The authentication used by the ssh package is much too heavyweight
(public key calculations require a lot of CPU as well as rather much
code for the multi-precision math) and not needed here. A simple keyed
MD5 authentication is enough. I.e. take a string X as key and a
non-constant string Z as message, and put (Z, MD5(X+Z)) in the
datagram. Simple, fast, requires exactly one datagram and doesn't fall
under US export restrictions (no encryption is done). Use a timer for
Z to protect against replay.
There's not much of a wheel to re-invent here. (And we don't need
tractor wheels for a bicycle. :-)
olaf