1.3 Policy and Mechanism
Critical to our study of security is the distinction between policy and mechanism.
Definition 11. A security policy is a statement of what is, and what is not, allowed.
Definition 12. A security mechanism is a method, tool, or procedure for enforcing a security policy.
Mechanisms can be nontechnical, such as requiring proof of identity before changing a password; in fact, policies often require some procedural mechanisms that technology cannot enforce.
As an example, suppose a university's computer science laboratory has a policy that prohibits any student from copying another student's homework files. The computer system provides mechanisms for preventing others from reading a user's files. Anna fails to use these mechanisms to protect her homework files, and Bill copies them. A breach of security has occurred, because Bill has violated the security policy. Anna's failure to protect her files does not authorize Bill to copy them.
In this example, Anna could easily have protected her files. In other environments, such protection may not be easy. For example, the Internet provides only the most rudimentary security mechanisms, which are not adequate to protect information sent over that network. Nevertheless, acts such as the recording of passwords and other sensitive information violate an implicit security policy of most sites (specifically, that passwords are a user's confidential property and cannot be recorded by anyone).
Policies may be presented mathematically, as a list of allowed (secure) and disallowed (nonsecure) states. For our purposes, we will assume that any given policy provides an axiomatic description of secure states and nonsecure states. In practice, policies are rarely so precise; they normally describe in English what users and staff are allowed to do. The ambiguity inherent in such a description leads to states that are not classified as "allowed" or "disallowed." For example, consider the homework policy discussed above. If someone looks through another user's directory without copying homework files, is that a violation of security? The answer depends on site custom, rules, regulations, and laws, all of which are outside our focus and may change over time.
When two different sites communicate or cooperate, the entity they compose has a security policy based on the security policies of the two entities. If those policies are inconsistent, either or both sites must decide what the security policy for the combined site should be. The inconsistency often manifests itself as a security breach. For example, if proprietary documents were given to a university, the policy of confidentiality in the corporation would conflict with the more open policies of most universities. The university and the company must develop a mutual security policy that meets both their needs in order to produce a consistent policy. When the two sites communicate through an independent third party, such as an Internet service provider, the complexity of the situation grows rapidly.
1.3.1 Goals of Security
Given a security policy's specification of "secure" and "nonsecure" actions, these security mechanisms can prevent the attack, detect the attack, or recover from the attack. The strategies may be used together or separately.
Prevention means that an attack will fail. For example, if one attempts to break into a host over the Internet and that host is not connected to the Internet, the attack has been prevented. Typically, prevention involves implementation of mechanisms that users cannot override and that are trusted to be implemented in a correct, unalterable way, so that the attacker cannot defeat the mechanism by changing it. Preventative mechanisms often are very cumbersome and interfere with system use to the point that they hinder normal use of the system. But some simple preventative mechanisms, such as passwords (which aim to prevent unauthorized users from accessing the system), have become widely accepted. Prevention mechanisms can prevent compromise of parts of the system; once in place, the resource protected by the mechanism need not be monitored for security problems, at least in theory.
Detection is most useful when an attack cannot be prevented, but it can also indicate the effectiveness of preventative measures. Detection mechanisms accept that an attack will occur; the goal is to determine that an attack is under way, or has occurred, and report it. The attack may be monitored, however, to provide data about its nature, severity, and results. Typical detection mechanisms monitor various aspects of the system, looking for actions or information indicating an attack. A good example of such a mechanism is one that gives a warning when a user enters an incorrect password three times. The login may continue, but an error message in a system log reports the unusually high number of mistyped passwords. Detection mechanisms do not prevent compromise of parts of the system, which is a serious drawback. The resource protected by the detection mechanism is continuously or periodically monitored for security problems.
Recovery has two forms. The first is to stop an attack and to assess and repair any damage caused by that attack. As an example, if the attacker deletes a file, one recovery mechanism would be to restore the file from backup tapes. In practice, recovery is far more complex, because the nature of each attack is unique. Thus, the type and extent of any damage can be difficult to characterize completely. Moreover, the attacker may return, so recovery involves identification and fixing of the vulnerabilities used by the attacker to enter the system. In some cases, retaliation (by attacking the attacker's system or taking legal steps to hold the attacker accountable) is part of recovery. In all these cases, the system's functioning is inhibited by the attack. By definition, recovery requires resumption of correct operation.
In a second form of recovery, the system continues to function correctly while an attack is under way. This type of recovery is quite difficult to implement because of the complexity of computer systems. It draws on techniques of fault tolerance as well as techniques of security and is typically used in safety-critical systems. It differs from the first form of recovery, because at no point does the system function incorrectly. However, the system may disable nonessential functionality. Of course, this type of recovery is often implemented in a weaker form whereby the system detects incorrect functioning automatically and then corrects (or attempts to correct) the error.