Foil Notes

Foil Notes allows you to encrypt notes with a password stronger than the lock code. Strictly speaking, each note 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 notes, they would have to crack the AES key (different for each note) or the RSA key (shared by all notes 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 - it's the same format as used by Foil Pics. Open (unencrypted) notes are stored in the format understood by Jolla Notes.

The RSA key is shared with Foil Pics. 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.

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