Privacy controls how an asset is displayed or exchanged between parties, so it cannot be known beyond those parties. Therefore, to protect secrets with privacy controls, the means of exchange must be protected. Unfortunately, this is extremely difficult to do without also using confidentiality and subjugation controls as well because the user will want to be able to use the same process repeatedly and that hinders good privacy controls.
Currently, some types of privacy controls are inherent in many services that communicate by UDP. If the service request does not match the service, then no response is sent. However, once the service request is known, that same service can be sent repeatedly for any and every system that has that service. Privacy controls require the service request to change every time the secret is revealed, even by authorized users, because there is no way to ensure that someone wasn't watching the interaction that one time. This, however, makes for a lousy protocol.
A famous technique, port knocking, attempts to enhance the use of privacy controls in networking. However, port knocking requires the use of an encrypted tunnel; otherwise, the sequence would have to change each time. You can also change the backend sequence so that even if a third party monitors the request, the result is still not obvious. This technique is used by some certification bodies like ISECOM's OSSTMM
Professional Security Tester (OPST) and Analyst certifications (OPSA), respectively, and most notably when you take the driving portion of the driver's license exam. In this part of the exam, the information that the driver is expected to know is generally known so there are no surprises. However, the test taker has no idea which streets or street conditions, weather conditions, or traffic conditions he or she must deal with. Only the examiner knows this. Therefore, if a technique like port knocking could be used for an important management or administrative service, the protocol to connect to the server could be protected—even if it is discovered because the port that it opens is a changing secret known only to the administrator. Furthermore, if the server receives no connection within a particular time limit then it closes again. This way the administrator needs to know only a limited number of ports to connect to and the attacker is befuddled by needing to find the listening port within a certain time limit.
Privacy controls, together with subjugation, integrity, authentication, and confidentiality controls, create a very tight process that is difficult to penetrate. A carefully constructed privacy control on its own, however, is still a formidable tool, even if just for skills-based certification exams.
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