Computer Security: A Practical Definition 

Computer Security: A Practical Definition

Defining "Computer Security" is not trivial. The difficulty lies in developing a definition that is broad enough to be valid regardless of the system being described, yet specific enough to describe what security really is. In a generic sense, security is "freedom from risk or danger." In the context of computer science, security is the prevention of, or protection against, access to information by unauthorized recipients, and intentional but unauthorized destruction or alteration of that information.
This can be re-stated: "Security is the ability of a system to protect information and system resources with respect to confidentiality and integrity." Note that the scope of this second definition includes system resources, which include CPUs, disks, and programs, in addition to information.
Taxonomy of Computer Security
Computer security is frequently associated with three core areas, which can be conveniently summarized by the acronym "CIA":
Confidentiality -- Ensuring that information is not accessed by unauthorized persons
Integrity -- Ensuring that information is not altered by unauthorized persons in a way that is not detectable by authorized users
Authentication -- Ensuring that users are the persons they claim to be
A strong security protocol addresses all three of these areas. It has enabled an explosion in ecommerce which is really about trust (or more precisely, about the lack of trust). SSL overcomes the lack of trust between transacting parties by ensuring confidentiality through encryption, integrity through checksums, and authentication via server certificates.
Computer security is not restricted to these three broad concepts. Additional ideas that are often considered part of the taxonomy of computer security include:
Access control -- Ensuring that users access only those resources and services that they are entitled to access and that qualified users are not denied access to services that they legitimately expect to receive
Nonrepudiation -- Ensuring that the originators of messages cannot deny that they in fact sent the messages.
Availability -- Ensuring that a system is operational and functional at a given moment, usually provided through redundancy; loss of availability is often referred to as "denial-of-service"
Privacy -- Ensuring that individuals maintain the right to control what information is collected about them, how it is used, who has used it, who maintains it, and what purpose it is used for
These additional elements don't neatly integrate into a singular definition. From one perspective, the concepts of privacy, confidentiality, and security are quite distinct and possess different attributes. Privacy is a property of individuals; confidentiality is a property of data; and security is a property assigned to computer hardware and software systems. From a practical perspective, the concepts are interwoven. A system that does not maintain data confidentiality or individual privacy could be theoretically or even mathematically "secure," but it wouldn't be wise to deploy anywhere in the real world.
A Functional View
Computer security can also be analyzed by function. It can be broken into five distinct functional areas
Risk avoidance -- A security fundamental that starts with questions like: Does my organization or business engage in activities that are too risky? Do we really need an unrestricted Internet connection? Do we really need to computerize that secure business process? Should we really standardize on a desktop operating system with no access control intrinsic?
Deterrence -- Reduces the threat to information assets through fear. Can consist of communication strategies designed to impress potential attackers of the likelihood of getting caught.
Prevention -- The traditional core of computer security. Consists of implementing safeguards like the tools covered Absolute prevention is theoretical, since there's a vanishing point where additional preventative measures are no longer cost-effective.
Detection -- Works best in conjunction with preventative measures. When prevention fails, detection should kick in, preferably while there's still time to prevent damage. Includes log-keeping and auditing activities
Recovery -- When all else fails, be prepared to pull out backup media and restore from scratch, or cut to backup servers and net connections, or fall back on a disaster recovery facility. Arguably, this function should be attended to before the others
Analyzing security by function can be a valuable part of the security planning process; a strong security policy will address all five areas, starting with recovery.

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