From: jim bell <jimbell@pacifier.com>
Date: Sun, 21 Jan 96 19:57:51 PST
To: cypherpunks@toad.com
Subject: Re: Wipe Swap File
Message-ID: <m0teD3N-00090UC@pacifier.com>
MIME-Version: 1.0
Content-Type: text/plain


-----BEGIN PGP SIGNED MESSAGE-----

At 12:56 PM 1/21/96 EST, Dr. Dimitri Vulis wrote:
>tcmay@got.net (Timothy C. May) writes:
>> >Degaussing the media (running a household magnet over it :-) may be an 
optio
>>
>> Ordinary household magnets fail for a couple of reasons:
>
>I've just established experimentally that thoroughly running a household 
magnet
>over a 3.5" floppy messed up less than 1/2 the sectors I tried to read.

Was that with the floppy in the case, or with the case popped open...
Also:  Did you rotate the disk physically to expose the data normally 
partially shielded by the door slider?

And was it an ordinary ferrite magnet, or an alnico, or...?

>Not a good option even for floppies.

Ditto!

>(Actually, there _was a smiley up there)

Yes, I noticed. <G>
[stuff deleted]


>Jim Bell mentioned the trick of hiding information into 'extra' tracks and
>sectors not used by the usual DOS formatting. It's very old too.

I will admit that at this point, even calling it a "trick" is giving it 
excessive credence.

Actually, I think it wasn't really used initially for "data hiding" 
purposes.  I'm talking about the early days of CP/M and other such systems, 
circa 1977 and such, when individuals "discovered" that floppy drives had no 
hard mechanical stop past the "last" good track, and they "stole" a few 
percent of extra capacity from a floppy by simply ignoring the recommended 
"last" track.  Naturally, it would work okay on some drives but not on 
others... which is why it was a bad idea.

In addition, I also discovered that it was possible to put a few more than 
26 sectors on each track of an 8" single-density (240 kilobytes!) floppy 
disk.  The main problem with using 
these "tricks" is that the floppy had no method of conveying formatting 
information to the system it was in, which meant that any floppy using this 
trick was by definition non-standard.  ("feature" or "bug" depending on your 
goal...)


> I think I saw
>copy protection schemes circa 1982 that hid important data on tracks 41--43.
>360K diskettes normally had 40 tracks. If the diskette was copies by DISKCOPY,
>it didn't know about the extra tracks, and the copy didn't have the info
>(usually, a piece of the program). It's very easy to do with just BIOS 
calls to
>format/read/write the track. Problem is, many cheap floppy drives these days
>aren't capable of seeking beyond track 80 when the FDC asks them to. You can
>write the data there and give the floppy to a friend who won't be able to read
>it from there.

I started building my own 12.5 MHz Z-80 -based CP/M system in 1978, fully 
designed and wire-wrapped by myself, and wrote my own BIOS.  (Used a WDC 
1791 FDC)  Had total 
control.  I didn't try this trick even then because of compatibility 
reasons, but one thing I _DID_ do was to write a floppy formatter that 
"undid" the 6-sector skewing that standard CP/M had to do to keep up with 
the data read/write. (in other words, I physically re-skewed the sector 
numbering to make the next "desired" sector come faster...) I ended up with an 
effective skew-factor of 2.  Even a skew factor of 1 worked on my system (no 
skew at all), but the problem was that when I gave the most extreme of these 
oddly skewed floppies to my friends with 8" floppies, they took A LONG TIME 
to read the data!  (Their systems always missed the next sector because their 
systems were too slow, so they only ended up being able to read one sector 
per disk rotation.)

All this helps to explain why I asked if PGP had ever been ported to CP/M.  
Nostalgia!


-----BEGIN PGP SIGNATURE-----
Version: 2.6.2

iQCVAwUBMQMF6vqHVDBboB2dAQG1+QP7BpyrLaVbTJISLo12rWMo9sqyfwtpv6A2
r7GGTvQTw6MwACA3pTh6HnnjpllveQSznNLpHaUeEjfpQX9NUXuJc4Z63E+EBFYw
Xp3c0rygdC4fHS2WJbrhn0JUpC1C5V+Cn/oEpL5qygfaoqE1mAvsw7cCAht44ne+
/dJvdnm+N9M=
=CbtQ
-----END PGP SIGNATURE-----