In my last post, I made comments about the vulnerabilities of cloud storage – specifically, that CrashPlan (like most cloud storage providers) stores a copy of the decryption key required to access your files on their servers, and that while this means that your files are still encrypted it also means that they can be decrypted without your knowledge by a malicious or inattentive system administrator, or under direction, coercion, or threat from a third party or government. In this post, I discuss one of the “zero-knowledge” cloud storage providers that eschews this behaviour: Tresorit.
Tresorit is a Dropbox-like cloud storage service with a client that manages the synchronization of folders called “tresors”. The key difference in the behaviour of the two is that the Tresorit client conducts encryption of all files on the client side using a key that is generated on the client device. All data is encrypted client-side; all transmissions between Tresorit’s servers and the client are secured with TLS; all data is encrypted at all times when stored on Tresorit’s servers. Access to synchronized files on new devices requires installation of and login to the client, at which point your Roaming Profile is downloaded and access to the encrypted files is granted. The authentication process is described as follows:
[T]he server generates a 160 bit random nonce and sends it back along with the authentication salt as a challenge. The client assembles a response value which contains the authentication salt received from the server, and a newly generated 160 bit client-side temporary salt value. It then calculates your authentication key again from your password and the authentication salt provided by the server and uses it to derive the actual response using the PBKDFv2 algorithm with SHA1 and 1 iteration and sends it back to the server.
As the server is also able to calculate the proper response value using the stored authentication key and the PBKDFv2 algorithm, the authentication process is finished successfully if these two values match.
That last link provides a lot of useful information about how the Roaming Profile is encrypted, what happens on password change, and how Tresorit manages to allow encrypted access to files when you generate sharing links. Further information, recommended for the geeks among the cryptographers, is available in the Tresorit Security Whitepaper (link opens PDF version of the document).
For security-conscious users that still require cloud storage (for collaborative work or for a small measure of backup), Tresorit is a good choice. Basic use is free with 3 GB of storage, and plans with increased storage start at $12.50/user/month.
This is no replacement for a full backup system, so if that’s what you’re seeking look elsewhere. But most people could benefit from having cloud storage that doesn’t have prying eyes as a de facto component, and Tresorit does a great job at that.