From: tcmay@got.net (Timothy C. May)
Date: Sun, 21 Jan 1996 02:32:28 +0800
To: cypherpunks@toad.com
Subject: Re: Wipe Swap File
Message-ID: <ad266bbe08021004727d@[205.199.118.202]>
MIME-Version: 1.0
Content-Type: text/plain


At 3:38 PM 1/20/96, Dr. Dimitri Vulis wrote:
>tallpaul@pipeline.com (tallpaul) writes:
>> Remember that one simple wipe is *not* secure. Current Department of
>> Defense security regs call for wiping the same space something like 8 or 9
>> times. Even then the wipe is not secure enough for higher level DofD
>> classified material. There the regs call for the physical destruction of
>> the medium after it has been wiped.
>
>Degaussing the media (running a household magnet over it :-) may be an option.

Ordinary household magnets fail for a couple of reasons:

1. Their field strength is not high enough to affect modern media, due to
the extremely high coercivity of modern media. (Try it out, you'll be
surprised at hard it is to really change a lot of bits with a household
magnet.)

2. Most "swap files," as used above, are of course on hard drives. Encased
in metal. In any case, the nearest a household magnet can get to the
surface is several centimeters. Unless the magnet is very large (such as
the 20-pounder I have from my childhood days), the field strength will drop
drastically in several centimeters.

(Modern disk drives, and even modern videotape machines, use very
high-coercivity coatings, including pure metal, and the heads must ride
very close to the media to flip the domains. A magnet several centimeters
away is effectively at infinity.)

3. A time-varying field is preferred. Bulk erasers work this way, by
plugging into an a.c. socket and generating a time-varying field. And even
these are getting harder to use to erase video tapes, for example, due to
the high coercivity of modern media. Most folks I know no longer even try
to bulk erase tapes.

>1. Does anyone know a cheap way to recover the traces of the previous
>(overwritten) recordings on the media?

There are custom drives for various media which have multiple heads, and
heads that can be "jogged" a little bit. This allows, I have read, the
subtle variations of multiple writes to be extracted.

Much more expensive would be various electron microscope-based imaging
methods to directly image the domains and extract subtle signs of past
write cycles.

--Tim May

Boycott espionage-enabled software!
We got computers, we're tapping phone lines, we know that that ain't allowed.
---------:---------:---------:---------:---------:---------:---------:----
Timothy C. May              | Crypto Anarchy: encryption, digital money,
tcmay@got.net  408-728-0152 | anonymous networks, digital pseudonyms, zero
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