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Access control is the ability to permit or deny the use of something by someone. Access Control in Computer SecurityIn computer security, access control includes authentication, authorization and audit. It also includes measures such as physical devices, including biometric scans and metal locks, hidden paths, digital signatures, encryption, social barriers, and monitoring by humans and automated systems.In any access control model, the entities that can perform actions in the system are called subjects, and the entities representing resources to which access may need to be controlled are called objects (see also Access Control Matrix). Subjects and objects should both be considered as software entities, rather than as human users: any human user can only have an effect on the system via the software entities that they control. Although some systems equate subjects with user IDs, so that all processes started by a user by default have the same authority, this level of control is not fine-grained enough to satisfy the Principle of least privilege, and arguably is responsible for the prevalence of malware in such systems (see computer insecurity). In some models, for example the object-capability model, any software entity can potentially act as both a subject and object. Access control models used by current systems tend to fall into one of two classes: those based on capabilities and those based on access control lists (ACLs). In a capability-based model, holding an unforgeable reference or capability to an object provides access to the object (roughly analogous to how possession of your house key grants you access to your house); access is conveyed to another party by transmitting such a capability over a secure channel. In an ACL-based model, a subject's access to an object depends on whether its identity is on a list associated with the object (roughly analogous to how a bouncer at a private party would check your ID to see if your name is on the guest list); access is conveyed by editing the list. (Different ACL systems have a variety of different conventions regarding who or what is responsible for editing the list and how it is edited.) Both capability-based and ACL-based models have mechanisms to allow access rights to be granted to all members of a group of subjects (often the group is itself modeled as a subject). Access control systems provide the essential services of identification and authentication (I&A), authorization, and accountability where:
Identification and authentication (I&A)Identification and authentication (I&A) is a two-step process that determines who can log on to a system. Identification is how a user tells a system who he or she is (for example, by using a username). The identification component of an access control system is normally a relatively simple mechanism based on either Username or User ID. In the case of a system or process, identification is usually based on:
AuthorizationAuthorization applies to subjects rather than to users (the association between a user and the subjects initially controlled by that user having been determined by I&A). Authorization determines what a subject can do on the system. Most modern operating systems define sets of permissions that are variations or extensions of three basic types of access:
AccountabilityAccountability uses such system components as audit trails (records) and logs to associate a subject with its actions. The information recorded should be sufficient to map the subject to a controlling user. Audit trails and logs are important for detecting or re-creating security violations. If no one is regularly reviewing your logs and they are not maintained in a secure and consistent manner, they may not be admissible as evidence.Many systems can generate automated reports based on certain predefined criteria or thresholds, known as clipping levels. For example, a clipping level may be set to generate a report for the following:
Access Control TechniquesAccess control techniques are sometimes categorized as either discretionary or mandatory.Discretionary Access ControlDiscretionary access control (DAC) is an access policy determined by the owner of an object. The owner decides who is allowed access to the object and what privileges they have.Two important concepts in DAC are:
Mandatory Access ControlMandatory access control (MAC) is an access policy determined by the system, not the owner. MAC is used in multilevel systems that process highly sensitive data, such as classified government and military information. A multilevel system is a single computer system that handles multiple classification levels between subjects and objects.
Rule-based access controls: This type of control further defines specific conditions for access to a requested object. All MAC-based systems implement a simple form of rule-based access control to determine whether access should be granted or denied by matching:
Few systems implement MAC. XTS-400 is an example of one that does. Access Control in TelecommunicationsIn telecommunication, the term access control is defined in U.S. Federal Standard 1037C with the following meanings:
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