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I’m searching for an easy to use symmetric encryption tool with a GUI or, even better, one that has the possibility to be added to the file browser's context menu. Gnome already ships a encryption GUI named Seahorse but it only allows asymmetric encryption.
Using CLI commands based on OpenSSL or GPG is OK for me, but it makes it difficult for my family to encrypt or decrypt a file. A front end for GPG or OpenSSL will work too.
Any suggestions?
This is already built into the file browser (Nautilus)! Only it's combined with another feature, so it isn't straightforwardly discoverable.
To encrypt, select one or more files. Right-click and select Compress. Pick one of the extensions that support encryption — I recommend .7z. Mnemonic, so your family knows what format to pick: take the first item in the list. Expand “Other Options” and enter a password.
Why .7z? .zip has the advantage that the files can be read out of the box on Windows, but I don't know whether an encrypted file produced in this way uses the newer, strong encryption algorithm (based on AES and PBKDF2) or the older, broken algorithm (compatible with older software). So I recommend .7z instead. The 7z format supports encryption (AES, with decent key stretching). Note that neither zip nor 7z provide cryptographic integrity verification (i.e. someone malicious can change the archive and it won't be detected).
On Ubuntu, make sure you have the p7zip or p7zip-full package installed. It provides the command line tool 7zr (p7zip package, supports only 7z archives) or 7z (supports most common archive formats as well). If you need this on Windows, 7-Zip is available (and is a good archive manager software beyond 7z format support).
To decrypt an archive, just expand it and enter the password when prompted.
Open source and free. It encrypts binary (non-textual) files (second screenshot below) but works even better with text because it has a built-in editor. It edits text without saving temporary files on disk whereas file archivers usually do. So if you exchange text, I would definitely recommend EncryptPad because you won't have to worry about finding and erasing disk space under temp files.
The file format EncryptPad saves is GPG. If your recipient does not have EncryptPad installed, he or she can use GPG by simply right clicking on a file in Windows Explorer. GPG also comes with popular Linux distributions.
Another important benefit is support of key files. You can exchange key files with your relatives and use them instead of relying on passwords that can be broken with a dictionary attack. Key files are stored encrypted on the disk as additional measure to increase security.
Windows, Mac, Linux
You can verify that the application comes from the author unchanged with an authority certificate and a GPG signature.
EncryptPad is an application for viewing and editing symmetrically encrypted text. It also provides a tool for encrypting and decrypting binary files on disk. The application offers effective measures for protecting information with simple and convenient graphical and command line interface. The editor uses most widely chosen quality file format OpenPGP RFC 4880. Unlike other OpenPGP software which main purpose is asymmetric encryption, the primary focus of EncryptPad is symmetric encryption.
