At present, common security threats on Internet can be categorized into the following types: unauthorized use, Denial of Service (DoS), information theft and data tampering.

These four types of security threats are represented as follows:

• Unauthorized use

  Resources are used by an unauthorized user (also called illegal user) or in unauthorized mode (also called illegal authorization).

  For example, an attacker makes out the user name and password to access a computer system and use resources illegally.

• DoS

  The server denies legal access requests from legal users.

  For example, an attacker sends a large number of data packets to the server within a short time to prevent the server from processing legal tasks due to overload.

• Information theft

  An attacker eavesdrops the system and obtains significant data or information from the network instead of attacking the system directly.

• Data tampering

  An attacker undermines data integrity by modifying, deleting, delaying, reordering system data or message streams, or inserting false messag