Private Cloud Storage: Client-Side Encryption and Usable Secure Utility Functions.

With the development of cloud environments and smartphones, and increasing awareness of security and privacy, client-side encryption, represented by end-to-end encryption (E2E Encryption), has made rapid progress over the last 10 years. When client-side encryption is adopted, a wide variety of utility functions such as search and sorting provided by the cloud side, utilization on multiple terminals, and data sharing with other users are restricted. To solve this problem, there has been a great deal of interest in technologies such as searchable encryption and order preserving encryption, which allow data to be processed while being encrypted. However, there are few examples in which the effectiveness was discussed by applying these actually to the application. In particular, these technologies were rarely discussed from the viewpoint of usability. Therefore, we focus on cloud storage and propose an application that combines multiple encryption technologies on the client side to realize secure and usable cloud storage that can be closely linked with existing cloud storage services. The proposed application is then evaluated to demonstrate its usability. The application we proposed provides file encryption on the client side, secure retrieval, sorting, and folder sharing with other users. As a result of the user study, it was shown that the usability of the prototype application did not differ from that of the unencrypted application developed for comparison, and the usability of the proposed application was high. Furthermore, implementation and user experiments have revealed a number of new challenges in securely implementing utility functions while providing client-side encryption for contents, and have newly demonstrated the need for applied research in this field.