DalSemi: Add-Only Memory for Storage of Digital Cash

[Dave Emery]

Roy Silvernail writes:

> This comes from the fact that PROMs are typically mask-programmed at the
> fabrication stage.  EPROMS are programmed in the field.  And yes, if you
> could get the top of the chip off cleanly, you could erase and reprogram
> them.  (not likely, and hardly undetectable, but it's ever so slightly
> possible)  More often, I see these devices called OTP (for One Time
> Programmable).  In the more standard types (27C256, for example),
> they're a fraction of the cost of the erasable ones.  Ceramic cases and
> quartz windows are spendy.

	Perhaps someone with more semiconductor physics background
can correct me, but my understanding is that some kinds of nuclear
radiation can be used to erase OTP EPROMS.   I suppose this might damage
the crystal lattice badly enough to render the device useless in
some bit positions or reduce data retention time a lot, but I sure
wouldn't bet any security on devices out there not being 
arbitrarily reprogrammable (thus using bits to represent digital
coins in a wallet that get reset when they are spent is not a
good idea).

						Dave Emery
						die at die.com

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