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Because the military operates many computers at what is called system high, in which all users are cleared for the highest level of information the network possesses, a sophisticated insider who became a spy would have considerable access. The spy could transmit information to a less closely watched part of the network -- or to an outsider -- without appearing to do so by using what is known as a covert channel. This involves signaling the secret message the agent wants to send in binary code by making minute changes in the speed or the order in which the "bits" of other, entirely innocent messages are transmitted. According to Walker, covert channels have been found that are capable of carrying as much as 1 million bits of information per second. Walker and other experts say they know of no cases in which U.S. covert channels were actually used.
Some steps have been taken to deal with the problem of malicious insiders. "We have put protective mechanisms into systems that are very, very closely held so that very few people know something is keeping track," says Donald Latham. Walker and others now in the private sector are also working to develop "trusted systems" designed to make sure that users obtain only information they are entitled to see and do only things they are authorized to do.
Advocates say such systems will allow computers to be linked in more useful ways without endangering security. Says Walker: "The lack of trusted computer systems is the largest impediment to the effective use of computers in the U.S. today." Until such systems are developed and put in place, computer networks will continue to be at risk -- although the threat cuts both ways. "If you believe the Soviet Union can get into our systems and change them at will," asks a former senior Government expert, "what do you think they think we can do to them?" In the hidden world of computer espionage, the battle may just be gearing up.