Re: DalSemi: Add-Only Memory for Storage of Digital Cash (fwd)

[tcmay@got.net (Timothy C. May)]

At 1:46 PM 10/19/95, Nathan Loofbourrow wrote:
(quoting someone else...)

> >      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

Sure, x-rays can erase EPROM cells. (Not the dosages found in airport x-ray
machines, anticipating the usual question someone raises when they hear
this.)

> > 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).

The details of whether one can x-ray (or gamma-ray, etc.) erase a one-time
programmable (OTP) device without semi-permanently damaging it depend on a
lot of factors. Generally, an EPROM-based OTP device will erase under
irradiation before it is damaged. (At least this was my experience. Your
erasage may vary.)

>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.

Whether a floating gate empty of electrons ("erased") is considered to be a
"zero" or a "one" is wholly dependent on conventions in the design....one
manufacturer may treat "erased" bits as zeroes, another as ones. (The sense
of the charge may invert several times on the way through the sense
amplifiers and decoding logic.)

Bottom line: I'd take a bet that I could bulk-erase a Dallas Semi chip
without damaging it permanently, even if lacked a transparent lid. But I
wouldn't take a bet that I can then reprogram it so as to spoof another
such chip.

Continuing on to some other points:

>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?

There are several kinds of "programmable read-only memories" (PROMs):

* Fusible-link PROMs, in which a silicon, nichrome, or other fuse is
"blown" by passing an appropriate current through the fuse. These are
indeed nonreversible changes (though a failure mechanism is for fuses to
"grow back"). And there are variants involving "anti-fuses."

* _Erasable_ PROMs, in two main categories:

- UV-EPROMs, or just EPROMs, in which a dosage of UV light through a
transparent window (quartz or sapphire) erases the EPROM cells. This works
by the UV photons supply enough energy to the electrons stored in isolated
("floating") polysilicon gates so tha they leave the floating gates and are
conducted away. Programming is done by forcing electrons to be
avalanche-injected onto the floating gates.

- EE-PROMs, or EEPROMs, or "E-squared PROMs," in which UV light is not
needed. A suitable electrical bias can cause a cell to be erased (i.e., to
allow electrons on floating gates to leave).

* And there are all sorts of wrinkles, variants, and other issues:

- Transparent lids vs. no lids. EEPROMs obviously need no transparent lids.
Even EPROMs may be built without transparent lids, if the intended use is
for customers to program them _once_ and only once ("OTP").

- Bit-erase vs. block-erase. Whether individual bits in an EEPROM can be
erased without neighboring bits being erased. A design issue. (Obviously
UV-EPROMs are only used in block-erase situations, where the block is the
entire memory.)

Intel built a "Key-PROM," circa 1983-5, in which the innards were somewhat
encrypted against outside-reading. My electron-beam testing lab was able to
image the internal states of these devices and so bypass the encryption.
(But it was enough for casual use to foil certain types of copying.)

I hope this helps. Semiconductor physics is its own specialty, like crypto.

--Tim May

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