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Subliminal channels and information security in Computer Systems

While constructing information security systems, one should allow for existence of the socalled Troyan horses and other hidden holes to attack. In this paper, we model such a situation on the base of a pair of automata, legal and illegal, which are controlled by a common input. We give a definition of a subliminal illegal control channel. Existence conditions and examples of subliminal channels are given. This research was supported by the Russian Foundation for Basic Research, grant 97-01-00162. The following ideology lies in the foundation of constructing a guaranteed information security system [1]. To prevent attacks, a set of rules how to deal with the information is formulated, which ensures the security. These rules are referred to as a security policy. An information security system is intended to realize this policy. Such a system is good if it gives a reliable support for the security policy, and is bad otherwise. Since the most part of the computer hardware and software is imported from abroad, they should be considered as untrustworthy, because they can include bugs (Troyan horses) intended to crack the security. We therefore assume that the use of imported equipment cannot guarantee the fulfillment of security policy. To investigate possibilities to construct a trusted computer system on the base of untrustworthy elements, we should define and analyze some models of attacks. The approach evolved in this paper consists of the following. We consider the attack in the form of an illegal automaton A2 (Troyan horse, bug, etc.) which functions in a legal computer system Al. It seems likely that the bugs (Troyan horses) are not oriented to specific applications and do not possess considerable intellectual capabilities. We can therefore assume that the illegal automaton A2 is initiated by some kind of a logic bomb or by a special signal through control channel, and then receives the application-oriented software, allocates it into memory and executes it. Then, there should be a considerable information flow from the intruder to A2 (illegal programs, auxiliary data, etc.); if such a flow is detected, it is easy to remove the bug. It is clear that the transmission of any information for the automaton A2 is realized through the same input flow which feed the system A1. Some subject U observes the computer system A1, and possesses some function of the current state of Al and of the job fed to the input. *UDC 519.7. Originally published in Diskretnaya Matematika (1998) 10, No. 1, 3-9 (in Russian). Received December 3, 1997. Translated by A. V. Kolchin.