On Tue, Jul 14, 2015 at 6:21 PM, R0b0t1 <r03...@gmail.com> wrote: > > On Sun, Jul 12, 2015 at 7:18 PM, Rich Freeman <ri...@gentoo.org> wrote: > >I think that assumes that the two get averaged together in some way > >and cannot be separated. If you could determine the orientation of > >individual magnetic domains it is possible that you might be able to > >determine which ones are which. For example, if in a given location > >you found 90% of the grains had one orientation, and 10% had another, > >you might be able to infer that the 10% was the previous value of that > >location. > > Every bit on the disk will have this ghost inverse behind it. If you > flip bits at random - what overwriting the drive with random data > effectively does - then it's impossible to tell which ones were > flipped recently and which ones were flipped before the last write.
If a disk head moves across a track and lays down a pattern of magnetic fields, I imagine that the intensity of those fields will vary with distance from the head. If the head makes a second pass writing a different pattern of magnetic fields following a path not identical to the first, I imagine that those field intensities will also vary with distance from the head, but particles on ones die of the track will probably retain more of the former pattern and particles on the other side of the track would tend to retain more of the second pattern. I'm just not seeing anything that suggests that such an attack is physically impossible. It might be impractical today. It might be impractical forever. However, impossible is a very high bar to clear. Whether somebody with a technical capability so advanced that it is so debatable today fits within your threat model is a different story. Clearly these techniques are not available commercially/etc. If you're afraid of the NSA and you have unencrypted data on a disk in the first place they've probably already defeated your security in 100 different ways already. So, I'll agree there is a practical argument to be made. However, I can't really agree that something is physically impossible unless you can prove it from first principles. -- Rich
