Bảo mật hệ thống mạng part 5 ppsx

In order to perform this service properly, the confidentiality service must work with the accountability service to properly identify individuals.. The confidentiality ser-vice must take

CHAPTER 3

Information Security Services

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Information security services are the base-level services that are used to combat the

at-tacks defined in Chapter 2 Each of the four security services combats specific atat-tacks (see Table 3-1) The services defined here should not be confused with security mecha-nisms, which are the actual implementations of these services

The specifics of how information security services are used within an organization de-pend upon proper risk assessment and security planning (see Chapters 6 and 7) However,

to understand the basic requirements for security within an organization, it is important to understand how security services can be used to counter specific types of attacks

CONFIDENTIALITY

The confidentiality service provides for the secrecy of information When properly used, confidentiality only allows authorized users to have access to information In order to perform this service properly, the confidentiality service must work with the accountability service to properly identify individuals In performing this function, the confidentiality service protects against the access attack The confidentiality ser-vice must take into account the fact that information may reside in physical form in paper files, in electronic form in electronic files, and in transit

Confidentiality of Files

There are different ways to provide for the confidentiality of files depending upon the way in which the file exists For paper files, the physical paper file must be protected The physical file must exist at a particular location; therefore, access to this location must be controlled The confidentiality service for paper files relies on physical access controls This includes locks on file cabinets or desk drawers, restricted rooms within a site, or ac-cess restrictions on the site itself

If the files are electronic, they have different characteristics First, the files may exist in several locations at the same time (backup tapes, various computer systems, floppy disks or

Security Service Attack Confidentiality Integrity Availability Accountability

Denial of service X

Table 3-1. Information Security Services vs Attacks

CDs, and so on) Second, physical access to the file’s physical location may not be necessary.

Handling the confidentiality of tapes and disks is similar to handling the physical security of

paper files Since an attacker must physically access the tape or disk, confidentiality requires

physical access controls Access to electronic files on computer systems relies on some type

of computer access control (this may include the encryption of files) Computer access

con-trol relies on proper identification and authentication (an accountability service) and proper

system configuration so that an unauthorized user cannot become an authorized user by

by-passing the identification and authentication function (such as via a system vulnerability)

Table 3-2 shows the mechanisms and requirements for the confidentiality of files

Confidentiality of Information in Transmission

Only protecting information stored in files is not sufficient to properly protect the

infor-mation Information can also be attacked while in transmission Therefore, protecting the

confidentiality of information in transmission may also be necessary (see Figure 3-1); this

is done through the use of encryption

Information can be protected on a per-message basis or by encrypting all traffic on a link

Encryption by itself can prevent eavesdropping but it cannot completely prevent

intercep-tion In order to protect information from being intercepted, proper identification and

au-thentication must be used to determine the identity of the remote end point (see Figure 3-2)

Traffic Flow Confidentiality

Unlike other confidentiality services, traffic flow confidentiality is not concerned with the

actual information being stored or transmitted Traffic flow confidentiality is concerned

with the fact that some form of traffic is occurring between two end points (see

Fig-ure 3-3) This type of information can be used (by a traffic analyst) to identify

organiza-tions that are communicating The amount of traffic flowing between the two end points

may also indicate some information For example, many news organizations watch

deliv-eries of pizza to the White House and the Pentagon The idea is that an increase in the

number of pizzas may indicate a crisis is occurring

Chapter 3: Information Security Services 29

Confidentiality mechanisms Physical security controls

Computer file access control Encryption of files

File confidentiality requirements Identification and authentication

Proper computer system configuration Proper key management if encryption is used

Table 3-2. File Confidentiality Mechanisms and Requirements

Traffic flow confidentiality can be provided by obscuring information flows between two end points within a much larger flow of traffic In the military, two sites may set up communications and then send a constant flow of traffic regardless of the number of mes-sages that are actually sent (the remainder is filled up with garbage) In this way, the amount

of traffic remains constant and any changes to the message rate will not be detected

Attacks That Can Be Prevented

Confidentiality can prevent access attacks However, confidentiality by itself cannot completely solve the problem The confidentiality service must work with the account-ability service to establish the identity of the individual who is attempting to access infor-mation Combined, the confidentiality and accountability services can reduce the risk of unauthorized access

INTEGRITY

The integrity service provides for the correctness of information When properly used, in-tegrity allows users to have confidence that the information is correct and has not been modified by an unauthorized individual As with confidentiality, this service must work

30 Network Security: A Beginner’s Guide

Figure 3-1. Encryption can protect information in transmission

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with the accountability service to properly identify individuals The integrity service

pro-tects against modification attacks Information to be protected by the integrity service

may exist in physical paper form, in electronic form, or in transit

Integrity of Files

Information may exist in paper or electronic files Paper files are generally easier to

pro-tect for integrity than electronic files, and it is generally easier to identify when a paper

file was modified I say “generally” here as there is some amount of skill required to

mod-ify a paper file in such a way that it will pass inspection while an electronic file can be

modified by anyone with access to it

There are several ways to protect paper files from modification These include using

sig-nature pages, initialing every page, binding the information in a book, and distributing

mul-tiple copies of the file in question The integrity mechanisms are used to make it very

difficult for a modification to go unnoticed Certainly forgers can copy signatures but this is

a difficult skill Initialing every page makes a simple page replacement difficult Binding

documents into books makes the insertion or deletion of entries or pages difficult Making

multiple copies of the information and distributing the copies to interested parties makes it

difficult to successfully change all of the documents at the same time

Chapter 3: Information Security Services 31

Figure 3-2. Encryption coupled with identification and authentication can protect against

interception

Of course, another way to prevent the modification of paper documents is to prevent unauthorized access completely This can be accomplished through the same mecha-nisms used for confidentiality (that is, physical security measures)

Electronic files are generally easier to modify In many cases, all it takes is to bring the file up in a word processor and insert or delete the appropriate information When the file

is saved, the new information takes the place of the old The primary method of protect-ing the integrity of electronic information files is the same as for protectprotect-ing the confidenti-ality of the information, computer file access control In this case, however, the access

Figure 3-3. Traffic flows can identify which organizations are working together

control mechanism is not configured to completely deny access but instead is configured

to allow for the reading of the file but not for the writing of changes Also, as with

confi-dentiality, it is very important to correctly identify the individual seeking to make a

change This can only be performed through the use of identification and authentication

The use of computer file access controls works well if the files reside on a single

com-puter system or a network within the control of the organization What if the file is to be

copied to other parties or organizations? In this case, it is clear that the access controls on a

single computer system or network are insufficient to provide protection Therefore,

there must be a mechanism that can identify when an unauthorized change has been

made to the file That mechanism is a digital signature (see Chapter 12 for more detail on

digital signatures) A digital signature on a file can identify if the file has been modified

since the signature was created In order to be worthwhile, the digital signature must be

identified with a particular user; thus, the integrity service must work with the

identifica-tion and authenticaidentifica-tion funcidentifica-tion

Integrity of Information Transmission

Information can be modified during transmission However, it is extremely difficult to

modify traffic without performing an interception attack Encryption can prevent most

forms of modification attacks during transmission When coupled with a strong

identifi-cation and authentiidentifi-cation function, even interception attacks can be thwarted (look back

to Figure 3-2)

Attacks That Can Be Prevented

The integrity service can prevent successful modification and repudiation attacks While

any modification attack may change a file or information in transit, modification attacks

cannot be successful if the integrity service is functioning properly as the unauthorized

change will be detected When coupled with a good identification and authentication

ser-vice, even changes to files outside of the organization can be detected

Successful repudiation attacks cannot be prevented without both a good integrity

ser-vice and good identification and authentication In this case, the mechanism to detect the

attack is a digital signature

AVAILABILITY

The availability service provides for information to be useful Availability allows users to

access computer systems, the information on the systems, and the applications that

per-form operations on the inper-formation Availability also provides for the communications

systems to transmit information between locations or computer systems The

informa-tion and capabilities most often thought of when we speak of availability are all

elec-tronic However, the availability of paper information files can also be protected

Chapter 3: Information Security Services 33