With the progress of time, computer

With the progress of time, computer technology has been greatly developed and today's network communication
system has spread to every corner of the world, involving political, economic, military and all walks of social life. It
plays an extremely important role. However, besides fun and convenience, computer also brings to us a lot of
security risks due to its openness and connectivity. Users are now faced with a large number of security threats. Is
computer network safe? Criminal cases are frequently visitors of domestic and international coverage. Reports on
systematic security vulnerabilities are never rare. Table 1 shows the report on security vulnerabilities of information
system by the U.S. security organization CERT / CC.