From: John Deters <jad@dsddhc.com>
Date: Sat, 7 Sep 1996 05:06:57 +0800
To: Bruce Schneier <cypherpunks@toad.com
Subject: Re: What the NSA is patenting
Message-ID: <2.2.32.19960906165918.0073d060@labg30>
MIME-Version: 1.0
Content-Type: text/plain


At 01:26 AM 9/3/96 -0500, Bruce Schneier wrote:
>I just spent a pleasant hour or so searching a patent database for all
>patents assigned to the NSA.  There's some interesting stuff:
>
>	"Self-locking, tamper-evident package"
>	Method of retrieving documents that concern the same topic"
>
>Fifty-Four patents total.  (Used to be they just kept stuff secret; now
>they patent some of it.)  Attached is the most interesting thing I found: a
>patent on techniques for reading data off overwritten magnetic media.
>
>Bruce

[ Interesting patent deleted ]

This method implies that they have the ability to scan the entire platter
surface at resolution level that is basically atomic.

>From lots of experience gained from working with metalworking machinery, it
sounds like some of the old magnetic data might be leaving traces behind by
a process called "backlash".  It happens because machines don't realign
themselves precisely, unless the lead-in steps are repeated every time.  

A fairly simple demonstration of this can be found on most dot-matrix
printers.  They usually have a mode called "uni-directional" printing, where
the printhead puts dots on the paper only when travelling from
left-to-right.  This is used to improve the quality of a graphic image.
Print a simple pattern of repeating vertical bars (||||||) across the page
and down several lines with this mode turned off, and you'll probably notice
the lines tend to not line up perfectly.  Turn uni-directional printing on,
and watch the behavior of the printhead.  It will "home" itself to the far
left side before printing the next line of bars.  The bars will then be
lined up "better" than in the bi-directional print.

We should be able to use this feature to our advantage to write a "backlash-
enhanced" wipedisk driver.

The wipedisk utilities I've seen today primarily consist of repeatedly
writing a pattern such as 0x55555555, then 0xAAAAAAAA, then 0xFFFFFFFF, then
0x00000000.  While this will probably eradicate most of the traces of the
original data, it's all happening "unidirectionally" -- starting at the
first sector of the file,  write this data till all the sectors have been
overwritten.  

Given that the original data may have been written in reverse sector order,
or reverse cylinder motion order, or after a large cylinder change, the
wipedisk might still leave traces remaining on the disk.  Using the above
example of printing vertical bars, imagine having each line print three
times using the unidirectional mode, and randomly picking one line out of
the entire array to print bi-directionally.  It'll stand out like a sore
thumb.  That's what I think they're looking for with their data recovery method.

What would probably make for a more secure wipe utility would be to alter
the "head approach path" prior to making each of the passes described above.
So, before overwriting the sectors in order from 0 to EOF full of
0x55555555s, have the head move to the 0th cylinder first.  Before
overwriting the sectors in order from 0 to EOF with 0xAAAAAAAAs, have the
head move to the last cylinder beforehand.  Repeat for the 0xFFFFFFFF and
0x00000000 sectors, except overwrite the sectors in order from EOF to 0.

All this pre-writing motion could theoretically reduce the repeatability of
the drive head positioning arm as well as possibly hitting different
rotational sync  points, using the backlash effect to its fullest extent.

Of course, the biggest problem will be that of overcoming intelligent disk
controllers.  No self-respecting SCSI drive is going to voluntarily swing
the disk head around inefficiently, and I don't know enough about how IDE
works to say anything different about it.

I hope some hardware hacker who knows their low-level stuff will be able to
write a secure disk wiper.

John
--
J. Deters  "Captain's log, stardate 25970-point-5.  I am nailed to the hull."
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