(1) create plain.txt

(2) Create a public/private keypair if you don't already have one.
    I use gpg, the Gnu Privacy Guard, successor to pgp (Pretty Good Privacy)
    See http://www.gnupg.org/(en)/documentation/howtos.html
    and http://www.gnupg.org/gph/en/manual.html

    Here's how to do a few things from the command line.

         * generating a key :                         gpg --gen-key
           and then answer all the questions.

            Sample: in "milo" account
            passphrase = "silk, velvet"

         * listing keys                               gpg --list-keys
         * edit your key (needs passphrase)           gpg --edit-key UID
         * fetching keys from remote server           gpg --search mahoney@marlboro.edu
           (default server on cs is subkeys.pgp.net)
           see for example www.keyserver.net/en/
         * uploading to a server                      gpg --send-keys
         * exporting public keys                      gpg --export UID > public.txt
	 * encrypting and signing a file
	     you need the public key of the recipient


     My email client (Mozilla Thunderbird) uses gpg to do things, 
     so much of this can happen through a GUI.

 (3) Use your PGP/MIME or S/MIME or otherwise crypto enabled email
     client to sign or encrypt and sign a message...