Public key crypto by hand

[Robert J. Hansen]

> Sorry, this is a bit of topic. But I wonder if anyone knows of a
> public key system that can realistically be done by pencil and paper.
> I realize anything a computer can calculate could conceivably be done
> by hand, but I'm looking for something realistic. I don't consider
> raising very large numbers to very large powers realistic.

I am aware of none, and there are some good theoretical reasons to  
think there are none.

That said, beware of all theories: when theory conflicts with the real  
world, theory must give way.