Availability

Definition

A service is of no practical use if no one is able to access it. Availability is the property that legitimate principals are able to access a service within a timely manner whenever they may need to do so.

Theory

Availability is typically expressed numerically as the fraction of a total time period during which a service is available. Although one of the keystones of computer security, availability has historically not been emphasized as much as other properties of security such as confidentiality and integrity. This lack of emphasis on availability has changed recently with the rise of open Internet services.

Decreased availability can occur both inadvertently, through failure of hardware, software, or infrastructure, or intentionally, through attacks on the service or infrastructure. The first can be mitigated through redundancy, where the probability of all backups experiencing a failure simultaneously is (hopefully) very low. It is in regard to these random failures...