THE OSI SECURITY ARCHITECTURE

To assess effectively the security needs of an organization and to evaluate and
choose various security products and policies, the manager responsible for computer
and network security needs some systematic way of defining the requirements
for security and characterizing the approaches to satisfying those requirements.This
is difficult enough in a centralized data processing environment; with the use of
local and wide area networks, the problems are compounded.
ITU-T4 Recommendation X.800, Security Architecture for OSI, defines such a
systematic approach.5 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 architecture
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.




SECURITY ATTACKS

A useful means of classifying security attacks, used both in X.800 and RFC 2828, is
in terms of passive attacks and active attacks. A passive attack attempts to learn or
make use of information from the system but does not affect system resources. An
active attack attempts to alter system resources or affect their operation.
Passive Attacks
Passive attacks 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
conversation, 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 still might 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 apparently normal fashion, and neither the sender nor the
receiver is aware that a third party has read the messages or observed the
traffic pattern. However, it is feasible to prevent the success of these
attacks, usually by means of encryption. Thus, the emphasis in dealing with
passive attacks is on prevention rather than detection.



Active Attacks

Active attacks 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.
A masquerade takes place when one entity pretends to be a different
entity (Figure 1.3a). 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
retransmission to produce an unauthorized effect (Figure 1.3b).
Modification of messages simply means that some portion of a
legitimate message is altered, or that messages are delayed or reordered,
to produce an unauthorized effect (Figure 1.3c). 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.”
The denial of service prevents or inhibits the normal use or management
of communications facilities (Figure 1.3d). 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 messages 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 also may
contribute to prevention.