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<div class="moz-cite-prefix">Dne 08. 01. 26 v 4:17 Peter Gutmann via
cryptography napsal(a):<br>
</div>
<blockquote type="cite"
cite="mid:SYCPR01MB36617EEBCA5B120F8C1C4B4BEE85A@SYCPR01MB3661.ausprd01.prod.outlook.com">
<pre wrap="" class="moz-quote-pre">Yup, it's pretty much completely irrelevant. One is a collection of clearly-
labelled/identified bits of paper with the labelling visible to the naked eye,
the other is a mini-skip full of, let's say ten million because I'm too lazy
to sit down and do the volumetric calculation, identical fragments that have
all suffered severe mechanical trauma. And it's not like when I looked at it
30+ years ago where you could use ferrofluid to get an idea of the geometry
and then try and recover the MFM data signal with an MFM (magnetic force
microscope) and a lot of effort, you're dealing with exotic recording
technologies where, and this is the bit I'm not sure about, you may need to
actually physically reconstruct the platter to be able to pull a signal off
it, or at least recover the encoding for the signal. The bit density on a
fragment won't matter much if the only way to recover what's on there is to
have it moving under a read head at a given velocity with a flying height of a
few nanometers... on a shredded fragment with burrs and bends and other
damage.
Peter.
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<p>Peter,</p>
<p>In past I need to digging deeper in that area, because I have
participated in several discussions on this topic with customers.
They have concerned both magnetic media and solid state drives.
The discussion is challenging but interesting because it balances
data protection, sometimes cryptography, physics, technology and
engineering disciplines. I would like to summarize few points, may
could be useful for someone, beside there are lack of relation to
cryptography.</p>
Hard Drive:<br>
Hard drive are fantastic engineering masterpiece. Could be provided
with rotation per minute from list of 5400,7200,10000,15000. Able to
read up to four segments per rotation (depends on optimalization).
Able to precisely move fast enough to relevant track with time
shorter than half of one rotation. And read extremely small bit of
data. That bit now contain few magnetic domains (sometimes tenths,
sometime only few of them) and depends on technology has a size
about 100nm in length and 50nm wide. I do not know exact properties
from SMR devices (Shingled Magnetic Recording). This can create a
problem how to recover data. Methods used in past could be
summarized as:<br>
- Mount plates to another disk and read pieces of data. This is
natural attitude with low probability of success. Without
reprogramming of controller to read raw data is unrealistic. More,
the head itself is too wide to get appropriate "remnants". Pieces of
data could be read thanks to imperfection of technology, like small
vibration of head, not quite accurate exposure of the header to the
same position as in the previous read/write case, wrong behave of
domains on the threshold of magnetic field enforcing bit to change
and other deviations.<br>
- Kerr effect - tilting of plane polarized light. For infrared light
this is about 5°, the higher the frequency, the lower the angle.
More, you need to use magnetic domain size bigger than wavelength.
That kind of attitude is relevant for past technology.<br>
- Magnetic force microscopy (MFM). That technology require mostly
two pass, where first one mapping relief (AFM) thanks to Wan der
Walls forces, second path is about collecting information from
distance (in nanometers scale) by magnetic force. That kind of
mapping require vacuum and it is slow. But able to read orientation
of magnetic domains ... include gaps between.<br>
- In situation of shreeding, cutting material to pieces eradicate
piece of data in the mechanical stress zone due to mechanical stress
and possible reorganization due to indentation into the material.
Furthermore, shearing or removal of material thermally and
mechanically stresses individual domains, in addition to
reorganization, some may pass the Curie temperature and lose the
ability to hold information. The information from them is then
random, mostly follow closest field force (could be engines or earth
magnetic field).<br>
- In situation of burning, crossing the Curie temperature has the
same properties, domains stay randomly oriented.<br>
- Demagnetizing of media, exactly enforcing media to use specific or
random orientation probably will be tough to achieve for
technologies like HAMR or MAMR.<br>
Technology;Size of bit;Domains per bit;Size of domain;Curie point<br>
LMR;200-100nm;10-50;200-80nm;720-870K<br>
PMR;100-20nm;5-20;20-10nm;720-920K<br>
SMR;80-15nm;5-15;15-10nm;720-920K<br>
HAMR;50-5nm;2-8;8-5nm;970-1020K<br>
MAMR;60-7nm;3-10;10-7nm;770–920 K<br>
This is a reason, why I enforcing disk encryption, mainly on disk
level to save a computation power. I do not think so that I need to
explain advantages, because erasing of key material will be enough
(for paranoid, you can reprovision the new key and overwrite whole
content, delete that key again after).<br>
For regular disk wipe, there are quite good list of standards. Peter
Gutman introduced me to this area 20 years ago, but he is not aware
about (thanks Peter, I was curios about your reasons for 35 pass).
From my perspective, 3 phases is enough for most situation, 7 for
sensitive data. But any remnants on the edge, caused by random head
moving (vibration, temperature) can achieve attacker to read that
data. Mainly pieces of data, broken by another write operations.
Something you would like to read 10 letters writen on the same paper
by the same pencil. Standards in that area is bellow.<br>
NIST SP 800-88 (Rev. 1)<br>
DoD 5220.22-M<br>
DoD 5220.22-M ECE<br>
Air Force (AFSSI 5020)<br>
German BSI (BSI-2011-VS)<br>
Peter Gutmann Method<br>
Cryptographic Erasure (Crypto Erase)<br>
IEEE 2883-2022<br>
<br>
Solid State Drive:<br>
<p>This is probably most funny part. Solid state disks are memories,
which, depends of technology (and architecture), could have
complete different behave. And because small, could end in funny
stories about mechanical destruction, which destruct case, but not
media inside. </p>
<p>SSD disk should not use wipping like HDD, because of statistical
properties of balancing utilization. Security should depend on
cryptography, nothing else.</p>
<div class="moz-cite-prefix"><span
style="color: rgb(230, 232, 240); font-family: "Google Sans", Arial, sans-serif; font-size: 16px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; white-space: normal; background-color: rgb(28, 30, 36); text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial; display: inline !important; float: none;">I
hope this can help to someone. And, please apologize me for my
gramma, I still work on it.</span></div>
<div class="moz-cite-prefix"><span
style="color: rgb(230, 232, 240); font-family: "Google Sans", Arial, sans-serif; font-size: 16px; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; white-space: normal; background-color: rgb(28, 30, 36); text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial; display: inline !important; float: none;"><br>
</span></div>
<div class="moz-cite-prefix">Regards</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Jan</div>
<br>
<pre class="moz-signature" cols="72">--
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Jan Dušátko
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