I take it you mean a PGP like piece of software since the heart of PGP is the work of Phil Zimmerman. You might want to have a look over at
Wikipedia and read up about it, tis quite a lengthy read but probably has some good
information that would be of interest. Now if you want to go straight to the goods I suggest GnuPG which thankfully
is both *nix and Windows friendly with several plugins for various thinks like instant messengers and popular email clients. And best of all since
its using OpenPGP its free to use and modify, so if your brave enough have a poke about in the source code.