File decryption for Boxcryptor. For a single file decryption, there is also a Go version. Here you have both a single file or a directory decryptor. The single file is bcdecryptor.py, the directory decryptor is dir_bcdecryptor.py.
Boxcryptor is commercial software (though you can use it freely with limitations) that encrypts files in the cloud in its largest an acceptance, which means OneDrive (Microsoft), Google Drive, iCloud (Apple), ownCloud, Box, Dropbox, etc. All compatibles products are listed here.
Because I'm paranoid (I'm working in a big European bank as a security officer/expert), and I wanted to understand the crypto process of the Boxcryptor solution (zero-knowledge based). The best way to do that is to write my decrypting program in Python 3 (because I like Python 3). Secondly, like every commercial program, you can't access the source code to audit it, and the documentation is often too incomplete for auditing purposes. And, at last, in the unlikely case of the company disappear, or if the executable files are lost or too old to be used, you may want a program to decrypt your files. That's what is done here.
Of course, the Boxcryptor solution encrypts and decrypts the files on the fly, and seems to work well. But as I've said, you can't audit the code. To prevent this, the editor published a decryptor (in Java and C++) here:
Like my work, their programs are not really intended to be production-grade programs, but having this source code is a good way to understand how their crypto process works. As a challenge, I wanted to do a decryptor by myself, and because I have no relation with the editor, you have here an independent way of understanding it and decrypting your files.
You need to get a copy of the crypto information stored by Boxcryptor, More information here:
- https://www.boxcryptor.com/en/key-management/
- https://www.boxcryptor.com/en/help/boxcryptor-account/windows/#export-your-keys/
You'll get a .bckey file. This file is very important and needs some protection, even if you still need your Boxcryptor account's password to decrypt your data (see below).
As a zero-knowledge solution, even with the previous file, you won't be able to decrypt anything. So always remember your (strong) password and keep it in a safe place (like KeePass).
To be completed, but Python 3 with the Cryptography module is mandatory.
First, I'm proud of myself: I've understood all the underlying crypto! And coded all that into Python! And it works!
Besides, I can see that Boxcryptor do what they say: they have implemented a zero-knowledge solution. That means that, without your password, nobody can decrypt your files, even them. But as usual, the ultimate proof of confidence would be to assess that your password is stored nowhere...
With my laptop, I've decrypted an .iso file (1.81Go) in 163 sec.
