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 characterizing the approaches to satisfying those requirements. The OSI security architecture was developed in the context of the OSI protocol architecture, which is described in Appendix H. However, for our purposes in this chapter, an understanding of the OSI protocol architecture is not required.
purposes, the OSI security architecture provides a useful, if abstract,
overview of many of the concepts.. The OSI security architecture focuses on
security attacks, mechanisms, and services. These can be defined briefly as
and Attacks (RFC 2828)
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.
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.
Security Attacks, Services And
the security needs of an organization effectively, the manager responsible for
security needs some systematic way of defining the requirements for security
and characterization of approaches to satisfy those requirements. One approach
is to consider three aspects of information security:
attack – Any action that compromises
the security of information owned by an
mechanism – A mechanism that is
designed to detect, prevent or recover from
a security attack.
service – A 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.
classification of security services are as follows:
that the information in a computer system and transmitted information are accessible only for reading by authorized parties.
printing, displaying and other forms of disclosure.
that the origin of a message or electronic document is correctly identified, with an assurance that the identity is not
that only authorized parties are able to modify computer system assets and transmitted information.
Modification includes writing, changing status, deleting, creating and delaying
or replaying of transmitted messages.
repudiation: Requires that neither the sender nor the receiver
of a message be able to deny the
control: Requires that access to information resources may be controlled by or the target system.
Requires that computer system assets be available to authorized parties when needed.
assurance that the communicating entity is the one that it claims to be.
association with a logical connection to provide confidence in the identity of
the entities connected.
connectionless transfer, provides assurance that the source of received data is
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).
protection of data from unauthorized disclosure.
protection of all user data on a connection.
protection of all user data in a single data block
confidentiality of selected fields within the user data on a connection or in a
single data block.
protection of the information that might be derived from observation of traffic
Integrity with Recovery
for the integrity of all user data on a connection and detects any
modification, insertion, deletion, or replay of any data within an entire data
sequence, with recovery attempted.
Integrity without Recovery
but provides only detection without recovery.
for the integrity of selected fields within the user data of a data block
transferred over a connection and takes the form of determination of whether
the selected fields have been modified, inserted, deleted, or replayed.
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.
the integrity of selected fields within a single connectionless data block;
takes the form of determination of whether the selected fields have been
protection against denial by one of the entities involved in a communication of
having participated in all or part of the communication.
that the message was sent by the specified party.
the most specific security mechanisms in use is cryptographic techniques.
Encryption or encryption-like transformations of information are the most
common means of providing security. Some of the mechanisms are:
four general categories of attack which are listed below.
of the system is destroyed or becomes unavailable or unusable. This is an
attack on availability.
destruction of piece of hardware, cutting of a communication line or disabling
of file management system.
unauthorized party gains access to an asset. This is an attack on
confidentiality. Unauthorized party could be a person, a program or a
computer.e.g., wire tapping to capture data in the network, illicit copying of
unauthorized party not only gains access to but tampers with an asset. This is
an attack on integrity.
changing values in data file, altering a program, modifying the contents of messages
being transmitted in a network.
unauthorized party inserts counterfeit objects into the system. This is an
attack on authenticity.
insertion of spurious message in a network or addition of records to a file.
A useful categorization of these
attacks is in terms of
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.
Passive attacks are of two types:
message contents: A telephone conversation, an e-mail message and a transferred file may contain sensitive
or confidential information. We would like to prevent the opponent from
learning the contents of these transmissions.
analysis: If we had encryption protection in place, an opponent might still be able to observe the pattern of the
message. The opponent could determine the location and identity of communication
hosts and could observe the frequency and length of messages being exchanged.
This information might be useful in guessing the nature of
that was taking place.
attacks are very difficult to detect because they do not involve any alteration
of data. However, it is feasible to prevent the success of these attacks.
attacks involve some modification of the data stream or the creation of a false
attacks can be classified in to four categories:
– One entity pretends to be a different
Replay – involves passive capture of a data
unit and its subsequent transmission to produce
an unauthorized effect.
of messages – Some
portion of message is altered or the messages are delayed or recorded, to produce an unauthorized effect.
service – Prevents or inhibits the
normal use or management of communication
facilities. Another form of service denial is the disruption of an entire
network, either by disabling the network or overloading it with messages so as
to degrade performance.
quite difficult to prevent active attacks absolutely, because to do so would
require physical protection of all communication facilities and paths at all
times. Instead, the goal is to detect them and to recover from any disruption
or delays caused by them.
Symmetric and public key
methods fall into two categories.
symmetric key algorithms, the encryption and decryption keys are known both to
sender and receiver. The encryption key is shared and the decryption key is
easily calculated from it. In many cases, the encryption and decryption keys
are the same.
key cryptography, encryption key is made public, but it is computationally
infeasible to find the decryption key without the information known to the