[dm-crypt] (OT) Secure data wipe

Arno Wagner arno at wagner.name
Sun Dec 9 17:38:29 CET 2012

On Sun, Dec 09, 2012 at 03:48:01PM +0100, Karol Babioch wrote:
> Hi,
> Am 09.12.2012 13:26, schrieb Javier Juan Martínez Cabezón:
> > overwritting unallocated space several times
> this basically is a myth. Overwriting the whole drive just one time will
> make any recovery practically impossible, see [1].
> The "multiple times" aspect to my best knowledge originates from the
> good old days, where the density of tracks on floppies wasn't as high as
> it is with todays hard drives and it was possible to "miss" the track by
> some amount.

Actually, the background is as follows (track density, bit-density, 
etc, are secondary): A magnetic medium has a s/n (signal-to-noise) 
ratio and a ratio an overwrite weakens an older singnal by. 

As a simplified example, you may get 10mV of noise and a maximum 
signal of 100mV. Then you have signal-to-noise of 10:1. An overwrite 
always weakens the original signal, bit it is still there. Say, it 
gets weakened by a factor of 3. Then you have the n-th (overwriting)
signal at 100mV singal strengt, and the n-1 (older) signal at
30mV. You can hence read signal n, reconstruct (in analog) how it 
was written, subtract it from an analog media read and get the
n-1 signal. 

For floppies, I did some measurements 20 years back, and I 
could clearly see the n-1 signal and maybe the n-2 signal 
would also have been possible to read. The s/n ratio was 
was very high.

For modern disks, the s/n ratio is low enough that the n-1 
signal vanishes in the noise after a single overwrite. The 
background is that modern electronics can read the signal 
almost perfectly in analog and modern signal processors can 
do all the math needed to get the maximum possible decoding 
quality in real-time. If the HDD manufacturers could cram
in more bits, they would, but the surfaces can simply not 
hold them reliably. They certainly cannot hold twice as
much data, as in a signal and the overwritten signal from
before. Ordinary 1.44MB floppy disks coukld be made to 
hold > 20MB with special equipment back in the day.

Rumour has it that after one overwrite you may get lucky and 
pull off single bits, like occasionally recognizing a keyword, 
using very expensive equipment and a lot of time. For encrypted 
data that is pretty worthless. 

Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno at wagner.name
GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D 9718
One of the painful things about our time is that those who feel certainty
are stupid, and those with any imagination and understanding are filled
with doubt and indecision. -- Bertrand Russell

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