Gpg (GNU Privacy Guard) is a OpenPGP implementation.
There are several frontend applications, although I never tried any of them; I prefer the command line.
gpg --symmetric --armor --cipher-algo AES 12-words.txt
It will prompt for a passphrase and create a 12-words.txt.asc file with encrypted data (ASCII characters).
A more bare-bones means of symmetric encryption with Rijndael is ccrypt. You can use Base64 to format the binary data in a way that's relatively easy to read and write by hand.
$ printf 'first second third fourth fifth banana orange pineapple watermelon aircraft tree blue' | ccrypt | base64
Enter encryption key: abc
Enter encryption key: (repeat) abc