Due to the increasing use of encrypted communication, such as Transport Layer Security (TLS), encrypted cache systems are a promising approach for providing communication efficiency and privacy. Cache-22 is an encrypted cache system (Emura et al. ISITA 2020) that makes it possible to significantly reduce communication between a cache server and a service provider. In the final procedure of Cache-22, the service provider sends the corresponding decryption key to the user via TLS and this procedure allows the service provider to control which users can access the contents. For example, if a user has downloaded ciphertexts of several episodes of a show, the service provider can decide to provide some of the contents (e.g., the first episode) available for free while requiring a fee for the remaining contents. However, no concrete access control method has been implemented in the original Cache-22 system. In this paper, we add a scalable access control protocol to Cache-22. Specifically, we propose a time-dependent access control that requires a communication cost of $O(\log T_{\sf max})$ where $T_{\sf max}$ is the maximum time period. Although the protocol is stateful, we can provide time-dependent access control with scalability at the expense of this key management. We present experimental results and demonstrate that the modified system is effective for controlling access rights. We also observe a relationship between cache capacity and network traffic because the number of duplicated contents is higher than that in the original Cache-22 system, due to time-dependent access control.

翻译：由于传输层安全协议（TLS）等加密通信的日益普及，加密缓存系统成为提升通信效率与隐私保护的有效途径。Cache-22 是一种加密缓存系统（Emura 等，ISITA 2020），可显著减少缓存服务器与服务提供商之间的通信量。在 Cache-22 的最终流程中，服务提供商通过 TLS 将对应的解密密钥发送给用户，这一流程使得服务提供商能够控制哪些用户可以访问内容。例如，若用户下载了某节目多集的密文，服务提供商可决定提供部分内容（如第一集）免费访问，而剩余内容则需付费。然而，原始 Cache-22 系统并未实现具体的访问控制方法。本文向 Cache-22 添加了一种可扩展的访问控制协议。具体而言，我们提出了一种时间依赖访问控制，其通信开销为 $O(\log T_{\sf max})$，其中 $T_{\sf max}$ 为最大时间段。尽管该协议是有状态的，但通过牺牲密钥管理开销，我们能够提供具有可扩展性的时间依赖访问控制。我们通过实验结果表明，改进后的系统能有效控制访问权限。同时，由于时间依赖访问控制导致重复内容数量高于原始 Cache-22 系统，我们还观察到缓存容量与网络流量之间的关系。