Re: How many overwrites for secure erase?

bz <bz+spr@xxxxxxxxxxxxxxxxxxxx> writes:

"Sebastian G." <seppi@xxxxxxxxx> wrote in
news:62qjtgF2484t3U3@xxxxxxxxxxxxx:

bealoid wrote:

"Sebastian G." <seppi@xxxxxxxxx> wrote in
news:62q4cpF2486s5U1@xxxxxxxxxxxxx:

[snip]

This is nothing special though, even Peter Gutmann mentions much more
potent ways like magnetic force scanning tunneling microscopy.

and even gutmann says that no longer works and that 3 random overwrites
is about as good as you can do.


And even I wrote that one overwrite is enough, but this doesn't change
the result that such techniques allow to retrieve information
statistically significantly better than random guessing. If the high
level data are encoded with redundancy as well, this might be a problem.

remember that you are allowed as many read passes as you want on each
track. This allows 'averaging out' much of the noise.

No it does not because the "noise" is on the disk. It is not random added
noise. It is part of the magnetism of the disk, which is the same on each
read. Now if you had 1000 disks all with the same data and all erased the
same way, then noise averageing may stqnd a chance.



You can use heads that are much narrower than the normal read head.

And yu think they do not use read heads which are as narrow as possible
now? How do you think they get terrabytes on a tiny disk that once held
10MB for the same area?



This allows an increased signal to noise ratio for the buried signal near
the outside of the overwrite band since the bands will not overlap exactly.

Ancient technology.
By using many reads from many 'lanes' through the data, you have a chance
of 'digging' the signal out of the noise.

I think I remember that, at one time, the CIA could dig down through 7
layers of overwriten data to recover the underlying information.

And once upon a time disks held 10MB of datq

Of course, that may take millions of passes per track.

Knowing what the data could be [a 1 or a 0] makes it much easier than
recovery of an analogue signal (nixon's missing minutes).


--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

bz+spr@xxxxxxxxxxxxxxxxxxxx remove ch100-5 to avoid spam trap
.

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