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Index > Heap > why several passes to shred file?

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sleepsleep



Joined: 05 Oct 2006
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sleepsleep
i see software to shred files provide several passes, but i don't understand why?

why it provide several passes?
is that after i zero the file bases on the file size, the file still can be recovered?
Post 14 Jul 2012, 11:27
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JohnFound



Joined: 16 Jun 2003
Posts: 3500
Location: Bulgaria
JohnFound
I am not very sure about the connection with the reality, but there is an urban myth (I don't know where I hear or read about it) that after one pass of writing, the magnetic carrier may still keep some remaining traces of the previous record and this record can be read with special hardware.

IMHO, it is possible but very improbable.
So, the explanation is maybe "marketing tricks". You know: "Our program writes 100 times in order to guarantee full irrecoverable erase!"
Post 14 Jul 2012, 12:18
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AsmGuru62



Joined: 28 Jan 2004
Posts: 1409
Location: Toronto, Canada
AsmGuru62
I once was talking to some CSI guy.
The special hardware they have is basically a hard drive with the ability to move the heads in a micron increments.
The normal drive heads are set to move by a larger increments, so the neighbouring drive tracks will not interfere.
So, when zeroes are written into a magnetic surface - the data can still be recovered by moving heads (which normal HD can't do - the one which zeroed that sector) by a microns on each side of the track.
The only way to destroy data is physically destroy magnetic plates in the drive, like dismantle it and then shred the plates, but that would kill ALL files!
Smile
Post 14 Jul 2012, 13:44
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sleepsleep



Joined: 05 Oct 2006
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sleepsleep
wow,
awesome information!! thanks to both of u!
Post 14 Jul 2012, 14:44
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17279
Location: In your JS exploiting you and your system
revolution
With modern HDDs one pass is enough. There is no way to recover anything even if you microstep the heads.
Post 14 Jul 2012, 14:51
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Madis731



Joined: 25 Sep 2003
Posts: 2141
Location: Estonia
Madis731
To extend revolution's answer: modern HDD-s already do very little steps to meet the growing need for data. I've read somewhere that they must deal with "not interfering with neighboring cells/tracks" on daily basis now.
Post 16 Jul 2012, 06:55
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r22



Joined: 27 Dec 2004
Posts: 805
r22
Would it be easier or harder to erase a specific file on an SSD than an HDD.
SSD's have over provisioning that acts as a cache/workspace of sorts, but on the other hand when flash is re-written there's no ghostly magnetic residue of the past like with magnetic storage.

With how relatively cheap DDR2/3 is I wish a board with 8-16 RAM slots and a battery backup that can power them during an outage or relocation would come down to a use friendly price. Volatile memory is faster and more fun than flash or magnets.
Post 16 Jul 2012, 15:24
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kalambong



Joined: 08 Nov 2008
Posts: 165
kalambong
AsmGuru62 wrote:
I once was talking to some CSI guy.
The special hardware they have is basically a hard drive with the ability to move the heads in a micron increments.
The normal drive heads are set to move by a larger increments, so the neighbouring drive tracks will not interfere.
So, when zeroes are written into a magnetic surface - the data can still be recovered by moving heads (which normal HD can't do - the one which zeroed that sector) by a microns on each side of the track.
The only way to destroy data is physically destroy magnetic plates in the drive, like dismantle it and then shred the plates, but that would kill ALL files!
Smile



If they truly want to read your HD by moving the drive head one micron at a time, they have to replace almost everything inside your HD - except for the plates, - in order to make it work

Normal drive heads are not made to move one micron at a time - at least not yet
Post 16 Jul 2012, 23:52
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AsmGuru62



Joined: 28 Jan 2004
Posts: 1409
Location: Toronto, Canada
AsmGuru62
They take off plates from the HD to be investigated and put them in THEIR system.
Then they move THEIR heads somehow and perform statistical analysis of magnetic domens leftovers.
However, if HD has been put into strong magnetic field - this may prevent CSI from seeing the data, but that is not a common thing.
Post 17 Jul 2012, 04:38
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17279
Location: In your JS exploiting you and your system
revolution
If you are concerned enough about some TLA reading some ghost image of past data on your HDDs then perhaps you should consider not putting your data on the HDD in the clear in the first place.

Instead, make sure that all data hitting the HDD is encrypted. Truecrypt, and others, can do this. So, problem solved. And you never have to erase it, just "forget" the passphrase (or kill the header(s) (~1kB only) with 10000 (or whatever other number you feel comfortable with) passes of random junk).

As for SSDs, it is more problematic to guarantee erasure of your data. As above the same solution applies, just use encryption, problem solved.
Post 17 Jul 2012, 05:34
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Xorpd!



Joined: 21 Dec 2006
Posts: 161
Xorpd!
HCl (for HD) or HF (for SSD) provides good encryption.
Post 17 Jul 2012, 23:30
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
Back in the very very old days, multiple passes were necessary - not so now. Read this, and be sure to read the epilogue. Lots of people seem to be stuck with the voodoo of the past Smile
Post 22 Jul 2012, 00:28
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