Secure hard-drive data wiping

Date: 25 Jan, 2012
Posted by: admin
In: hints & tips|linux, open source & software|recycle

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How to securely delete data from a hard-drive (or USB pendrive) so that the information is virtually unrecoverable even to someone with unlimited funds.

Myth: Over-written hard-drive data can be [easily] recovered

It’s easier to guess the data than recover it!

Over-write it and then smash it with a hammer?!?

“If you are bloody minded, hard-drive data can still be read off the platters since the smashing will not completely destroy the magnetic domain information.”
(Friend-of-a-friend on Facebook)

Sorry but this just isn’t true. The success rate of restoring data from a full zero-pass (setting all bits to zero) is so low and the cost so prohibitive that no-one would attempt it without expecting a multi-million dollar return. This used to be almost feasible but on a modern drive the magnetic domains are so tiny …

“The fallacy that data can be forensically recovered using an electron microscope or related means needs to be put to rest.”
(Wright, et al., “Overwriting Hard Drive Data: The Great Wiping Controversy” [online viewer])

Of course if you’re an active secret agent harbouring data that can bring down a super-power then you’d use a hidden encrypted partition (on a USB pendrive) and then just thermite the drives for speed and certainty.

Options for a secure overwrite?

  • Do “dd if=/dev/urandom of=/dev/sda“; if you don’t know what that menas then this isn’t the option for you. If you do know, please, please, check the dev address is the correct one – best idea, remove all the drives except the one to be deleted and use a bootable linux distro on CD/USB pendrive (that has no personal data on to be lost).
  • Use DBAN. DBAN is a free open-source option. There are closed source /paid options, such as KillDisk (which has a DoD certificated wipe process); they do have a freeware KillDisk.
  • Use a ‘secure erase’ utility like CMRR. These access the drive and instruct the drive itself to perform a secure wipe – be sure to get the right drive, there’s no going back!

Once you’re done then you can donate the drive on Freecycle/Craigslist or somewhere.

Example recovery attempt

The above quoted Wright, et al., source includes an example of attempting a recovery using a Magnetic Force Microscopy device to look at overwritten magnetic domains on a recently produced hard-disk drive. Here’s their results:

Before  wipe (p.255 in www.vidarholen.net/~vidar/overwriting_hard_drive_data.pdf)

Secure deletion of data - Peter Gutmann - 1996
Abstract
With the use of increasingly sophisticated encryption systems, an attacker
wishing to gain access to sensitive data is forced to look elsewhere for in-
formation. One avenue of attack is the recovery of supposedly erased data
from magnetic media or random-access memory.

After single-pass wipe

¡ÄuÜtÞdM@ª""îFnFã:à•ÅÒ̾‘¨L‘¿ôPÙ!#¯ -×LˆÙÆ!mC
2´³„‡·}NŽýñêZØ^›l©þì®·äÖŒv¿^œº0TÏ[ªHÝBš¸ð
7zô|»òëÖ/""º[ýÀ†,kR¿xt¸÷\Í2$Iå""•ÑU%TóÁ’ØoxÈ$i
Wï^™oËS²Œ,Ê%ñ ÖeS» eüB®Èk‹|YrÍȶ=ÏÌSáöp¥D
ôÈŽ"|ûÚA6¸œ÷U•$µM¢;Òæe•ÏÏMÀùœç]#•Q
Á¹Ù""—OX“h
ÍýïÉûË Ã""W$5Ä=rB+5•ö–GßÜä9ïõNë-ߨYa“–ì%×Ó¿Ô[Mãü
·†Î‚ƒ‚…[Ä‚KDnFJˆ·×ÅŒ¿êäd¬sPÖí8'v0æ#!)YÐúÆ©
k-‹HÈø$°•Ø°Ïm/Wîc@Û»Ì"„zbíþ00000000000000000

To be fair they do have an example using a brand-new drive where the data was the only thing to be written to the drive after manufacture. In that optimal example there were a couple of partial words that it was possible to recover.

Recovering an overwritten MS Word document?

One last quote from Wright, et al., on attempting to recover an MS Word document.

The bit-by-bit chance of recovery lies between 0.92 (+/- 0.15)10 and 0.54 (+/-0.16)11. We have used the higher probability in the calculations to add an additional level of confidence in our conclusions. This demonstrates that the chances of recovering a single 8-bit character on the pristine drive are 51.32%. The recovery rate of a 32-bit word is 0.06937619 (just under 7%). As such, the chances of finding a single 4 letter word correctly from a Microsoft Word document file is 2.3166E-05 (0.00002317%).

Thermite!

If you’re still not convinced there’s always thermite!

 

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