Docurity: A New Cryptographic Primitive for Collaborative Cloud Systems

Recently, there has been a sudden shift from using traditional office applications to the collaborative cloud-based office suite such as Microsoft Office 365. Such cloud-based systems allow users to work together on the same docu-ment stored in a cloud server at once, by which users can effectively collaborate with each other. However, there are security concerns unsolved in using cloud col-laboration. One of the major concerns is the security of data stored in cloud ser-vers, which comes from the fact that data that multiple users are working together cannot be stored in encrypted form because of the dynamic characteristic of cloud collaboration. In this paper, we propose a novel mode of operation, DL-ECB, for AES by which we can modify, insert, and delete the ciphertext based on changes in plaintext. Therefore, we can use encrypted data in collaborative cloud-based platforms. To demonstrate that the DL-ECB mode can preserve the confidential-ity, integrity, and auditability of data used in collaborative cloud systems from adversaries, we implement and evaluate the prototype of the DL-ECB mode.