by http://www.rcarey.org/cion.html
Caesarion is a (64-bit clean) professional quality, open source, and completely
free E-mail security tool for the FreeBSD, OpenBSD and Linux operating systems.
It should also compile on NetBSD and other UNIX-like operating systems without
difficulty, since the source code is written in ANSI C++ and is mostly
ANSI/ISO/POSIX compliant.
The software provides communications secrecy, user authentication and
data-integrity verification.
Caesarion is very simple to use and comes with complete documentation and
examples showing how to use it.
The cryptography algorithms used in Caesarion are those which are currently
considered secure by the experts; the RSA public-key cryptosystem and the ARC5
algorithm.