为实现信息系统的安全目标，需要系统能够提供相关的安全服务：身份标识与认证服务、授权与访问控制服务、非否认服务、机密性服务、完整性服务。
Services such as identification and authentication, authority and access control, non-deniability, confidentiality and integrity are the key elements for the security of the information system.
加密可以保证有关某个主题的数据机密性，原因是只有拥有正确私钥的服务提供者才能正确执行解密，因此也能读懂该数据。
The encryption guarantees the data confidentiality on the topic, because only the service provider owning the right private key can perform decryption correctly and, thus, understand the data.
对于一对一交互，我们通常使用服务公钥来加密信息并提供机密性。
Typically for one-to-one interactions we use the service public key to encrypt information (and provide confidentiality).
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