On Consistency of Encrypted Files

In this paper we address the problem of consistency for cryptographic file systems. A cryptographic file system protects the users’ data from the file server, which is possibly untrusted and might exhibit Byzantine behavior, by encrypting the data before sending it to the server. The consistency of the encrypted file objects that implement a cryptographic file system relies on the consistency of the two components used to implement them: the file storage protocol and the key distribution protocol. We first formally define consistency for encrypted file objects in a generic way: for any consistency conditions for the key and file objects belonging to one of the two classes of consistency conditions considered, we define a corresponding consistency condition for encrypted file objects. We then provide, in our main result, necessary and sufficient conditions for the consistency of the key distribution and file storage protocols under which the encrypted storage is consistent. Lastly, we give an example implementation of a consistent encrypted file object, utilizing a fork consistent file access protocol and a sequentially consistent key distribution protocol. The proof of consistency of the implementation builds from our main theorem.