函数加密体制（一） (13-4)

我们认为，现在是时候采用加密系统的一个新的广阔视野。为此，我们探讨了函数加密（功能加密）的概念。在一个函数加密系统内，解密密钥允许用户学习到加密数据的一个函数。简而言之，在函数加密系统中对于一个功能F(·，·) （模拟为一个图灵机），一个持有主密钥的权威机构可以生成一个私钥 skₖ ，这个私钥有能力在加密数据上计算出函数值 F(k，·) 。更准确的来说，解密者使用私钥 skₖ 可以从 x 的密文中计算出函数值 F(k，x) 。直观来看，该系统的安全性是保证不可以学习到关于 x 更多的东西。但是正如我们将要看到的那样，严格的刻画这个安全是非常具有挑战的。

For many emerging applications such as “cloud"services this notion of public-key encryption is insufficient. For example, there is often a need to specify a decryption policy in the ciphertext and only individuals who satisfy the policy can decrypt.More generally, we may want to only give access to a function of the plaintext,depending on the decryptor's authorization. As a concreteexample,consider a cloudservice stor- ing encrypted images.Law enforcement may require the cloud to search for images containing a particular face.Thus, the cloud needs a restricted secret key that decrypts images that contain the target face, but reveals nothing about other images. More gen- erally,the secret key may only reveal a function of the plaintext image,for example an image that is blurred everywhere except for the target face. Traditional public-key cryptography cannot help with such tasks.

We believe that it is time to adopt a new broad vision of encryption systems. To this

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