardware, software, firmware, information/data, and telecommunications).

This definition introduces three key objectives that are at the heart of com- puter security:

■ Confidentiality: This term covers two related concepts:

Data1 confidentiality: Assures that private or confidential information is not made available or disclosed to unauthorized individuals.

Privacy: Assures that individuals control or influence what information re- lated to them may be collected and stored and by whom and to whom that information may be disclosed.

1RFC 4949 defines information as “facts and ideas, which can be represented (encoded) as various forms of data,” and data as “information in a specific physical representation, usually a sequence of symbols that have meaning; especially a representation of information that can be processed or produced by a computer.” Security literature typically does not make much of a distinction, nor does this book.


■ Integrity: This term covers two related concepts:

Data integrity: Assures that information (both stored and in transmit- ted packets) and programs are changed only in a specified and authorized manner.

System integrity: Assures that a system performs its intended function in an unimpaired manner, free from deliberate or inadvertent unauthorized manipulation of the system.

■ Availability: Assures that systems work promptly and service is not denied to authorized users.

These three concepts form what is often referred to as the CIA triad. The three concepts embody the fundamental security objectives for both data and for information and computing services. For example, the NIST standard FIPS 199 (Standards for Security Categorization of Federal Information and Information Systems) lists confidentiality, integrity, and availability as the three security objec- tives for information and for information systems. FIPS 199 provides a useful char- acterization of these three objectives in terms of requirements and the definition of a loss of security in each category:

■ Confidentiality: Preserving authorized restrictions on information access and disclosure, including means for protecting personal privacy and propri- etary information. A loss of confidentiality is the unauthorized disclosure of information.

■ Integrity: Guarding against improper information modification or destruc- tion, including ensuring information nonrepudiation and authenticity. A loss of integrity is the unauthorized modification or destruction of information.

■ Availability: Ensuring timely and reliable access to and use of information. A loss of availability is the disruption of access to or use of information or an information system.

Although the use of the CIA triad to define security objectives is well estab- lished, some in the security field feel that additional concepts are needed to present a complete picture (Figure 1.1). Two of the most commonly mentioned are as follows:

Figure 1.1 Essential Network and Computer Security Requirements

Data and




A ccountability

A ut

he nt

ic ity

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■ Authenticity: The property of being genuine and being able to be verified and trusted; confidence in the validity of a transmission, a message, or message originator. This means verifying that users are who they say they are and that each input arriving at the system came from a trusted source.

■ Accountability: The security goal that generates the requirement for actions of an entity to be traced uniquely to that entity. This supports nonrepudia- tion, deterrence, fault isolation, intrusion detection and prevention, and after- action recovery and legal action. Because truly secure systems are not yet an achievable goal, we must be able to trace a security breach to a responsible party. Systems must keep records of their activities to permit later forensic analysis to trace security breaches or to aid in transaction disputes.


We now provide some examples of applications that illustrate the requirements just enumerated.2 For these examples, we use three levels of impact on organizations or individuals should there be a breach of security (i.e., a loss of confidentiality, integ- rity, or availability). These levels are defined in FIPS PUB 199:

■ Low: The loss could be expected to have a limited adverse effect on organi- zational operations, organizational assets, or individuals. A limited adverse effect means that, for example, the loss of confidentiality, integrity, or avail- ability might (i) cause a degradation in mission capability to an extent and duration that the organization is able to perform its primary functions, but the effectiveness of the functions is noticeably reduced; (ii) result in minor dam- age to organizational assets; (iii) result in minor financial loss; or (iv) result in minor harm to individuals.

■ Moderate: The loss could be expected to have a serious adverse effect on organizational operations, organizational assets, or individuals. A serious adverse effect means that, for example, the loss might (i) cause a signifi- cant degradation in mission capability to an extent and duration that the organization is able to perform its primary functions, but the effectiveness of the functions is significantly reduced; (ii) result in significant damage to organizational assets; (iii) result in significant financial loss; or (iv) result in significant harm to individuals that does not involve loss of life or serious, life-threatening injuries.

■ High: The loss could be expected to have a severe or catastrophic adverse effect on organizational operations, organizational assets, or individuals. A  severe or catastrophic adverse effect means that, for example, the loss might (i) cause a severe degradation in or loss of mission capability to an extent and duration that the organization is not able to perform one or more of its primary functions; (ii) result in major damage to organizational assets; (iii) result in major financial loss; or (iv) result in severe or catastrophic harm to individuals involving loss of life or serious, life-threatening injuries.

2These examples are taken from a security policy document published by the Information Technology Security and Privacy Office at Purdue University.


CONFIDENTIALITY Student grade information is an asset whose confidentiality is considered to be highly important by students. In the United States, the release of such information is regulated by the Family Educational Rights and Privacy Act (FERPA). Grade information should only be available to students, their parents, and employees that require the information to do their job. Student enrollment information may have a moderate confidentiality rating. While still covered by FERPA, this information is seen by more people on a daily basis, is less likely to be targeted than grade information, and results in less damage if disclosed. Directory information, such as lists of students or faculty or departmental lists, may be as- signed a low confidentiality rating or indeed no rating. This information is typically freely available to the public and published on a school’s Web site.

INTEGRITY Several aspects of integrity are illustrated by the example of a hospital patient’s allergy information stored in a database. The doctor should be able to trust that the information is correct and current. Now suppose that an employee (e.g., a nurse) who is authorized to view and update this information deliberately falsifies the data to cause harm to the hospital. The database needs to be restored to a trusted basis quickly, and it should be possible to trace the error back to the person responsible. Patient allergy information is an example of an asset with a high requirement for integrity. Inaccurate information could result in serious harm or death to a patient and expose the hospital to massive liability.

An example of an asset that may be assigned a moderate level of integrity requirement is a Web site that offers a forum to registered users to discuss some specific topic. Either a registered user or a hacker could falsify some entries or deface the Web site. If the forum exists only for the enjoyment of the users, brings in little or no advertising revenue, and is not used for something important such as research, then potential damage is not severe. The Web master may experience some data, financial, and time loss.

An example of a low integrity requirement is an anonymous online poll. Many Web sites, such as news organizations, offer these polls to their users with very few safeguards. However, the inaccuracy and unscientific nature of such polls is well understood.

AVAILABILITY The more critical a component or service, the higher is the level of availability required. Consider a system that provides authentication services for critical systems, applications, and devices. An interruption of service results in the inability for customers to access computing resources and staff to access the re- sources they need to perform critical tasks. The loss of the service translates into a large financial loss in lost employee productivity and potential customer loss.

An example of an asset that would typically be rated as having a moderate availability requirement is a public Web site for a university; the Web site provides information for current and prospective students and donors. Such a site is not a critical component of the university’s information system, but its unavailability will cause some embarrassment.

An online telephone directory lookup application would be classified as a low availability requirement. Although the temporary loss of the application may be an annoyance, there are other ways to access the information, such as a hardcopy directory or the operator.


The Challenges of Computer Security

Computer and network security is both fascinating and complex. Some of the reasons follow:

1. Security is not as simple as it might first appear to the novice. The require- ments seem to be straightforward; indeed, most of the major requirements for security services can be given self-explanatory, one-word labels: confidential- ity, authentication, nonrepudiation, or integrity. But the mechanisms used to meet those requirements can be quite complex, and understanding them may involve rather subtle reasoning.

2. In developing a particular security mechanism or algorithm, one must always consider potential attacks on those security features. In many cases, successful attacks are designed by looking at the problem in a completely different way, therefore exploiting an unexpected weakness in the mechanism.

3. Because of point 2, the procedures used to provide particular services are often counterintuitive. Typically, a security mechanism is complex, and it is not obvious from the statement of a particular requirement that such elaborate measures are needed. It is only when the various aspects of the threat are con- sidered that elaborate security mechanisms make sense.

4. Having designed various security mechanisms, it is necessary to decide where to use them. This is true both in terms of physical placement (e.g., at what points in a network are certain security mechanisms needed) and in a logical sense (e.g., at what layer or layers of an architecture such as TCP/IP [Transmission Control Protocol/Internet Protocol] should mechanisms be placed).

5. Security mechanisms typically involve more than a particular algorithm or protocol. They also require that participants be in possession of some secret in- formation (e.g., an encryption key), which raises questions about the creation, distribution, and protection of that secret information. There also may be a re- liance on communications protocols whose behavior may complicate the task of developing the security mechanism. For example, if the proper functioning of the security mechanism requires setting time limits on the transit time of a message from sender to receiver, then any protocol or network that introduces variable, unpredictable delays may render such time limits meaningless.

6. Computer and network security is essentially a battle of wits between a per- petrator who tries to find holes and the designer or administrator who tries to close them. The great advantage that the attacker has is that he or she need only find a single weakness, while the designer must find and eliminate all weaknesses to achieve perfect security.

7. There is a natural tendency on the part of users and system managers to per- ceive little benefit from security investment until a security failure occurs.

8. Security requires regular, even constant, monitoring, and this is difficult in today’s short-term, overloaded environment.

9. Security is still too often an afterthought to be incorporated into a system after the design is complete rather than being an integral part of the design process.


10. Many users and even security administrators view strong security as an impediment to efficient and user-friendly operation of an information system or use of information.

The difficulties just enumerated will be encountered in numerous ways as we examine the various security threats and mechanisms throughout this book.


To assess effectively the security needs of an organization and to evaluate and choose various security products and policies, the manager responsible for security needs some systematic way of defining the requirements for security and character- izing the approaches to satisfying those requirements. This is difficult enough in a centralized data processing environment; with the use of local and wide area net- works, the problems are compounded.

ITU-T3 Recommendation X.800, Security Architecture for OSI, defines such a systematic approach.4 The OSI security architecture is useful to managers as a way of organizing the task of providing security. Furthermore, because this architecture was developed as an international standard, computer and communications vendors have developed security features for their products and services that relate to this structured definition of services and mechanisms.

For our purposes, the OSI security architecture provides a useful, if abstract, overview of many of the concepts that this book deals with. The OSI security archi- tecture focuses on security attacks, mechanisms, and services. These can be defined briefly as

■ Security attack: Any action that compromises the security of information owned by an organization.

■ Security mechanism: A process (or a device incorporating such a process) that is designed to detect, prevent, or recover from a security attack.

■ Security service: A processing or communication service that enhances the security of the data processing systems and the information transfers of an organization. The services are intended to counter security attacks, and they make use of one or more security mechanisms to provide the service.

In the literature, the terms threat and attack are commonly used to mean more or less the same thing. Table 1.1 provides definitions taken from RFC 4949, Internet Security Glossary.

3The International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T) is a United Nations-sponsored agency that develops standards, called Recommendations, relating to tele- communications and to open systems interconnection (OSI). 4The OSI security architecture was developed in the context of the OSI protocol architecture, which is described in Appendix L. However, for our purposes in this chapter, an understanding of the OSI proto- col architecture is not required.



A useful means of classifying security attacks, used both in X.800 and RFC 4949, is in terms of passive attacks and active attacks (Figure 1.2). A passive attack attempts to learn or make use of information from the system but does not affect system re- sources. An active attack attempts to alter system resources or affect their operation.

Passive Attacks

Passive attacks (Figure 1.2a) are in the nature of eavesdropping on, or monitoring of, transmissions. The goal of the opponent is to obtain information that is being transmitted. Two types of passive attacks are the release of message contents and traffic analysis.

The release of message contents is easily understood. A telephone conver- sation, an electronic mail message, and a transferred file may contain sensitive or confidential information. We would like to prevent an opponent from learning the contents of these transmissions.

A second type of passive attack, traffic analysis, is subtler. Suppose that we had a way of masking the contents of messages or other information traffic so that opponents, even if they captured the message, could not extract the information from the message. The common technique for masking contents is encryption. If we had encryption protection in place, an opponent might still be able to observe the pattern of these messages. The opponent could determine the location and identity of communicating hosts and could observe the frequency and length of messages being exchanged. This information might be useful in guessing the nature of the communication that was taking place.

Passive attacks are very difficult to detect, because they do not involve any alteration of the data. Typically, the message traffic is sent and received in an appar- ently normal fashion, and neither the sender nor receiver is aware that a third party has read the messages or observed the traffic pattern. However, it is feasible to pre- vent the success of these attacks, usually by means of encryption. Thus, the empha- sis in dealing with passive attacks is on prevention rather than detection.

Active Attacks

Active attacks (Figure 1.2b) involve some modification of the data stream or the creation of a false stream and can be subdivided into four categories: masquerade, replay, modification of messages, and denial of service.

Threat A potential for violation of security, which exists when there is a circumstance, capability, action, or event that could breach security and cause harm. That is, a threat is a possible danger that might exploit a vulnerability.

Attack An assault on system security that derives from an intelligent threat; that is, an intelligent act that is a deliberate attempt (especially in the sense of a method or technique) to evade security services and violate the security policy of a system.

Table 1.1 Threats and Attacks (RFC 4949)


A masquerade takes place when one entity pretends to be a different entity (path 2 of Figure 1.2b is active). A masquerade attack usually includes one of the other forms of active attack. For example, authentication sequences can be captured and replayed after a valid authentication sequence has taken place, thus enabling an authorized entity with few privileges to obtain extra privileges by impersonating an entity that has those privileges.

Replay involves the passive capture of a data unit and its subsequent retrans- mission to produce an unauthorized effect (paths 1, 2, and 3 active).

Modification of messages simply means that some portion of a legitimate mes- sage is altered, or that messages are delayed or reordered, to produce an unauthor- ized effect (paths 1 and 2 active). For example, a message meaning “Allow John Smith to read confidential file accounts” is modified to mean “Allow Fred Brown to read confidential file accounts.”

Figure 1.2 Security Attacks

(a) Passive attacks


(b) Active attacks






Internet or other communications facility

Internet or other communications facility

1 2 3


The denial of service prevents or inhibits the normal use or management of communications facilities (path 3 active). This attack may have a specific target; for example, an entity may suppress all messages directed to a particular destination (e.g., the security audit service). Another form of service denial is the disruption of an entire network, either by disabling the network or by overloading it with mes- sages so as to degrade performance.

Active attacks present the opposite characteristics of passive attacks. Whereas passive attacks are difficult to detect, measures are available to prevent their success. On the other hand, it is quite difficult to prevent active attacks absolutely because of the wide variety of potential physical, software, and network vulnerabilities. Instead, the goal is to detect active attacks and to recover from any disruption or delays caused by them. If the detection has a deterrent effect, it may also contribute to prevention.


X.800 defines a security service as a service that is provided by a protocol layer of communicating open systems and that ensures adequate security of the systems or of data transfers. Perhaps a clearer definition is found in RFC 4949, which provides the following definition: a processing or communication service that is provided by a system to give a specific kind of protection to system resources; security services implement security policies and are implemented by security mechanisms.

X.800 divides these services into five categories and fourteen specific services (Table 1.2). We look at each category in turn.5


The authentication service is concerned with assuring that a communication is au- thentic. In the case of a single message, such as a warning or alarm signal, the function of the authentication service is to assure the recipient that the message is from the source that it claims to be from. In the case of an ongoing interaction, such as the con- nection of a terminal to a host, two aspects are involved. First, at the time of connec- tion initiation, the service assures that the two entities are authentic, that is, that each is the entity that it claims to be. Second, the service must assure that the connection is not interfered with in such a way that a third party can masquerade as one of the two legitimate parties for the purposes of unauthorized transmission or reception.

Two specific authentication services are defined in X.800:

■ Peer entity authentication: Provides for the corroboration of the identity of a peer entity in an association. Two entities are considered peers if they imple- ment to same protocol in different systems; for example two TCP modules in two communicating systems. Peer entity authentication is provided for

5There is no universal agreement about many of the terms used in the security literature. For example, the term integrity is sometimes used to refer to all aspects of information security. The term authentication is sometimes used to refer both to verification of identity and to the various functions listed under integrity in this chapter. Our usage here agrees with both X.800 and RFC 4949.



The assurance that the communicating entity is the one that it claims to be.

Peer Entity Authentication Used in association with a logical connection to provide confidence in the identity of the entities connected.

Data-Origin Authentication In a connectionless transfer, provides assurance that the source of received data is as claimed.


The prevention of unauthorized use of a resource (i.e., this service controls who can have access to a resource, under what conditions access can occur, and what those accessing the resource are allowed to do).


The protection of data from unauthorized disclosure.

Connection Confidentiality The protection of all user data on a connection.

Connectionless Confidentiality The protection of all user data in a single data block.

Selective-Field Confidentiality The confidentiality of selected fields within the user data on a connection or in a single data block.

Traffic-Flow Confidentiality The protection of the information that might be derived from observation of traffic flows.


The assurance that data received are exactly as sent by an authorized entity (i.e., contain no modi- fication, insertion, deletion, or replay).

Connection Integrity with Recovery Provides for the integrity of all user data on a connec- tion and detects any modification, insertion, deletion, or replay of any data within an entire data sequence, with recovery attempted.

Connection Integrity without Recovery As above, but provides only detection without recovery.

Selective-Field Connection Integrity Provides for the integrity of selected fields within the user data of a data block transferred over a connec- tion and takes the form of determination of whether the selected fields have been modified, inserted, deleted, or replayed.

Connectionless Integrity Provides for the integrity of a single connectionless data block and may take the form of detection of data modification. Additionally, a limited form of replay detection may be provided.

Selective-Field Connectionless Integrity Provides for the integrity of selected fields within a single connectionless data block; takes the form of determination of whether the selected fields have been modified.


Provides protection against denial by one of the entities involved in a communication of having par- ticipated in all or part of the communication.

Nonrepudiation, Origin Proof that the message was sent by the specified party.

Nonrepudiation, Destination Proof that the message was received by the specified party.

Table 1.2 Security Services (X.800)

use at the establishment of, or at times during the data transfer phase of, a connection. It attempts to provide confidence that an entity is not performing either a masquerade or an unauthorized replay of a previous connection.

Data origin authentication: Provides for the corroboration of the source of a data unit. It does not provide protection against the duplication or modifica- tion of data units. This type of service supports applications like electronic mail, where there are no prior interactions between the communicating entities.


Access Control

In the context of network security, access control is the ability to limit and control the access to host systems and applications via communications links. To achieve this, each entity trying to gain access must first be identified, or authenticated, so that access rights can be tailored to the individual.

Data Confidentiality

Confidentiality is the protection of transmitted data from passive attacks. With re- spect to the content of a data transmission, several levels of protection can be iden- tified. The broadest service protects all user data transmitted between two users over a period of time. For example, when a TCP connection is set up between two systems, this broad protection prevents the release of any user data transmitted over the TCP connection. Narrower forms of this service can also be defined, including the protection of a single message or even specific fields within a message. These refinements are less useful than the broad approach and may even be more complex and expensive to implement.

The other aspect of confidentiality is the protection of traffic flow from analysis. This requires that an attacker not be able to observe the source and desti- nation, frequency, length, or other characteristics of the traffic on a communications facility.

Data Integrity

As with confidentiality, integrity can apply to a stream of messages, a single mes- sage, or selected fields within a message. Again, the most useful and straightforward approach is total stream protection.

A connection-oriented integrity service, one that deals with a stream of mes- sages, assures that messages are received as sent with no duplication, insertion, modification, reordering, or replays. The destruction of data is also covered under this service. Thus, the connection-oriented integrity service addresses both mes- sage stream modification and denial of service. On the other hand, a connection- less integrity service, one that deals with individual messages without regard to any larger context, generally provides protection against message modification only.

We can make a distinction between service with and without recovery. Because the integrity service relates to active attacks, we are concerned with detection rather than prevention. If a violation of integrity is detected, then the service may simply report this violation, and some other portion of software or human intervention is required to recover from the violation. Alternatively, there are mechanisms avail- able to recover from the loss of integrity of data, as we will review subsequently. The incorporation of automated recovery mechanisms is, in general, the more attractive alternative.


Nonrepudiation prevents either sender or receiver from denying a transmitted mes- sage. Thus, when a message is sent, the receiver can prove that the alleged sender in fact sent the message. Similarly, when a message is received, the sender can prove that the alleged receiver in fact received the message.


Availability Service

Both X.800 and RFC 4949 define availability to be the property of a system or a system resource being accessible and usable upon demand by an authorized system entity, according to performance specifications for the system (i.e., a system is avail- able if it provides services according to the system design whenever users request them). A variety of attacks can result in the loss of or reduction in availability. Some of these attacks are amenable to automated countermeasures, such as authentica- tion and encryption, whereas others require some sort of physical action to prevent or recover from loss of availability of elements of a distributed system.

X.800 treats availability as a property to be associated with various security services. However, it makes sense to call out specifically an availability service. An availability service is one that protects a system to ensure its availability. This ser- vice addresses the security concerns raised by denial-of-service attacks. It depends on proper management and control of system resources and thus depends on access control service and other security services.


Table 1.3 lists the security mechanisms defined in X.800. The mechanisms are divided into those that are implemented in a specific protocol layer, such as TCP or an application-layer protocol, and those that are not specific to any particular pro- tocol layer or security service. These mechanisms will be covered in the appropriate

SPECIFIC SECURITY MECHANISMS May be incorporated into the appropriate protocol layer in order to provide some of the OSI security services.

Encipherment The use of mathematical algorithms to transform data into a form that is not readily intelligible. The transformation and subsequent recovery of the data depend on an algorithm and zero or more encryption keys.

Digital Signature Data appended to, or a cryptographic transformation of, a data unit that allows a recipient of the data unit to prove the source and integrity of the data unit and protect against forgery (e.g., by the recipient).

Access Control A variety of mechanisms that enforce access rights to resources.

Data Integrity A variety of mechanisms used to assure the integrity of a data unit or stream of data units.


Mechanisms that are not specific to any particular OSI security service or protocol layer.

Trusted Functionality That which is perceived to be correct with respect to some criteria (e.g., as established by a security policy).

Security Label The marking bound to a resource (which may be a data unit) that names or designates the security attri- butes of that resource.

Event Detection Detection of security-relevant events.

Security Audit Trail Data collected and potentially used to facilitate a security audit, which is an independent review and examination of system records and activities.

Security Recovery Deals with requests from mechanisms, such as event handling and management functions, and takes recovery actions.

Table 1.3 Security Mechanisms (X.800)


places in the book. So we do not elaborate now, except to comment on the defini- tion of encipherment. X.800 distinguishes between reversible encipherment mech- anisms and irreversible encipherment mechanisms. A reversible encipherment mechanism is simply an encryption algorithm that allows data to be encrypted and subsequently decrypted. Irreversible encipherment mechanisms include hash algo- rithms and message authentication codes, which are used in digital signature and message authentication applications.

Table 1.4, based on one in X.800, indicates the relationship between security services and security mechanisms.


Authentication Exchange A mechanism intended to ensure the identity of an entity by means of information exchange.

Traffic Padding The insertion of bits into gaps in a data stream to frustrate traffic analysis attempts.

Routing Control Enables selection of particular physically secure routes for certain data and allows routing changes, especially when a breach of security is suspected.

Notarization The use of a trusted third party to assure certain properties of a data exchange.




















Peer entity authentication



En cip

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Di git

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Ac ces

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Tr affi

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Data origin authentication

Access control


Traffic flow confidentiality

Data integrity



Table 1.4 Relationship Between Security Services and Mechanisms



Despite years of research and development, it has not been possible to develop security design and implementation techniques that systematically exclude security flaws and prevent all unauthorized actions. In the absence of such foolproof tech- niques, it is useful to have a set of widely agreed design principles that can guide the development of protection mechanisms. The National Centers of Academic Excellence in Information Assurance/Cyber Defense, which is jointly sponsored by the U.S. National Security Agency and the U.S. Department of Homeland Security, list the following as fundamental security design principles [NCAE13]:

■ Economy of mechanism

■ Fail-safe defaults

■ Complete mediation

■ Open design

■ Separation of privilege

■ Least privilege

■ Least common mechanism

■ Psychological acceptability

■ Isolation

■ Encapsulation

■ Modularity

■ Layering

■ Least astonishment

The first eight listed principles were first proposed in [SALT75] and have withstood the test of time. In this section, we briefly discuss each principle.

Economy of mechanism means that the design of security measures embod- ied in both hardware and software should be as simple and small as possible. The motivation for this principle is that relatively simple, small design is eas- ier to test and verify thoroughly. With a complex design, there are many more opportunities for an adversary to discover subtle weaknesses to exploit that may be difficult to spot ahead of time. The more complex the mechanism, the more likely it is to possess exploitable flaws. Simple mechanisms tend to have fewer exploitable flaws and require less maintenance. Further, because configuration management issues are simplified, updating or replacing a simple mechanism becomes a less intensive process. In practice, this is perhaps the most difficult principle to honor. There is a constant demand for new features in both hard- ware and software, complicating the security design task. The best that can be done is to keep this principle in mind during system design to try to eliminate unnecessary complexity.

Fail-safe defaults means that access decisions should be based on permission rather than exclusion. That is, the default situation is lack of access, and the protec- tion scheme identifies conditions under which access is permitted. This approach


exhibits a better failure mode than the alternative approach, where the default is to permit access. A design or implementation mistake in a mechanism that gives explicit permission tends to fail by refusing permission, a safe situation that can be quickly detected. On the other hand, a design or implementation mistake in a mechanism that explicitly excludes access tends to fail by allowing access, a failure that may long go unnoticed in normal use. Most file access systems and virtually all protected services on client/server systems use fail-safe defaults.

Complete mediation means that every access must be checked against the access control mechanism. Systems should not rely on access decisions retrieved from a cache. In a system designed to operate continuously, this principle requires that, if access decisions are remembered for future use, careful consideration be given to how changes in authority are propagated into such local memories. File access systems appear to provide an example of a system that complies with this principle. However, typically, once a user has opened a file, no check is made to see if permissions change. To fully implement complete mediation, every time a user reads a field or record in a file, or a data item in a database, the system must exercise access control. This resource-intensive approach is rarely used.

Open design means that the design of a security mechanism should be open rather than secret. For example, although encryption keys must be secret, encryption algorithms should be open to public scrutiny. The algorithms can then be reviewed by many experts, and users can therefore have high confidence in them. This is the philosophy behind the National Institute of Standards and Technology (NIST) program of standardizing encryption and hash algorithms, and has led to the wide- spread adoption of NIST-approved algorithms.

Separation of privilege is defined in [SALT75] as a practice in which mul- tiple privilege attributes are required to achieve access to a restricted resource. A good example of this is multifactor user authentication, which requires the use of multiple techniques, such as a password and a smart card, to authorize a user. The term is also now applied to any technique in which a program is divided into parts that are limited to the specific privileges they require in order to perform a specific task. This is used to mitigate the potential damage of a computer security attack. One example of this latter interpretation of the principle is removing high privilege operations to another process and running that process with the higher privileges required to perform its tasks. Day-to-day interfaces are executed in a lower privi- leged process.

Least privilege means that every process and every user of the system should operate using the least set of privileges necessary to perform the task. A good example of the use of this principle is role-based access control. The system security policy can identify and define the various roles of users or processes. Each role is assigned only those permissions needed to perform its functions. Each permission specifies a permitted access to a particular resource (such as read and write access to a specified file or directory, connect access to a given host and port). Unless a permission is granted explicitly, the user or process should not be able to access the protected resource. More generally, any access control system should allow each user only the privileges that are authorized for that user. There is also a temporal aspect to the least privilege principle. For example, system programs or administra- tors who have special privileges should have those privileges only when necessary;


when they are doing ordinary activities the privileges should be withdrawn. Leaving them in place just opens the door to accidents.

Least common mechanism means that the design should minimize the func- tions shared by different users, providing mutual security. This principle helps reduce the number of unintended communication paths and reduces the amount of hardware and software on which all users depend, thus making it easier to verify if there are any undesirable security implications.

Psychological acceptability implies that the security mechanisms should not interfere unduly with the work of users, while at the same time meeting the needs of those who authorize access. If security mechanisms hinder the usability or accessibil- ity of resources, then users may opt to turn off those mechanisms. Where possible, security mechanisms should be transparent to the users of the system or at most introduce minimal obstruction. In addition to not being intrusive or burdensome, security procedures must reflect the user’s mental model of protection. If the protec- tion procedures do not make sense to the user or if the user must translate his image of protection into a substantially different protocol, the user is likely to make errors.

Isolation is a principle that applies in three contexts. First, public access sys- tems should be isolated from critical resources (data, processes, etc.) to prevent dis- closure or tampering. In cases where the sensitivity or criticality of the information is high, organizations may want to limit the number of systems on which that data is stored and isolate them, either physically or logically. Physical isolation may include ensuring that no physical connection exists between an organization’s public access information resources and an organization’s critical information. When implement- ing logical isolation solutions, layers of security services and mechanisms should be established between public systems and secure systems responsible for protecting critical resources. Second, the processes and files of individual users should be iso- lated from one another except where it is explicitly desired. All modern operating systems provide facilities for such isolation, so that individual users have separate, isolated process space, memory space, and file space, with protections for prevent- ing unauthorized access. And finally, security mechanisms should be isolated in the sense of preventing access to those mechanisms. For example, logical access control may provide a means of isolating cryptographic software from other parts of the host system and for protecting cryptographic software from tampering and the keys from replacement or disclosure.

Encapsulation can be viewed as a specific form of isolation based on object- oriented functionality. Protection is provided by encapsulating a collection of pro- cedures and data objects in a domain of its own so that the internal structure of a data object is accessible only to the procedures of the protected subsystem, and the procedures may be called only at designated domain entry points.

Modularity in the context of security refers both to the development of security functions as separate, protected modules and to the use of a modular architecture for mechanism design and implementation. With respect to the use of separate security modules, the design goal here is to provide common security functions and services, such as cryptographic functions, as common modules. For example, numerous proto- cols and applications make use of cryptographic functions. Rather than implement- ing such functions in each protocol or application, a more secure design is provided by developing a common cryptographic module that can be invoked by numerous


protocols and applications. The design and implementation effort can then focus on the secure design and implementation of a single cryptographic module and includ- ing mechanisms to protect the module from tampering. With respect to the use of a modular architecture, each security mechanism should be able to support migration to new technology or upgrade of new features without requiring an entire system redesign. The security design should be modular so that individual parts of the secu- rity design can be upgraded without the requirement to modify the entire system.

Layering refers to the use of multiple, overlapping protection approaches addressing the people, technology, and operational aspects of information systems. By using multiple, overlapping protection approaches, the failure or circumven- tion of any individual protection approach will not leave the system unprotected. We will see throughout this book that a layering approach is often used to provide multiple barriers between an adversary and protected information or services. This technique is often referred to as defense in depth.

Least astonishment means that a program or user interface should always respond in the way that is least likely to astonish the user. For example, the mechanism for authorization should be transparent enough to a user that the user has a good intui- tive understanding of how the security goals map to the provided security mechanism.


In Section 1.3, we provided an overview of the spectrum of security threats and attacks facing computer and network systems. Section 22.1 goes into more detail about the nature of attacks and the types of adversaries that present security threats. In this section, we elaborate on two concepts that are useful in evaluating and clas- sifying threats: attack surfaces and attack trees.

Attack Surfaces

An attack surface consists of the reachable and exploitable vulnerabilities in a sys- tem [MANA11, HOWA03]. Examples of attack surfaces are the following:

■ Open ports on outward facing Web and other servers, and code listening on those ports

■ Services available on the inside of a firewall

■ Code that processes incoming data, email, XML, office documents, and indus- try-specific custom data exchange formats

■ Interfaces, SQL, and Web forms

■ An employee with access to sensitive information vulnerable to a social engineering attack

Attack surfaces can be categorized as follows:

■ Network attack surface: This category refers to vulnerabilities over an enterprise network, wide-area network, or the Internet. Included in this category are net- work protocol vulnerabilities, such as those used for a denial-of-service attack, disruption of communications links, and various forms of intruder attacks.


■ Software attack surface: This refers to vulnerabilities in application, utility, or operating system code. A particular focus in this category is Web server software.

■ Human attack surface: This category refers to vulnerabilities created by personnel or outsiders, such as social engineering, human error, and trusted insiders.

An attack surface analysis is a useful technique for assessing the scale and severity of threats to a system. A systematic analysis of points of vulnerability makes developers and security analysts aware of where security mechanisms are required. Once an attack surface is defined, designers may be able to find ways to make the surface smaller, thus making the task of the adversary more difficult. The attack surface also provides guidance on setting priorities for testing, strengthening security measures, and modifying the service or application.

As illustrated in Figure 1.3, the use of layering, or defense in depth, and attack surface reduction complement each other in mitigating security risk.

Attack Trees

An attack tree is a branching, hierarchical data structure that represents a set of poten- tial techniques for exploiting security vulnerabilities [MAUW05, MOOR01, SCHN99]. The security incident that is the goal of the attack is represented as the root node of the tree, and the ways that an attacker could reach that goal are iteratively and incre- mentally represented as branches and subnodes of the tree. Each subnode defines a subgoal, and each subgoal may have its own set of further subgoals, and so on. The final nodes on the paths outward from the root, that is, the leaf nodes, represent differ- ent ways to initiate an attack. Each node other than a leaf is either an AND-node or an OR-node. To achieve the goal represented by an AND-node, the subgoals represented by all of that node’s subnodes must be achieved; and for an OR-node, at least one of the subgoals must be achieved. Branches can be labeled with values representing dif- ficulty, cost, or other attack attributes, so that alternative attacks can be compared.

Figure 1.3 Defense in Depth and Attack Surface

Attack surface

Medium security risk

High security risk

Low security riskD

ee p

L ay

er in


Sh al

lo w

Small Large

Medium security risk


The motivation for the use of attack trees is to effectively exploit the infor- mation available on attack patterns. Organizations such as CERT publish security advisories that have enabled the development of a body of knowledge about both general attack strategies and specific attack patterns. Security analysts can use the attack tree to document security attacks in a structured form that reveals key vul- nerabilities. The attack tree can guide both the design of systems and applications, and the choice and strength of countermeasures.

Figure 1.4, based on a figure in [DIMI07], is an example of an attack tree analysis for an Internet banking authentication application. The root of the tree is the objective of the attacker, which is to compromise a user’s account. The shaded boxes on the tree are the leaf nodes, which represent events that comprise the attacks. Note that in this tree, all the nodes other than leaf nodes are OR-nodes. The analysis to generate this tree considered the three components involved in authentication:

Figure 1.4 An Attack Tree for Internet Banking Authentication

Bank account compromise

User credential compromise

User credential guessing

UT/U1a User surveillance

UT/U1b Theft of token and handwritten notes

Malicious software installation Vulnerability exploit

UT/U2a Hidden code

UT/U2b Worms

UT/U3a Smartcard analyzers

UT/U2c Emails with malicious code

UT/U3b Smartcard reader manipulator

UT/U3c Brute force attacks with PIN calculators

CC2 Sniffing

UT/U4a Social engineering

IBS3 Web site manipulation

UT/U4b Web page obfuscation

CC1 Pharming

Redirection of communication toward fraudulent site

CC3 Active man-in-the middle attacks

IBS1 Brute force attacks

User communication with attacker

Injection of commands

Use of known authenticated session by attacker

Normal user authentication with specified session ID

CC4 Pre-defined session IDs (session hijacking)

IBS2 Security policy violation


■ User terminal and user (UT/U): These attacks target the user equipment, including the tokens that may be involved, such as smartcards or other pass- word generators, as well as the actions of the user.

■ Communications channel (CC): This type of attack focuses on communica- tion links.

■ Internet banking server (IBS): These types of attacks are offline attacks against the servers that host the Internet banking application.

Five overall attack strategies can be identified, each of which exploits one or more of the three components. The five strategies are as follows:

■ User credential compromise: This strategy can be used against many ele- ments of the attack surface. There are procedural attacks, such as monitoring a user’s action to observe a PIN or other credential, or theft of the user’s token or handwritten notes. An adversary may also compromise token information using a variety of token attack tools, such as hacking the smart- card or using a brute force approach to guess the PIN. Another possible strategy is to embed malicious software to compromise the user’s login and password. An adversary may also attempt to obtain credential information via the communication channel (sniffing). Finally, an adversary may use various means to engage in communication with the target user, as shown in Figure 1.4.

■ Injection of commands: In this type of attack, the attacker is able to intercept communication between the UT and the IBS. Various schemes can be used to be able to impersonate the valid user and so gain access to the banking system.

■ User credential guessing: It is reported in [HILT06] that brute force attacks against some banking authentication schemes are feasible by sending ran- dom usernames and passwords. The attack mechanism is based on distributed zombie personal computers, hosting automated programs for username- or password-based calculation.

■ Security policy violation: For example, violating the bank’s security policy in combination with weak access control and logging mechanisms, an em- ployee may cause an internal security incident and expose a customer’s account.

■ Use of known authenticated session: This type of attack persuades or forces the user to connect to the IBS with a preset session ID. Once the user authen- ticates to the server, the attacker may utilize the known session ID to send packets to the IBS, spoofing the user’s identity.

Figure 1.4 provides a thorough view of the different types of attacks on an Internet banking authentication application. Using this tree as a starting point, secu- rity analysts can assess the risk of each attack and, using the design principles out- lined in the preceding section, design a comprehensive security facility. [DIMO07] provides a good account of the results of this design effort.



A model for much of what we will be discussing is captured, in very general terms, in Figure 1.5. A message is to be transferred from one party to another across some sort of Internet service. The two parties, who are the principals in this transaction, must cooperate for the exchange to take place. A logical information channel is established by defining a route through the Internet from source to destination and by the coop- erative use of communication protocols (e.g., TCP/IP) by the two principals.

Security aspects come into play when it is necessary or desirable to protect the information transmission from an opponent who may present a threat to confidentiality, authenticity, and so on. All the techniques for providing security have two components:

■ A security-related transformation on the information to be sent. Examples include the encryption of the message, which scrambles the message so that it is unreadable by the opponent, and the addition of a code based on the con- tents of the message, which can be used to verify the identity of the sender.

■ Some secret information shared by the two principals and, it is hoped, unknown to the opponent. An example is an encryption key used in conjunc- tion with the transformation to scramble the message before transmission and unscramble it on reception.6

A trusted third party may be needed to achieve secure transmission. For example, a third party may be responsible for distributing the secret information

6Part Two discusses a form of encryption, known as a symmetric encryption, in which only one of the two principals needs to have the secret information.

Figure 1.5 Model for Network Security

Information channelSecurity-related



Secret information

M es

sa ge

M es

sa ge

Se cu

re m

es sa


Se cu

re m

es sa




Trusted third party (e.g., arbiter, distributer of secret information)

Security-related transformation

Secret information


to the two principals while keeping it from any opponent. Or a third party may be needed to arbitrate disputes between the two principals concerning the authenticity of a message transmission.

This general model shows that there are four basic tasks in designing a par- ticular security service:

1. Design an algorithm for performing the security-related transformation. The algorithm should be such that an opponent cannot defeat its purpose.

2. Generate the secret information to be used with the algorithm.

3. Develop methods for the distribution and sharing of the secret information.

4. Specify a protocol to be used by the two principals that makes use of the security algorithm and the secret information to achieve a particular security service.

Parts One through Five of this book concentrate on the types of security mechanisms and services that fit into the model shown in Figure 1.5. However, there are other security-related situations of interest that do not neatly fit this model but are considered in this book. A general model of these other situations is illustrated in Figure 1.6, which reflects a concern for protecting an information system from unwanted access. Most readers are familiar with the concerns caused by the existence of hackers, who attempt to penetrate systems that can be accessed over a network. The hacker can be someone who, with no malign intent, simply gets satisfaction from breaking and entering a computer system. The intruder can be a disgruntled employee who wishes to do damage or a criminal who seeks to exploit computer assets for financial gain (e.g., obtaining credit card numbers or perform- ing illegal money transfers).

Another type of unwanted access is the placement in a computer system of logic that exploits vulnerabilities in the system and that can affect application pro- grams as well as utility programs, such as editors and compilers. Programs can pres- ent two kinds of threats:

■ Information access threats: Intercept or modify data on behalf of users who should not have access to that data.

■ Service threats: Exploit service flaws in computers to inhibit use by legitimate users.

Figure 1.6 Network Access Security Model

Computing resources (processor, memory, I/O)




Internal security controls

Information system

Gatekeeper function

Opponent —human (e.g., hacker) —software (e.g., virus, worm)

Access channel

1.9 / STANDARDS 43

Viruses and worms are two examples of software attacks. Such attacks can be introduced into a system by means of a disk that contains the unwanted logic con- cealed in otherwise useful software. They can also be inserted into a system across a network; this latter mechanism is of more concern in network security.

The security mechanisms needed to cope with unwanted access fall into two broad categories (see Figure 1.6). The first category might be termed a gatekeeper function. It includes password-based login procedures that are designed to deny access to all but authorized users and screening logic that is designed to detect and reject worms, viruses, and other similar attacks. Once either an unwanted user or unwanted software gains access, the second line of defense consists of a vari- ety of internal controls that monitor activity and analyze stored information in an attempt to detect the presence of unwanted intruders. These issues are explored in Part Six.


Many of the security techniques and applications described in this book have been specified as standards. Additionally, standards have been developed to cover man- agement practices and the overall architecture of security mechanisms and services. Throughout this book, we describe the most important standards in use or that are being developed for various aspects of cryptography and network security. Various organizations have been involved in the development or promotion of these stan- dards. The most important (in the current context) of these organizations are as follows:

■ National Institute of Standards and Technology: NIST is a U.S. federal agency that deals with measurement science, standards, and technology related to U.S. government use and to the promotion of U.S. private-sector innovation. Despite its national scope, NIST Federal Information Processing Standards (FIPS) and Special Publications (SP) have a worldwide impact.

■ Internet Society: ISOC is a professional membership society with world- wide organizational and individual membership. It provides leadership in addressing issues that confront the future of the Internet and is the organiza- tion home for the groups responsible for Internet infrastructure standards, including the Internet Engineering Task Force (IETF) and the Internet Architecture Board (IAB). These organizations develop Internet stan- dards and related specifications, all of which are published as Requests for Comments (RFCs).

■ ITU-T: The International Telecommunication Union (ITU) is an interna- tional organization within the United Nations System in which governments and the private sector coordinate global telecom networks and services. The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors of the ITU. ITU-T’s mission is the development of technical standards covering all fields of telecommunications. ITU-T standards are referred to as Recommendations.


■ ISO: The International Organization for Standardization (ISO)7 is a world- wide federation of national standards bodies from more than 140 countries, one from each country. ISO is a nongovernmental organization that promotes the development of standardization and related activities with a view to fa- cilitating the international exchange of goods and services and to developing cooperation in the spheres of intellectual, scientific, technological, and eco- nomic activity. ISO’s work results in international agreements that are pub- lished as International Standards.

A more detailed discussion of these organizations is contained in Appendix D.


7ISO is not an acronym (in which case it would be IOS), but it is a word, derived from the Greek, mean- ing equal.

Key Terms

access control active attack authentication authenticity availability data confidentiality data integrity

denial of service encryption integrity intruder masquerade nonrepudiation OSI security architecture

passive attack replay security attacks security mechanisms security services traffic analysis

Review Questions

1.1 What is the OSI security architecture? 1.2 List and briefly define the three key objectives of computer security. 1.3 List and briefly define categories of passive and active security attacks. 1.4 List and briefly define categories of security services. 1.5 List and briefly define categories of security mechanisms. 1.6 List and briefly define the fundamental security design principles. 1.7 Explain the difference between an attack surface and an attack tree.


1.1 Consider an automated cash deposit machine in which users provide a card or an ac- count number to deposit cash. Give examples of confidentiality, integrity, and avail- ability requirements associated with the system, and, in each case, indicate the degree of importance of the requirement.

1.2 Repeat Problem 1.1 for a payment gateway system where a user pays for an item using their account via the payment gateway.


1.3 Consider a financial report publishing system used to produce reports for various organizations. a. Give an example of a type of publication in which confidentiality of the stored

data is the most important requirement. b. Give an example of a type of publication in which data integrity is the most im-

portant requirement. c. Give an example in which system availability is the most important requirement.

1.4 For each of the following assets, assign a low, moderate, or high impact level for the loss of confidentiality, availability, and integrity, respectively. Justify your answers. a. A student maintaining a blog to post public information. b. An examination section of a university that is managing sensitive information

about exam papers. c. An information system in a pathological laboratory maintaining the patient’s data. d. A student information system used for maintaining student data in a university

that contains both personal, academic information and routine administrative in- formation (not privacy related). Assess the impact for the two data sets separately and the information system as a whole.

e. A University library contains a library management system which controls the distribution of books amongst the students of various departments. The library management system contains both the student data and the book data. Assess the impact for the two data sets separately and the information system as a whole.

1.5 Draw a matrix similar to Table 1.4 that shows the relationship between security ser- vices and attacks.

1.6 Draw a matrix similar to Table 1.4 that shows the relationship between security mechanisms and attacks.

1.7 Develop an attack tree for gaining access to the contents of a physical safe. 1.8 Consider a company whose operations are housed in two buildings on the same prop-

erty; one building is headquarters, the other building contains network and computer services. The property is physically protected by a fence around the perimeter, and the only entrance to the property is through this fenced perimeter. In addition to the perimeter fence, physical security consists of a guarded front gate. The local net- works are split between the Headquarters’ LAN and the Network Services’ LAN. Internet users connect to the Web server through a firewall. Dial-up users get access to a particular server on the Network Services’ LAN. Develop an attack tree in which the root node represents disclosure of proprietary secrets. Include physical, social engineering, and technical attacks. The tree may contain both AND and OR nodes. Develop a tree that has at least 15 leaf nodes.

1.9 Read all of the classic papers cited in the Recommended Reading section for this chapter, available at the Author Web site at The papers are available at Compose a 500–1000 word paper (or 8–12 slide PowerPoint presentation) that summarizes the key concepts that emerge from these papers, emphasizing concepts that are common to most or all of the papers.

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