数字证书采用公钥体制进行加密和解密。每个用户有一个私钥，用它进行(46)；同时每个用户还有一个公钥，用于(47)。X.509标准规定，数字证书由(48)发放，将其放入公共目录中，以供用户访问。X.509数字证书的签名字段是指(49)，如果用户UA从A地的发证机构取得了证书，用户UB从B地的发证机构取得了证书，那么(50)。

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In the following essay, each blank has four choices. Choose the most suitable one from the four choices and write down in the answer sheet.

 Open Shortest Path First(OSPF)is a(71)routing algorithm that(72)work done on the OSI IS-IS intradomain routing protocol. This routing, as compared to distance-vector routing, requires(73)processing power. The(74)algorithm is used to calculate routes. OSPF routing table updates only take place when necessary,(75)at regular intervals.

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Packet-switching wireless networks are preferable(66)when transmissions are(67)bemuse of the way charges are(68)per packet. Circuit-switched networks are preferable for transferring large files or for other lengthy transmissions because customers are(69)for the(70)of time they use the network.