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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>数字签名和证书</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 5 页（共8 页）</nobr></font></td>
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<p>可以用非对称密钥验证发送者。如果消息可以用一方的公钥解密，那么这就确保了它是用发送者的私钥加密的。这一事实可以用来验证发送者的身份，因为他/她是惟一可以使用自己私钥的人（只要该没有私钥被其他人所窃取）。</p>
<p>但是如何得到个人的公钥以，以及如何知道这就是他/她的公钥呢？包含个人名字、失效日期和个人的公钥副本的证书解决了这个难题。一个称为证书权威机构（CA）的中央证书颁发机构验证人们的身份，并授予他们证书。授予的证书将通过 CA 数字签名，以确保它们的可信性。默认情况下，CA 证书通常安装在像 Internet 浏览器这样的应用程序中。CA 是值得信任的，因此它所信任的个人也是值得信任的。这种信任链使整个系统可以正确工作。</p>
<p> CA 根据对个人的信任程度，颁发不同强度的证书。可以只通过证明拥有有效的电子邮件 ID 来获得最低级的证书，而商业级的证书可能需要高级的标识技术，如 DNA 测试。 </p>
<p>CA 的工作不限于发布证书。它还负责维护证书的状态。如果某人的私钥丢失，那么他或者她可以向颁发了这个证书的 CA 报告失窃的情况。CA 将这个证书加入到 CA 的撤销清单（CRL）中，那里列出了失效的证书。所有人都可以访问这个数据库，以确保他们所信任的证书没有问题。</p>
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