4 ms·

Is the obvious reason securely erasing data from a flash drive? I did not know that that is not possible.

by witty_username 10y ago

raverbashing 10y ago
dd if=/dev/urandom of=/dev/sdx bs=1024 count=(size of drive in K) Do it for as many times as you want.
- teddyh 10y ago
  That does not really work. https://en.wikipedia.org/wiki/Data_remanence#Data_on_solid-state_drives https://en.wikipedia.org/wiki/Data_remanence#Data_on_solid-s...
  - raverbashing 10y ago
    > Second, overwriting the entire visible address space of an SSD twice is usually, but not always, sufficient to sanitize the drive. Overwrite you whole SSD 10 times I doubt anyone but the most serious attacker can find anything, and even then.
  - mikeash 10y ago
    Even ignoring that substantial problem, dd also tends not to work when the hardware is broken. (And you can't be sure it's sufficiently broken to prevent data recovery.)
    - setq 10y ago
      Yes. Incidentally when you get a nice fresh SAN for example, you can dd 20TiB volumes in a couple of seconds. Doesn't mean they have been written to.
- michaelt 10y ago
  As flash drives do automatic wear levelling and remapping of bad sectors, some people worry that their data might remain on the flash, in sectors they can't read or write because of those features.
  - zaroth 10y ago
    If someone is that concerned perhaps they should have been using an encrypted volume on the flash drive in the first place.
    - setq 10y ago
      This is true. However you have to trade off the risk of an entire volume failing due to a partial cell failure which would mean only the loss of a single file if it wasn't encrypted. This device was stored securely in a fire safe at home, not used as a portable device.
  - pkaye 10y ago
    I've worked on SSD firmware for many years and for most of the implementation algorithms I'm aware of, if you write the full drive range twice with random data, that should overwrite all the original data.
    - janc_ 10y ago
      "Most" implies "not all"…
  - raverbashing 10y ago
    I find these arguments funny Where do you think the drive will keep data after saving an amount of data multiple times the size of the drive? What you describe might happen if you "delete" a file and save a new one. Then it is most likely recoverable.
    - setq 10y ago
      Not all drives allocate all flash cells for storage. Some are metadata and some are spare so they wear leveling has some headroom. In fact if you use Samsung disks you can leave some headroom extra on the device to handle these scenarios.
joncrocks 10y ago
The 'problem' is that most SSDs don't necessarily erase data when you 'delete' something. SSDs have a number of 'tricks' that make them subtly different to hard drives, which include things like: * Only whole 'segments' of the drive can be 'cleared' at a time, meaning the actual removal of data may not occur until some time after the file is no longer accessible through the file-system * Often SSDs will have 'over provision' meaning that although your drive is labelled "32Gb", it might have more than that available physically (e.g. https://www.kingston.com/en/ssd/overprovisioning https://www.kingston.com/en/ssd/overprovisioning). This is used to augment the capacity as areas of the drive degrade over the life of the drive. See http://arstechnica.com/security/2011/03/ask-ars-how-can-i-safely-erase-the-data-from-my-ssd-drive/ http://arstechnica.com/security/2011/03/ask-ars-how-can-i-sa... for a bit more on this topic.
- deleted 10y ago
  [deleted]

Is the obvious reason securely erasing data from a flash drive? I did not know that that is not possible.

raverbashing 10y ago

dd if=/dev/urandom of=/dev/sdx bs=1024 count=(size of drive in K) Do it for as many times as you want.

teddyh 10y ago
That does not really work. https://en.wikipedia.org/wiki/Data_remanence#Data_on_solid-state_drives https://en.wikipedia.org/wiki/Data_remanence#Data_on_solid-s...
- raverbashing 10y ago
  > Second, overwriting the entire visible address space of an SSD twice is usually, but not always, sufficient to sanitize the drive. Overwrite you whole SSD 10 times I doubt anyone but the most serious attacker can find anything, and even then.
- mikeash 10y ago
  Even ignoring that substantial problem, dd also tends not to work when the hardware is broken. (And you can't be sure it's sufficiently broken to prevent data recovery.)
  - setq 10y ago
    Yes. Incidentally when you get a nice fresh SAN for example, you can dd 20TiB volumes in a couple of seconds. Doesn't mean they have been written to.
michaelt 10y ago
As flash drives do automatic wear levelling and remapping of bad sectors, some people worry that their data might remain on the flash, in sectors they can't read or write because of those features.
- zaroth 10y ago
  If someone is that concerned perhaps they should have been using an encrypted volume on the flash drive in the first place.
  - setq 10y ago
    This is true. However you have to trade off the risk of an entire volume failing due to a partial cell failure which would mean only the loss of a single file if it wasn't encrypted. This device was stored securely in a fire safe at home, not used as a portable device.
- pkaye 10y ago
  I've worked on SSD firmware for many years and for most of the implementation algorithms I'm aware of, if you write the full drive range twice with random data, that should overwrite all the original data.
  - janc_ 10y ago
    "Most" implies "not all"…
- raverbashing 10y ago
  I find these arguments funny Where do you think the drive will keep data after saving an amount of data multiple times the size of the drive? What you describe might happen if you "delete" a file and save a new one. Then it is most likely recoverable.
  - setq 10y ago
    Not all drives allocate all flash cells for storage. Some are metadata and some are spare so they wear leveling has some headroom. In fact if you use Samsung disks you can leave some headroom extra on the device to handle these scenarios.

joncrocks 10y ago

The 'problem' is that most SSDs don't necessarily erase data when you 'delete' something. SSDs have a number of 'tricks' that make them subtly different to hard drives, which include things like: * Only whole 'segments' of the drive can be 'cleared' at a time, meaning the actual removal of data may not occur until some time after the file is no longer accessible through the file-system * Often SSDs will have 'over provision' meaning that although your drive is labelled "32Gb", it might have more than that available physically (e.g. https://www.kingston.com/en/ssd/overprovisioning https://www.kingston.com/en/ssd/overprovisioning). This is used to augment the capacity as areas of the drive degrade over the life of the drive. See http://arstechnica.com/security/2011/03/ask-ars-how-can-i-safely-erase-the-data-from-my-ssd-drive/ http://arstechnica.com/security/2011/03/ask-ars-how-can-i-sa... for a bit more on this topic.

deleted 10y ago
[deleted]