[ Thanks to Noel
for this link. ]
“With the use of increasingly sophisticated encryption systems,
an attacker wishing to gain access to sensitive data is forced to
look elsewhere for information. One avenue of attack is the
recovery of supposedly erased data from magnetic media or
random-access memory. This paper covers some of the methods
available to recover erased data and presents schemes to make this
recovery significantly more difficult.”
“Faced with techniques such as MFM, truly deleting data from
magnetic media is very difficult. … In conventional terms, when a
one is written to disk the media records a one, and when a zero is
written the media records a zero. However the actual effect is
closer to obtaining a 0.95 when a zero is overwritten with a one,
and a 1.05 when a one is overwritten with a one. Normal disk
circuitry is set up so that both these values are read as ones, but
using specialised circuitry it is possible to work out what
previous “layers” contained. The recovery of at least one or two
layers of overwritten data isn’t too hard to perform by reading the
signal from the analog head electronics with a high-quality digital
sampling oscilloscope, downloading the sampled waveform to a PC,
and analysing it in software to recover the previously recorded
signal.”
“To erase magnetic media, we need to overwrite it many times
with alternating patterns in order to expose it to a magnetic field
oscillating fast enough that it does the desired flipping of the
magnetic domains in a reasonable amount of time. Unfortunately,
there is a complication in that we need to saturate the disk
surface to the greatest depth possible, and very high frequency
signals only “scratch the surface” of the magnetic medium. Disk
drive manufacturers, in trying to achieve ever-higher densities,
use the highest possible frequencies, whereas we really require the
lowest frequency a disk drive can produce. Even this is still
rather high. The best we can do is to use the lowest frequency
possible for overwrites, to penetrate as deeply as possible into
the recording medium.”