William Chops Westfield wrote:
>
>     [explosive chip destruction]
>     Goes like this;
>     Client has money.
>     Mark wants money.
>     Client says "Do this, and I'll pay you Mucho Money."
>     Mark does what they want, even if not MY style, fairly often <G>
>
> Better find out how much it will actually cost to make any explosive
> based destructive device legal.  Money doesn't go far when you need
> to pay lawyers to keep you out of jail.
>
> We have a crypto device that supposed to erase its keys if anyone tries to
> tamper with it.  I took one apart (very carefully, after noting no "danger:
> explosive" stickers :-) It didn't contain anything more impressive than a
> battery and some microswitches.
>
> BillW

That's why the way I'm actually doing this is the way it is;  I have to
keep exercising my weird sense of humor, though ("Use it or lose
it!")...

Using external program memory & a backup battery with series
microswitches (or similar) works pretty well.  You can use the keys as
part of program flow, or even have the PIC chip just run an interpreter,
fairly easy to do (you want a good way to install the code to interpret,
and not to have your laptop that code's on, to be stolen!)  i.e. write
your PIC code as a bunch of disconnected state machine routines, and
have it choose the state table based on a battery-backed RAM table of
how to decide which routine to run next.  That's pretty fast, and fairly
secure.  That PIC chip could be non-code protected (you don't care if
the competition knows how you code a "Raise this pin" or a "lower that
pin" or "Compare these items" routine!), the RAM holds the
decision-making data.

  Mark

--
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