Fancy encryption schemes

"how far in the future [will signatures have to remain airtight]?"

one of our customers, a foreign stock exchange, has stipulated that
their signatures will be archived and will be binding for 5-10 years
(sorry, i forget the exact number). billions of $US are at stake.
we believe that this requirement will prove to apply to our other
financial customers, too.
                                        -don davis, geer zolot assoc.