I have seen several variants of this idea over the years, one current
one is Dallas Semi's "One-Wire" (actually, two wires, counting
ground) system.  They make a series of chips which are intended to be
daisy chained on a twisted pair, and powered by the head end
controller.

They make a bunch of security keys using the same tech called
"ibutton"s.

see www.ibutton.com for the one-wire stuff, it has a link back to
Dallas's main web pages.

------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
barry@nrgsystems.com
"The witty saying has been deleted due to limited EPROM space"