Duarte, EduardoPinheiro, FilipeZúquete, AndréGomes, HélderHühnlein, DetlefRoßnagel, Heiko2017-06-302017-06-302014978-3-88579-631-2This 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.enText/Conference Paper1617-5468