From: Nathan Loofbourrow <loofbour@cis.ohio-state.edu>
Date: Thu, 19 Oct 95 06:49:05 PDT
To: die@die.com
Subject: Re: DalSemi: Add-Only Memory for Storage of Digital Cash (fwd)
In-Reply-To: <9510190432.AA04434@pig.die.com>
Message-ID: <199510191346.JAA10841@colon.cis.ohio-state.edu>
MIME-Version: 1.0
Content-Type: text/plain


 > 	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).

You might want to take a look at the paper I mentioned, then: I
believe that irradiation of the OTP EPROM to return all bits to "ones"
is too blunt a tool to do you any good. A virgin EPROM has a value of
zero in the suggested scheme. And, as mentioned, flipping random sets
of bits is strongly likely to get you caught.

Note also that the encoding is strongly tied to the laser-etched serial
number on the chip, so replay attacks between two different chips --
i.e. copying a fully loaded chip to a virgin chip -- won't work.

However, it does open up the scheme to replay attacks, if you can load
the chip with value, spend it, irradiate it, and then successfully
reprogram the chip with the exact same values you recorded.

Good point.

I seem to remember PROMs actually undergoing physical, rather than
electrical, state changes (that were presumably nonreversible). Am I
recalling old technology, or am I just plain mistaken?

nathan