Selective End-To-End Data-Sharing in the Cloud

Cloud-based services enable easy-to-use data-sharing between multiple parties, and, therefore, have been widely adopted over the last decade. Storage services by large cloud providers such as Drop-box or Google Drive as well as federated solutions such as Nextcloud have amassed millions of users. Nevertheless, privacy challenges hamper the adoption of such services for sensitive data: Firstly, rather than exposing their private data to a cloud service, users desire end-to-end confidentiality of the shared files without sacrificing usability, e.g., without repeatedly encrypting when sharing the same data set with multiple receivers. Secondly, only being able to expose complete (authenticated) files may force users to expose overmuch information. The receivers, as well as the requirements, might be unknown at issue-time, and thus the issued data set does not exactly match those requirements. This mismatch can be bridged by enabling cloud services to selectively disclose only relevant parts of a file without breaking the parts' authenticity. While both challenges have been solved individually, it is not trivial to combine these solutions and maintain their security intentions. In this paper, we tackle this issue and introduce selective end-to-end data-sharing by combining ideas from proxy re-encryption and redactable signature schemes. Proxy re-encryption provides us with the basis for end-to-end encrypted data-sharing, while redactable signatures enable to redact parts and selectively disclose only the remaining still authenticated parts. We overcome the issues encountered when naively combining these two concepts, introduce a security model, and present a modular instantiation together with implementations based on a selection of various building blocks. We conclude with an extensive performance evaluation of our instantiation.