Tcpcrypt is a protocol that attempts to encrypt (almost) all of your
network traffic. Unlike other security mechanisms, Tcpcrypt works out
of the box: it requires no configuration, no changes to applications,
and your network connections will continue to work even if the remote
end does not support Tcpcrypt, in which case connections will
gracefully fall back to standard clear-text TCP. Install Tcpcrypt and
you'll feel no difference in your every day user experience, but yet
your traffic will be more secure and you'll have made life much harder
for hackers.

WWW: http://www.tcpcrypt.org/