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It depends on the value of your data. It's theoretically possible to use specialized equipment to find remnants of the data even after having been zero'd out (

It depends on the value of your data. It's theoretically possible to use specialized equipment to find remnants of the data even after having been zero'd out (by taking out the platters and very carefully poring over the ever-so-slight analog-induced discrepancies in what "0" and "1" look like), but unless your attacker is the NSA you probably don't have to worry about that (and if your attacker is the NSA, you're probably already using full-disk encryption anyway, which largely renders disk erasure a moot point).

charcircuit 4y ago
>It's theoretically possible to use specialized equipment to find remnants of the data even after having been zero'd out No, this is not possible neither in theory nor in practice. >by taking out the platters and very carefully poring over the ever-so-slight analog-induced discrepancies in what "0" and "1" look like Even doing this is not enough to recover the data. What 0 and 1 look like isn't even constant. There is variability in what the current value is set to and their is variability in what the previous value was set to. There is too much randomness to recover the original data. >which largely renders disk erasure a moot point Disk erasure in this case can still be useful. In this case you only have to overwrite the master key. The idea that overwritten data can be recovered is an urban legend that is not based off reality, and simply wastes people's time and contributes to ewaste from people destroying harddrives. For just wiping data overwriting it with anything is fine. If you plan on using the disk for FDE you should overwrite the drive with random bytes so that you don't leak the size of the used disk space.
- yellowapple 4y ago
  > No, this is not possible neither in theory nor in practice. Then go tell that to Peter Gutmann: https://www.usenix.org/legacy/publications/library/proceedings/sec96/full_papers/gutmann/index.html https://www.usenix.org/legacy/publications/library/proceedin... The saving grace here is that newer drives are a lot denser than the ones in common use back then, so data recovery in this fashion is prohibitively expensive for anything manufactured in the last decade or so. You basically need to be more precise in your reads than hard drives themselves - and that's a pretty tall order. Still, technology does march on; there remains a risk that recovery techniques could catch up, and there are indeed folks who need absolute guarantees of erasure - and there ain't much more absolute than physical destruction. > What 0 and 1 look like isn't even constant. There is variability in what the current value is set to and their is variability in what the previous value was set to. It's exactly that variability that makes data recovery possible. > simply wastes people's time and contributes to ewaste from people destroying harddrives. It doesn't really do either. Again: this is really only relevant if you're being targeted by the sorts of nation-state-level intelligence agencies that have the time, resources, and expertise for this sort of thing - and when you're such a target, guaranteed data destruction matters a lot more than the amount of e-waste you generate. > If you plan on using the disk for FDE you should overwrite the drive with random bytes so that you don't leak the size of the used disk space. This doesn't always work - for example, if you want to use TRIM on SSDs, since unused blocks are zero'd out instead of left as-is. The usual recommendation there for maximum security is "just don't use TRIM", with the SSD performance and longevity implications that entails.
  - charcircuit 4y ago
    >Then go tell that to Peter Gutmann: Which is what the authors of "Overwriting Hard Drive Data: The Great Wiping Controversy" did. The highlight of their response is that for the recovery method described in Gutmann's paper to be accurate it requires a copy of the original data. When the authors tested a drive that existed at the time Gutmann wrote that paper in optimal conditions there was only a 92% chance of recovering a bit correctly. The chance to successfully recover exponentially decays with the number of bits you want to recover. >if you want to use TRIM on SSDs We were talking about HDDs. SSDs are out of scope.
    - yellowapple 4y ago
      > When the authors tested a drive that existed at the time Gutmann wrote that paper in optimal conditions there was only a 92% chance of recovering a bit correctly. The chance to successfully recover exponentially decays with the number of bits you want to recover. That's still a non-zero probability. For folks whose tolerance for data exfiltration is "zero", anything non-zero, no matter how infinitesimally small, is non-acceptable - hence: physical destruction rather than relying on overwriting. Thankfully, as mentioned previously, said folks are a pretty tiny minority of the total hard drive market, and are typically the folks encrypting everything at rest anyway. A random pass or two over encrypted data is more than enough to ensure that even if an attacker is able to accurately recover old bits, there's no telling whether or not those old bits are actually data. > We were talking about HDDs. SSDs are out of scope. If we're going to adjust our expectations for "overwriting is sufficient for data destruction" based on changes in technology, then SSDs are absolutely in scope - and indeed have their own set of implications when it comes to data retention after overwrite (like TRIM, as mentioned before, or the general case of two writes to the "same" block not necessarily being written to the exact same set of cells).
      - charcircuit 4y ago
        >That's still a non-zero probability Just as there is a nonzero chance of someone recovering your data by reading /dev/urandom. Technically your data could be outputed but the person reading /dev/urandom would not be able to tell if it was just random data or if it was what was wiped. Even if your tolerance for data exfiltration is 0 you can be okay with a non 0 chance for someone to randomly generate the data. >If we're going to adjust our expectations for "overwriting is sufficient for data destruction" based on changes in technology, then SSDs are absolutely in scope The process of wiping SSDs is different because you don't do it by overwriting all of the data, but instead by initiating a secure erase.

charcircuit 4y ago

>It's theoretically possible to use specialized equipment to find remnants of the data even after having been zero'd out No, this is not possible neither in theory nor in practice. >by taking out the platters and very carefully poring over the ever-so-slight analog-induced discrepancies in what "0" and "1" look like Even doing this is not enough to recover the data. What 0 and 1 look like isn't even constant. There is variability in what the current value is set to and their is variability in what the previous value was set to. There is too much randomness to recover the original data. >which largely renders disk erasure a moot point Disk erasure in this case can still be useful. In this case you only have to overwrite the master key. The idea that overwritten data can be recovered is an urban legend that is not based off reality, and simply wastes people's time and contributes to ewaste from people destroying harddrives. For just wiping data overwriting it with anything is fine. If you plan on using the disk for FDE you should overwrite the drive with random bytes so that you don't leak the size of the used disk space.

yellowapple 4y ago
> No, this is not possible neither in theory nor in practice. Then go tell that to Peter Gutmann: https://www.usenix.org/legacy/publications/library/proceedings/sec96/full_papers/gutmann/index.html https://www.usenix.org/legacy/publications/library/proceedin... The saving grace here is that newer drives are a lot denser than the ones in common use back then, so data recovery in this fashion is prohibitively expensive for anything manufactured in the last decade or so. You basically need to be more precise in your reads than hard drives themselves - and that's a pretty tall order. Still, technology does march on; there remains a risk that recovery techniques could catch up, and there are indeed folks who need absolute guarantees of erasure - and there ain't much more absolute than physical destruction. > What 0 and 1 look like isn't even constant. There is variability in what the current value is set to and their is variability in what the previous value was set to. It's exactly that variability that makes data recovery possible. > simply wastes people's time and contributes to ewaste from people destroying harddrives. It doesn't really do either. Again: this is really only relevant if you're being targeted by the sorts of nation-state-level intelligence agencies that have the time, resources, and expertise for this sort of thing - and when you're such a target, guaranteed data destruction matters a lot more than the amount of e-waste you generate. > If you plan on using the disk for FDE you should overwrite the drive with random bytes so that you don't leak the size of the used disk space. This doesn't always work - for example, if you want to use TRIM on SSDs, since unused blocks are zero'd out instead of left as-is. The usual recommendation there for maximum security is "just don't use TRIM", with the SSD performance and longevity implications that entails.
- charcircuit 4y ago
  >Then go tell that to Peter Gutmann: Which is what the authors of "Overwriting Hard Drive Data: The Great Wiping Controversy" did. The highlight of their response is that for the recovery method described in Gutmann's paper to be accurate it requires a copy of the original data. When the authors tested a drive that existed at the time Gutmann wrote that paper in optimal conditions there was only a 92% chance of recovering a bit correctly. The chance to successfully recover exponentially decays with the number of bits you want to recover. >if you want to use TRIM on SSDs We were talking about HDDs. SSDs are out of scope.
  - yellowapple 4y ago
    > When the authors tested a drive that existed at the time Gutmann wrote that paper in optimal conditions there was only a 92% chance of recovering a bit correctly. The chance to successfully recover exponentially decays with the number of bits you want to recover. That's still a non-zero probability. For folks whose tolerance for data exfiltration is "zero", anything non-zero, no matter how infinitesimally small, is non-acceptable - hence: physical destruction rather than relying on overwriting. Thankfully, as mentioned previously, said folks are a pretty tiny minority of the total hard drive market, and are typically the folks encrypting everything at rest anyway. A random pass or two over encrypted data is more than enough to ensure that even if an attacker is able to accurately recover old bits, there's no telling whether or not those old bits are actually data. > We were talking about HDDs. SSDs are out of scope. If we're going to adjust our expectations for "overwriting is sufficient for data destruction" based on changes in technology, then SSDs are absolutely in scope - and indeed have their own set of implications when it comes to data retention after overwrite (like TRIM, as mentioned before, or the general case of two writes to the "same" block not necessarily being written to the exact same set of cells).
    - charcircuit 4y ago
      >That's still a non-zero probability Just as there is a nonzero chance of someone recovering your data by reading /dev/urandom. Technically your data could be outputed but the person reading /dev/urandom would not be able to tell if it was just random data or if it was what was wiped. Even if your tolerance for data exfiltration is 0 you can be okay with a non 0 chance for someone to randomly generate the data. >If we're going to adjust our expectations for "overwriting is sufficient for data destruction" based on changes in technology, then SSDs are absolutely in scope The process of wiping SSDs is different because you don't do it by overwriting all of the data, but instead by initiating a secure erase.