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  3. A variety of simple techniques can prevent computer crime. For example, destroying printed information, protecting computer scre

A variety of simple techniques can prevent computer crime. For example, destroying printed information, protecting computer scre

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问题     A variety of simple techniques can prevent computer crime. For example, destroying printed information, protecting computer screens from observation. Keeping printed information and computers in locked cabinets, and clearing desktops of sensitive documents prevent access to confidential information. But more sophisticated methods are also necessary to prevent computer crimes.
    One technique to protect confidentiality is encryption. Information can be scrambled and unscrambled u-sing mathematical equations and a secret code called a key. Two keys ale 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 list In the future, passwords and tokens may be reinforced by biometrics, identification methods that use u-nique personal characteristics, such as fingerprints, retinal patterns, skin oils, deoxyribonucleic acid (DNA), voice variations, and keyboard-typingrhythms.
    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.
According to paragraph 2 , the keys are changed frequently in order to______.
选项 A、encode more data
B、decode more data
C、give people more private keys
D、make information difficult to decode
答案D
解析 文章指出，密钥定期改良，进一步阻止了未经允许的访问，并使加密信息难于解密。第二段Thekeys are modified periodically further hampering unauthorized access and makingthe encrypted infor-mation difficult to decode or forge．
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