Eliminating Covert Channels
The possibility of covert channels cannot be completely eliminated, although it can be significantly reduced by careful design and analysis.
The detection of a covert channel can be made more difficult by using characteristics of the communications medium for the legitimate channel that are never controlled or examined by legitimate users. For example, a file can be opened and closed by a program in a specific, timed pattern that can be detected by another program, and the pattern can be interpreted as a string of bits, forming a covert channel. Since it is unlikely that legitimate users will check for patterns of file opening and closing operations, this type of covert channel can remain undetected for long periods.
A similar case is port knocking. In usual communications the timing of requests is irrelevant and unwatched. Port knocking makes it significant.
Read more about this topic: Covert Channel
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