From: jim bell <jimbell@pacifier.com>
Date: Sun, 21 Jan 1996 12:47:46 +0800
To: cypherpunks@toad.com
Subject: Re: Wipe Swap File
Message-ID: <m0tdlVU-00091MC@pacifier.com>
MIME-Version: 1.0
Content-Type: text/plain


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At 10:38 AM 1/20/96 EST, 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.

Degaussing using a common, AC-driven bulk tape eraser is FAR FAR FAR better 
than using a permanent magnet.  DO NOT USE A 'HOUSEHOLD MAGNET"!!!!  (Except 
in an absolute dire emergency, such as when the government thugs are 
breaking down the door, and you have to wipe that disk in a second, and 
didn't think to keep the bulk eraser plugged in and immediately available, 
etc.  Even then, use a Neodymium Iron Boron magnet, because floppies are 
actually remarkably insensitive to demagnetization...)

Here is why:  Magnetic materials have "hysteresis curves."  If you merely 
apply a "DC" magnetic field to a floppy disk, this orients "all" the domains 
in one direction, but perhaps with a small residual bias based on the 
previously-magnetized direction.  Such data won't be readable on an ordinary 
floppy drive, of course, but it might be recovered, with substantial (read, 
"money") effort.  This gives uninformed people a false sense of security.

AC-powered tape demagnetizers, on the other hand, produce a 60-hertz
(actually, 120 
hertz, depending on how you look at it) pulsing magnetic field, which 
REPEATEDLY saturates and re-saturates the magnetic domains in one direction 
and then the other, taking a "trip around the hysteresis curve" 60 times per 
second.  Residual magnetic fields are repeatedly reversed and thus 
overwritten, and quickly become totally and completely unrecoverable in a 
second or so.  (actually, far less, I'm just not proposing you stand there 
for a minute degaussing a single floppy!!!)

And there is a far more practical reason to NOT use a permanent 
magnet, and CERTAINLY not on audio-quality tapes.  Read heads can get 
inadvertently magnetized, and if you insert a disk or tape with a 
DC-magnetization on it "who knows what" might go wrong. (it would take a 
reasonably technical audiophile to tell you how much of a problem this could 
be on audio cassette tapes.  It is possible that digital-writing floppy 
disks heads are comparatively immune from this effect, but don't count on 
it!)   (However, using an 
AC demagnetizer on a floppy after you've zapped it with a permanent magnet 
will remove whatever residual DC magnetiziation was present.


>Two semi-on-topic questions:
>
>1. Does anyone know a cheap way to recover the traces of the previous
>(overwritten) recordings on the media?

Cheap?  No.


>2. If a cheap way exists, has anyone considered stego use of it?

Doesn't sound particularly practical.

I can think of a slightly better way, MAYBE.  There are, what, 80 tracks on 
the typical floppy disk, right?  (okay, I  may be wrong about this...).   
But it would be physically possible to write a few more tracks onto the 
floppy before you hit a mechanical stop.  Putting data THERE while the 
typical system thinks there are "only" 80 tracks would hide it reasonably 
effectively.

Note:  I'm not over-rating the effectiveness of such a system.  It wouldn't 
faze the CIA or the NSA, but it would probably get by the local police, the 
state police, and maybe even the FBI unless they had written a program 
specifically designed to search "illegal" tracks.  Label the floppy, "Doom 
program, great game!" and they'll probably waste most of their time blasting 
monsters rather than looking for tracks 80, 81, 82, etc.)

Also, this is certainly not a new idea.

My public key.

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