Have you studied encryption? Do you occasionally need to send "private conversations," to friends?
GNU's Privacy Guard is the ticket. It's free. Works on any platform. Only problem may be setup, learning the way to use it, and "getting a second person" to understand it.
GnuPG is the GNU project's complete and free implementation of the OpenPGP standard as defined by RFC4880 . GnuPG allows to encrypt and sign your data and communication, features a versatile key managment system as well as access modules for all kind of public key directories. GnuPG, also known as GPG, is a command line tool with features for easy integration with other applications. A wealth of frontend applications and libraries are available. Version 2 of GnuPG also provides support for S/MIME.
Platforms: Linux, Windows, Mac OS X
Glen
