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On Thu, 09 Aug 2012 08:53:27 -0500 |
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Dale <rdalek1967@×××××.com> wrote: |
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|
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> Alex Schuster wrote: |
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> > Dale writes: |
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> > |
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> >> I have seen where people use dd to do this sort of thing to. I |
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> >> read somewhere that if you do a dd and put in all 1's, then all |
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> >> 0's then back again that it is very hard to get any data back off |
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> >> the drive. I think if you do it like over a dozen times, it is |
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> >> deemed impossible to get anything back. I think that is the |
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> >> Government standard of it's gone. |
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> > There's no need for multiple passes of dd with different values. |
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> > |
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> > http://www.h-online.com/security/news/item/Secure-deletion-a-single-overwrite-will-do-it-739699.html |
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> > |
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> > Wonko |
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> > |
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> > |
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> |
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> |
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> I wonder what some Government org like NSA would think about this? |
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> Then again, they may want us to believe this so they can get stuff |
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> back. ;-) ;-) |
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> |
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> That said, I always wondered how something can be there when it is |
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> erased. On paper, I can see that because it made a physical change to |
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> the paper but on magnetic media, it is magnetic not physical. |
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|
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It's quite simple once you understand how disks work. |
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|
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In textbooks we need to keep things simple, so we say things like "the |
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magnetic particles are all aligned this way for a 0 and that way for a |
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1". This gets the concept across but it also let's people believe that |
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bits on a disk are very much binary - like a light switch or a |
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transistor they are either on or off. |
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|
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In practice, nothing could be further from the truth. |
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|
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With disk magnetic media, you aren't dealing with a single isolated |
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thing (such as a chunk of disk that can only be one way), you are |
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dealing with a very very large number of magnetic particles that go to |
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make up one bit. It's how they average out that makes the drive think |
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it's a 0 or a 1. It all works much like tape cassettes - the head has a |
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little coil of wire in it and current flows through the coil. It passes |
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near a magnetic field on the tape, and the current in the coil changes. |
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Read the amount of change using fancy electronics, and voila! you have |
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audio. In the case of disks, it's voila! you have a stream of bits. |
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|
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Disk drives can't afford to be almost right like audio tape though, |
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they have to be exactly right so the drive has some amazing maths built |
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into it for error-correction and redundancy. I believe something like |
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40% of the space containing raw data is pure error checking (so your 3T |
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drive is actually 4.1T, but you will only ever get to use 3T) |
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|
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The trick is, when you overwrite an area of the disk, you don't erase |
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everything, there are some traces left behind. |
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|
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Pencil and paper is a good analogy. Write something in pencil. Erase |
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wit with a rubber eraser, and write something else in the same space. |
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Now hold the paper up to the light and if you know how to look you can |
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see the indents in the paper from the first thing you wrote. Train your |
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eyes to ignore what's written there now and only look at dented paper |
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with no lead marks, and you can read things quite clearly. James Bond |
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was especially good at this but that's a movie so real life isn't |
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*that* quick. |
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|
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Sekrit magic disk software does a very similar thing - it ignores the |
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current data and looks for traces left behind from the last write, and |
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the ones before that. |
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|
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This trick isn't universal of course. As drive technology advances and |
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IBM figures out new ways to do it, they come up with ideas like using |
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the _depth_ of the magnetic material too. Neat trick - you can double |
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the data stored in the same surface area. With each technology advance, |
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things change a lot, so the amount of reading backwards you can do is |
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always changing and depends very much on exactly what drive you have. |
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|
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|
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-- |
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Alan McKinnon |
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alan.mckinnon@×××××.com |