网084蒋超英1语论文.doc

湖南软件职业学院计算机专业英语（论文） AbstractComputer Network Security Theory and Practice introduces to the reader a complete and concise view of network security. It provides in-depth theoretical coverage of recent advancements and practical solutions to network security threats. This book can be used for a one-semester network security course for graduate and upper-level undergraduate students, as well as a reference for IT professionals.Computer SecurityI. Threats to Computer Security Threats to computer security are computer crimes （including viruses and electronic break-ins） and natural and other hazards. 1. Computer Criminals A computer crime is an illegal action in which the perpetrator uses special knowledge of computer technology. Computer criminals are of four types: （1） Employees The largest category of computer criminals consists of those with the easiest access to computers-namely, employees. Sometimes the employee is simply trying to steal something from the employer-equipment, software, electronic funds, proprietary information, or computer time. Sometimes the employee may be acting out of resentment and is trying to get back at the company. （2） Outside Users Not only employees but also some suppliers or clients may have access to a companys computer system. Examples are bank customers who use an automatic teller machine. Like employees, these authorized users mayobtain confidential passwords or find other ways of committing computer crimes. （3） “Hackers” and “Crackers” Some people think of these two groups as being the same, but they are not. Hackers are people who gain unauthorized access to a computer system for the fun and challenge of it. Crackers do the same thing but for malicious purposes. They may intend to steal technical information or to introduce what they call a “bomb”-a destructive computer program-into the system. （4） Organized Crime Organized crime has discovered that computers can be used just like legitimate business people use them, but for illegal purposes. For example, computers are useful for keeping track of stolen goods or illegal gambling debts. In addition, counterfeiters and forgers use microcomputers and printers to produce sophisticated-looking documents such as checks and drivers licenses.II. Computer systems and data besides criminals. They include the following: （1）Natural Hazards Natural forces include fires, floods, wind, hurricanes, tornadoes, and earthquakes. Even home computer users should store backup disks of programs and data in safe locations in case of fire or storm damage. （2） Civil Strife and Terrorism Wars, riots, and other forms of political unrest are real risks in some parts of the world. Even people in developed countries, however, must bemindful that acts of sabotage are possible. （3） Technological Failures Hardware and software dont always do what they are supposed to do. For instance, too little electricity, caused by a brownout or blackout,may cause the loss of data in primary storage. Too much electricity, as when lightning or other electrical disturbance affects a power line, maycause a voltage surge, or spike. This excess of electricity may destroychips or other electronic components of a computer. Most microcomputer users buy a surge protector, a device thats parates the computer from the power source of the wall outlet. When a voltage surge occurs, it activates a circuit breaker in the surge protector, protecting the computer system. Another technological catastrophe is when a hard-disk drive suddenly “crashes,” or fails, perhaps because it has been bumped inadvertently. If the user has forgotten to make backup copies of data on the hard disk, data may be lost. （4） Human Errors Human mistakes are inevitable. Data-entry errors are probably the most commonplace. Programmer errors also occur frequently. Some mistakes may result from faulty design, as when a software manufacturer makes a deletion command closely resembling another command. Some errors may be the result of sloppy procedures. One such example occurs when office workers keep important correspondence under filenames that no one else in the office knows.III. Measures to Protect Computer Security Security is concerned with protecting information, hardware, and software. Security measures consist of encryption, restricting access, anticipating disasters, and making backup copies1. Restricting Access Security experts are constantly devising ways to protect computer systems from access by unauthorized persons. Sometimes security is a matter of putting guards on company computer rooms and checking the identification of everyone admitted. Oftentimes it is a matter of being careful about assigning passwords to people and of changing them when people leave a company. Passwords are secret words or numbers that must be keyed into a computer system to gain access. In some “dial-back” computer systems, the user telephones the computer, punches in the correct password, and hangs up. The computer then calls back at a certain preauthorized number. Most major corporations today use special hardware and software called firewalls to control access to their internal computer networks. These firewalls act as a security buffer between the corporations private network and all external networks, including the Internet. All electronic communications coming into and leaving the corporation must be evaluated by the firewall. Security is maintained by denying access to unauthorized communications.2. Backing up Data Equipment can always be replaced. A companys data, however, maybe irreplaceable. Most companies have ways of trying to keep software and data from being tampered with in the first place. They include careful screening of job applicants, guarding of passwords, and auditing of data and programs from time to time. The safest procedure, however, is to make frequent backups of data and to store them in remote locations.3 Security for Microcomputers If you own a microcomputer system, there are several procedures to follow to keep it safe: （1） Avoid Extreme Conditions Dont expose the computer to extreme conditions. Direct sun, rain from an open window, extreme temperatures, cigarette smoke, and spilled drinks or food