This paper and the implemented desktop app were presented at the Open Identity Summit 2014 in Fraunhofer IAO Institute Center, Stuttgart, Germany.
In this paper we present Protbox, an open-source, cross-platform application for securely sharing files among strongly authenticated people through many different cloud storage services. The secure sharing includes four different protection features:
- confidentiality, to prevent non-authorized readings;
- integrity control, to detect malicious tampering;
- protection against un-wanted file removals, either by malicious or legitimate persons;
- access control to the shared data based on strong identification and authentication of people, using the nowadays widespread electronic, personal identity tokens (eIDs for short).
Many governments worldwide have been or are introducing eIDs to allow the identification of people in the scope of Internet interactions. Unfortunately, there are several kinds of eID types being deployed, which reduces the possibilities of using all of them in a single system requiring the authentication of persons. In our system we considered the case of the Portuguese eID (Cartão de Cidadão).
Comparing Protbox with similar solutions, it has two main distinctive characteristics:
- the key distribution between file sharing persons is performed by means of special files exchanged through the exact same cloud storage space used for file sharing, thus no extra services are required other than the trustworthy national PKIs (Public Key Infrastructures) used to validate eID signatures;
- the files exposed to others by means of cloud sharing are protected from malicious or involuntary tampering or removal.
Protbox has just two requirements regarding a cloud storage solution for folders and files:
- it should allow the sharing of folders by many persons;
- it should allow client operating systems to have a local mount point of the shared folder.
Nowadays, most file-oriented cloud storage solutions, if not all, fulfill these requirements; in our experiments we managed to explore it successfully with Dropbox, SkyDrive, Google Drive and SugarSync.
We developed an open-source prototype in Java, available here. It runs on any operating system with a suitable Java Virtual Machine (JVM) and is capable of recognizing any file system. It features a background folder synchronization engine and a graphical user interface for dealing with key distribution requests. Protbox randomly generates and uses a key per folder to protect all its contents, including files and sub-directories. Files are encrypted with AES and their integrity is ensured with HMAC-SHA1. Encrypted file names, which contain bytes that are not acceptable for naming files in existing file systems, are coded in a modified Base64 alphabet, which should work in most file systems. The prototype was successfully experimented in Windows, Mac OS X and Linux with all of the above referred cloud storage providers.
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