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network services, communications, and access control mechanisms—is functional and allows authorized users to gain authorized access.

      For availability to be maintained on a system, controls must be in place to ensure authorized access and an acceptable level of performance, to quickly handle interruptions, provide for redundancy, maintain reliable backups, and prevent data loss or destruction.

      There are numerous threats to availability. These include device failure, software errors, and environmental issues (heat, static electricity, flooding, power loss, and so on). Some forms of attack focus on the violation of availability, including DoS attacks, object destruction, and communication interruptions.

      Many availability breaches are caused by human error, oversight, or ineptitude. They can also occur because of an oversight in a security policy or a misconfigured security control.

      Numerous countermeasures can ensure availability against possible threats. These include designing intermediary delivery systems properly, using access controls effectively, monitoring performance and network traffic, using firewalls and routers to prevent DoS attacks, implementing redundancy for critical systems, and maintaining and testing backup systems. Most security policies, as well as business continuity planning (BCP), focus on the use of fault tolerance features at the various levels of access/storage/security (that is, disk, server, or site) with the goal of eliminating single points of failure to maintain availability of critical systems.

      Availability depends on both integrity and confidentiality. Without integrity and confidentiality, availability cannot be maintained. Concepts, conditions, and aspects of availability include the following:

       Usability: The state of being easy to use or learn or being able to be understood and controlled by a subject

       Accessibility: The assurance that the widest range of subjects can interact with a resource regardless of their capabilities or limitations

       Timeliness: Being prompt, on time, within a reasonable time frame, or providing low-latency response

      DAD, Overprotection, Authenticity, Non-repudiation, and AAA Services

      In addition to the CIA Triad, you need to consider a plethora of other security-related concepts and principles when designing a security policy and deploying a security solution. These include the DAD Triad, the risks of overprotection, authenticity, nonrepudiation, and AAA services.

One interesting security concept is the opposite of the CIA Triad, which is the DAD Triad. Disclosure, alteration, and destruction make up the DAD Triad. The DAD Triad represents the failures of security protections in the CIA Triad. It may be useful to recognize what to look for when a security mechanism fails. Disclosure occurs when sensitive or confidential material is accessed by unauthorized entities, it is a violation of confidentiality. Alternation occurs when data is either maliciously or accidentally changed, it is a violation of integrity. Destruction occurs when a resource is damaged or made inaccessible to authorized users (technically we usually call the later denial of service (DoS)), it is a violation of availability.

      It may also be worthwhile to know that too much security can be its own problem. Overprotecting confidentiality can result in a restriction of availability. Overprotecting integrity can result in a restriction of availability. Overproviding availability can result in a loss of confidentiality and integrity.

      Authenticity is the security concept that data is authentic or genuine and originates from its alleged source. This is related to integrity, but it's more closely related to verifying that it is from a claimed origin. When data has authenticity, the recipient can have a high level of confidence that the data is from whom it claims to be from and that it did not change in transit (or storage).

      Nonrepudiation ensures that the subject of an activity or who caused an event cannot deny that the event occurred. Nonrepudiation prevents a subject from claiming not to have sent a message, not to have performed an action, or not to have been the cause of an event. It is made possible through identification, authentication, authorization, accountability, and auditing. Nonrepudiation can be established using digital certificates, session identifiers, transaction logs, and numerous other transactional and access control mechanisms. A system built without proper enforcement of nonrepudiation does not provide verification that a specific entity performed a certain action. Nonrepudiation is an essential part of accountability. A suspect cannot be held accountable if they can repudiate the claim against them.

      AAA services is a core security mechanism of all security environments. The three As in this abbreviation refer to authentication, authorization, and accounting (or sometimes auditing). However, what is not as clear is that although there are three letters in the acronym, it actually refers to five elements: identification, authentication, authorization, auditing, and accounting. These five elements represent the following processes of security:

       Identification Identification is claiming to be an identity when attempting to access a secured area or system.

       Authentication Authentication is proving that you are that claimed identity.

       Authorization Authorization is defining the permissions (i.e., allow/grant and/or deny) of a resource and object access for a specific identity or subject.

       Auditing Auditing is recording a log of the events and activities related to the system and subjects.

       Accounting Accounting (aka accountability) is reviewing log files to check for compliance and violations in order to hold subjects accountable for their actions, especially violations of organizational security policy.

Although AAA is typically referenced in relation to authentication systems, it is actually a foundational concept for security. Missing any of these five elements can result in an incomplete security mechanism. The following sections discuss identification, authentication, authorization, auditing, and accountability (see Figure 1.2).

FIGURE 1.2 The five elements of AAA services

      Identification

A subject must perform identification to start the process of authentication, authorization, and accountability (AAA). Providing an identity can involve typing in a username; swiping a smartcard; waving a proximity device; speaking a phrase; or positioning your face, hand, or finger for a camera or scanning device. Without an identity, a system has no way to correlate an authentication factor with the subject.

      Once a subject has been identified (that is, once the subject's identity has been recognized and verified), the identity is accountable for any further actions by that subject. IT systems track activity by identities, not by the subjects themselves. A computer doesn't know one individual from another, but it does know that your user account is different from all other user accounts. Simply claiming an identity does not imply access or authority. The identity must be proven before use. That process is authentication.

      Authentication

      The process of verifying whether a claimed identity is valid is authentication. Authentication requires the subject to provide additional information that corresponds to the identity they are claiming. The most common form of authentication is using a password. Authentication verifies the identity of the subject by comparing one or more factors against the database of valid identities (that is, user accounts). The capability of the subject and system to maintain the secrecy of the authentication factors for identities directly reflects the level of security of that system.

      Identification and authentication are often used together as a single two-step process. Providing an identity is the first step, and providing the

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