Foil Pics

Foil Pics allows you to encrypt some pictures from the gallery with a password stronger than the lock code. Strictly speaking, each picture is encrypted with a unique random 256-bit AES key which in turn is encrypted with an RSA key which in turn is encrypted with your password. If the bad guys get your encrypted pictures, they would have to crack the AES key (different for each picture) or the RSA key (shared by all pictures but harder to crack) in order to extract the content. If they get the encrypted RSA key as well, then they can brute force your password. So in the end, the encryption is as strong as your password.

The format of the encrypted file is described here. The encrypted files are stored in ~/Documents/FoilPics.

The RSA key is shared with Foil Notes and Foil Auth apps. It's stored in ~/.local/share/foil/foil.key - you can copy it from one device to another. Using the same RSA key on different devices, protected with different passwords, is actually not such a bad idea. If you forget the password, you can use the backup key to decrypt the images, assuming that it has a different password that you still remember.

Note that you can add your encrypted files and the Foil key to the backup with the help of My Backup app.

Source code: https://github.com/monich/harbour-foilpics