1. Introduction
Computer attacks are an important security problem while the masquerade attack may be one of the most serious attacks [1] including the smart grid system. Literally masquerading is the attempt of substituting oneself for another. It can be a very serious menace to the computer system security. Some other intrusions may be less severe than Masquerading. Let's take DoS (Denial of Service) for example, DoS is also a kind of usual and serious attack, but its purpose is just stopping prevalent services. Different from the DoS, masquerading may obtain secure information or destroy the whole computer system after getting super privilege. From the view of user, masquerading may be one of the most serious threats. Moreover, the detection of masquerade attacks is more difficulty than that of other attacks. By mimicking the legitimate user's behavior, the masquerade attack won't be detected by most intrusion detection systems. In order to elude detection, some smart masqueraders insert some common or meaningless commands such as “Is” into their command sequences intentionally for concealing their actual purpose, resulting in the miss alarms of intrusion detection system.