  This paper presents a multi-platform, open-source application that aims to
protect data stored and shared in existing cloud storage services. The access
to the cryptographic material used to protect data is implemented using the
identification and authentication functionalities of national electronic
identity (eID) tokens. All peer to peer dialogs to exchange cryptographic
material is implemented using the cloud storage facilities. Furthermore, we
have included a set of mechanisms to prevent files from being permanently lost
or damaged due to concurrent modification, deletion and malicious tampering. We
have implemented a prototype in Java that is agnostic relatively to cloud
storage providers; it only manages local folders, one of them being the local
image of a cloud folder. We have successfully tested our prototype in Windows,
Mac OS X and Linux, with Dropbox, OneDrive, Google Drive and SugarSync.
