1.3.
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
1. release of message contents and
2. traffic analysis.
1. The release of message contents is easily understood (Figure 1.3a). 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.
Fig:1.3a
2. A second type of passive attack, traffic analysis, is subtler (Figure 1.3b).
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.
Fig 1.3b
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 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:
1. masquerade
2. replay
3.modification of messages and
4. denial of service
1. A masquerade takes place when one entity pretends to be a different entity.
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.
2. Replay involves the passive capture of a data unit and its subsequent
retransmission to produce an unauthorized effect.
3. 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. 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."
4. The denial of service prevents or inhibits the normal use or management of
communications communications facilities. 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.
