Enhancing Secure Cloud Data Sharing: Dynamic User Groups and Outsourced Decryption

Cloud computing, as a persuasive paradigm, offers on-demand data services. However, it faces various security threats during data sharing due to trust issues. To mitigate this problem, many cloud-based data-sharing systems employ cryptographic tools to guarantee the confidentiality of sensitive data. Nevertheless, fine-grained data sharing still suffers from many challenges, especially in complex cloud environments. In this paper, we introduce two cloud-based data-sharing systems with fine-grained access control. The first solution supports dynamic user groups, while the second solution further offers outsourced decryption, enabling compatibility with resource-constrained devices. To formalize our solution theoretically, we introduce the concept of ElGamal -type cryptosystem (ETC) and server-aided ETC with key encapsulation mechanism to generalize public-key encryption with specific features implicitly specified by ElGamal encryption. Through the application of ETC, we present generic constructions for revocable attribute-based encryption (RABE) and server-aided RABE (SR-ABE) with formal definitions and security analyses. These schemes serve as the fundamental mechanisms behind our proposed solutions. By applying the state-of-the-art attribute-based encryption scheme proposed in CCS'22, we introduce instantiations of RABE and SR-ABE with various desirable properties, including large universe, attribute multi-use, key exposure resistance, fast decryption, and more. Extensive experiments substantiate the superior performance of our proposed instantiations over previous solutions.