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What is considered the minimum amount of overwrites to sanitize media?

For example; there are 1, 2, 3, 7, and 35 overwrites methods to make data irrecoverable.

Using standard recovery methods (such as software such as Recuva or Foremost) what is the minimum amount of overwrites that make the data irrecoverable via software? What about with hardware?

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Here are a few docs on the subject: cryptome.org/afssi5020.htm ; www.thic.org/pdf/August07/UCSDCMRR.fspada.pdf –  Shadok Oct 3 '11 at 14:09
I had to resist the urge to answer "One pass in an autoclave should be sufficient." –  Shinrai Oct 3 '11 at 14:15
My HW-solution - a 20 lbs sledgehammer. Remember the goggles. –  Aki Oct 3 '11 at 15:53

5 Answers 5

up vote 11 down vote accepted

There's quite a lot of info in this question here

Only talking about whole discs:

There are two problems

  1. Is the data destroyed?

  2. Are people who need to know convinced that the data is destroyed?

1) No software claims to be able to recover data that has been overwritten just once. No data recovery company claims to be able to recover data that has been over written once. No university or other research exists which claims to have recovered data that has been over-written.

In these questions people usually mention Peter Gutmann. His 35 overwrite method was designed for use with ancient drives; there were a small set of passes for each different drive-controller type and you would only use all 35 passes if you didn't know what controller you had. Modern drives all use the same controller, so you don't need to do that many passes.

tl:dr - A single pass of all 0 with render the data unrecoverable to anyone. And that anyone includes governments with scanning tunneling electron microscopes.

2) But you can't prove the data has gone. There will always be doubts lingering. So, if you work with sensitive data you should just follow whatever the job specs say. Or, if you need to create the specs, follow some DOD standard (7 passes). Or if you want to be really sure, you can just send the drive for shredding.

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+1 for pointing out that 0ing the drive is enough... but -1 for saying you can't prove it lol. If you try physical recovery on a modern drive good luck. They are so densely packed it makes it pretty much impossible and it would take an extremely long time to recover even a small amount if it was... by small I mean really small as in not even worth trying small. –  Arctor Oct 3 '11 at 16:23
@Arctor - you can't prove a negative. While it's true that no-one can recover the data you'll always get people saying "But the NSA have these microscopes!", and while that's not likely you can't say that secret government agencies don't have secret alien tech. Obviously, if you're worried about the NSA then you need to get armed guards and shredding the discs will do. So, that's all I mean when I say "it can't be proved". –  DanBeale Oct 3 '11 at 16:26
If you want to properly erase the drive use MHDD and make sure its not the BIOS wipe. Use the one that requires you lock the drive and uses built in functions. This will erase bad sectors as well as good ones. DBAN can not wipe bad sectors so I wouldn't rely on it for a complete erase –  Arctor Oct 3 '11 at 16:38
@Arctor: How do you know someone else doesn't have some other kind of microscope that can recover the data? –  David Schwartz Oct 3 '11 at 18:51
7 passes is not actually the DoD standard. For sanitization, the DoD now requires degaussing, but considers a single overwrite sufficient for clearing data. –  Phong Oct 7 '11 at 18:14

I think you should consider adding details on the type of media you're talking about: in the case of nitrocellulose (aka "flash paper") a lighter is all you need to completely destroy any evidence ;)

Talking about computer storage:

It's very hard to protect against physical analysis provided the attackers have sufficient funds and technical knowledge as even degaussing is subject to many potential limitations (magnetic coercivity due to increasingly small and different alloys used for example).

In any way if the loss/theft of your data equals or exceed the price of the media you should physically destroys the media. Take care to use proper means doing so, you can have a look on this link.

Media is cheap, lost data may be awfully expensive if you are the defendant :D

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attacks including "magnetic coercivity" are a myth. –  DanBeale Oct 3 '11 at 16:14
"Citation needed." As far as I know the three types of degaussers in the US military are rated by their capacity to degauss medias with different magnetic coercivity. (source: look for "DEGAUSSERS" here: cryptome.org/afssi5020.htm) –  Shadok Oct 4 '11 at 10:14

In the modern context, a few wipes with random data will do according to Gutmann. The 35 cycle method was meant to cover every eventuality.

I'd just load up a bunch of disks and run DBAN on them overnight to be safe, or use the Linux shred command.

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Another answer specifically for your question:

Using standard tools a single pass will make the data unrecoverable. It's best to use the drive's built in mechanism for this, with an ATA command SECURE ERASE.

This will protect against software and hardware attacks.

If the drive is SSD (or other flash type memory) you need to physically shred it, because proprietary wear-levelling mechanisms mean you don't know if you've over-written data or not. And shredding gives peace of mind for conventional drives as well.

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The SSD issue is solved by encrypting the data, correct? –  josten Feb 23 '12 at 23:20

It's completely based on who you're trying to prevent from seeing the data:

If you want to prevent Uncle Arnold from finding your documents detailing arguments you've had with family members after you give this old computer to him, one or two passes should be fine.

If you're concerned the local immature nerd will buy your used computer from the local second-hand store and find your collection of anarchic writings, 5-10 passes should keep the G-men he'll call away.

If organized crime or the government have a vested interest in gaining access to your data after recovering the drive from the bottom of the Mariana's Trench where you dropped it during an afternoon cruise to that romantic location, you'll probably want to run 35 passes where you're not only erasing the data, but are actually writing over the data with random arrangements of 0's and 1's.

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"One OR TWO passes"? I'd say you give Uncle Arnold too much credit... –  haimg Oct 3 '11 at 14:42
Uncle Arnold gets his ideas about how computers work from shows like CSI, not from me or you (hopefully). And he's more concerned about forwarding chain emails (or now chain Facebook posts) than about what you may or may not have been doing with the computer when you owned it. –  music2myear Oct 3 '11 at 14:53
@haimg: I agree, most people are not even aware that data can be recovered. –  josten Oct 3 '11 at 15:24
If the Mafia or the Feds want your data I recommend thermite as a pretty reliably permanent means of erasure. –  CarlF Oct 3 '11 at 15:55
That works. Mythbusters has some good demonstrations on the proper amount of explosives to completely destroy cement trucks and other small vehicles. A similar amount should work for hard drives with sensitive data or local assassins, whichever you conceive to be the greater threat. –  music2myear Oct 3 '11 at 16:08

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