Directions: There are 4 passages in this part. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A., B., C. and D. You should decide on the best choice and write down your answer on the Answer Sheet.

Passage 1

A variety of simple techniques can prevent computer crime, but more sophisticated methods are also necessary to prevent computer crimes.

One technique to protect confidentiality is encryption(加密). Information can be scrambled and unscrambled using mathematical equations and a secret code called a key. Two keys are usually employed, one to encode and the other to decode the information. The key that encodes the data, called the private key, is possessed by only the sender. The key that decodes the data, called the public key, may be possessed by several receivers. The keys are modified periodically, further hampering unauthorized access and making the encrypted information difficult to decode or forge.

Another technique to prevent computer crime is to limit access of computer data files to approved users. Access- control software verifies computer users and limits their privileges to view and alter files. Records can be made of the files accessed, thereby making users accountable for their actions. Military organizations give access rights to classified, confidential, secret, or top secret information according to the corresponding security clearance level of the user. Passwords are confidential sequences of characters that give approved users access to computers. To be effective, passwords must be difficult to guess. Effective passwords contain a mixture of characters and symbols that are not real words.

Tokens are tamper-resistant plastic cards with microprocessor chips that contain a stored password that automatically and frequently changes. When a computer is accessed using a token, the computer reads the token’s password, as well as another password entered by the user, and matches these two to an identical token password generated by the computer and the user’s password, which is stored on a confidential list. In the future, passwords and tokens may be reinforced by biometrics (生物测定学), identification methods that use unique personal characteristics, such as fingerprints, skin oils, voice variations, and keyboard-typing rhythms.

Computer networks, multiple computers linked together, are particularly vulnerable to computer crimes. Information on networks can be protected by a firewall, a computer placed between the networked computers and the network. The firewall prevents unauthorized users from gaining access to the computers on a network, and it ensures that information received from an outside source does not contain computer viruses, self-replicating computer programs that interfere with a computer’s functions.