UPDATE (Jan 2019): Read this newer blog post instead https://www.peterbe.com/plog/encrypt-with-emacs-on-macos-ccrypt
With the ccrypt program for Linux and some Emacs settings you can achieve the following:
You create or open a file called passwords.cpt. Then Emacs asks you for a password. Once you've entered the password you can word with the file like any other file to for example write down all your passwords to various websites or bank accounts.
When you close Emacs and try to read the file with any other program you get the binary encrypted output which is useless. What's important to Emacs is that the filename ends in .cpt which is a bit clunky. In vi you can get the same effect simply by passing the -x option when opening a file. But I don't like vi.