what is Modification of messages

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.“

what is Denial of Service Dos Attacks

Denial of Service: prevents or inhibits the normal use or management of 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.

Types of 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.

Masquerade Attacks : 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.

replay attacks : An attack in which a service already authorized and completed is forged by another duplicate request" in an attempt to repeat authorized commands.

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.“

Denial of Service: prevents or inhibits the normal use or management of 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.

Explain TermThreat & Attack in Security perspective

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.


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

Security Mechanism the specific means of implementing one or more security services

• feature designed to detect, prevent, or recover from a security attack
• no single mechanism that will support all services required
• however one particular element underlies many of the security mechanisms in use:
• cryptographic techniques hence our focus on this topic

Note these mechanisms span a wide range of technical components, but one aspect seen in many is the use of cryptographic techniques.

Security Mechanisms (X.800)

• specific security mechanisms:encipherment, digital signatures, access controls, data integrity, authentication exchange, traffic padding, routing control, notarization
• pervasive security mechanisms:trusted functionality, security labels, event detection, security audit trails, security recovery

Security Services provided by (X.800)

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.

• Authentication - assurance that the communicating entity is the one claimed
• Access Control - prevention of the unauthorized use of a resource
• Data Confidentiality –protection of data from unauthorized disclosure
• Data Integrity - assurance that data received is as sent by an authorized entity
• Non-Repudiation - protection against denial by one of the parties in a communication

Passive Attacks

Have “passive attacks” which attempt to learn or make use of information from the system but does not affect system resources.

passive attacks

By eavesdropping on, or monitoring of, transmissions to:

+ obtain message contents (as shown above in Stallings Figure 1.3a), or

+ monitor traffic flows

Are difficult to detect because they do not involve any alteration of the data.

• 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 release of message contents and traffic analysis.
• 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:Also have “active attacks” which attempt to alter system resources or affect their operation.

By modification of data stream to:
+ masquerade of one entity as some other
+ replay previous messages (as shown above in Stallings Figure 1.4b)
+ modify messages in transit
+ denial of service

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.

• 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.

Security Attack: Expand on definition and use of “security attack”,

Security Attacks:

• any action that compromises the security of information owned by an organization
• information security is about how to prevent attacks, or failing that, to detect attacks on information-based systems
• often threat & attack used to mean same thing
• have a wide range of attacks
• can focus of generic types of attacks
• passive
• active

3 aspects of information security

consider 3 aspects of information security:

• security attack
• security mechanism
• security service

 The OSI security architecture focuses on security attacks,mechanisms,and services. These can be defined briefly as follows:

• 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.

OSI Security Architecture

• ITU-T X.800 “Security Architecture for OSI”
• defines a systematic way of defining and providing security requirements
• for us it provides a useful, if abstract, overview of concepts we will study

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. This is difficult enough in a centralized data processing environment; with the use of local and wide area networks,the problems are compounded. ITU-T Recommendation X.800, Security Architecture for OSI, defines such a systematic approach. The OSI security architecture is useful to managers as a way of organizing the task of providing security.