4 ms·

Or you encrypt the entire drive. Then the secure erasure is a matter of throwing away the keys, and you don't have to bother with thinking through your threat m

Or you encrypt the entire drive. Then the secure erasure is a matter of throwing away the keys, and you don't have to bother with thinking through your threat model.

wyager 4y ago
Two issues with that - you have to make sure you dispose of the key(s), and this doesn't let you securely erase just part of the drive.
- buildbot 4y ago
  3. Technically someone could uncover a flaw in whatever encryption you used.
  - rqtwteye 4y ago
    Once you have an attacker with that kind of technical skill, you are in a totally different world.
    - JosephRedfern 4y ago
      Not necessarily -- if the drive was found and decrypted after the exploit was leaked and made public (perhaps YEARS after it being a 0-day), it wouldn't necessarily be a skilled attack.
deleted 4y ago
[deleted]
cryptonector 4y ago
Throwing away the keys is surprisingly hard to do. Say you have one root key and you derive per-file or per-block keys using various metadata (inode numbers, generation numbers, block addresses, whatever). You can't throw away derived keys though! Or say you generate random keys per-file and you store those somewhere (it's annoying to have to store them). Now to throw those keys away you need to re-write the blocks containing them. If there's any behind-the-scenes copy-on-write going on then re-writing a block doesn't necessarily lose its old contents. The only thing that's easy to throw away -maybe- is the top-level / root keys. But that means deleting the entire filesystem.
- generalizations 4y ago
  > The only thing that's easy to throw away -maybe- is the top-level / root keys. But that means deleting the entire filesystem. Well, yeah, that's why I said you encrypt the entire drive. If you do things that make it hard to throw your keys away, then it's gonna be hard to throw your keys away.
- Nursie 4y ago
  > You can't throw away derived keys though! You can, AES-XTS does just this. Per-block keys can be derived on the fly from the root key and the block address. When the root key is gone, everything is gone.
  - cryptonector 4y ago
    Right, but clearly I meant that you can't arrange to delete individual files and lose their derived keys. The reason is that the bits you need to derive the keys may still be somewhere on disk, and one could -having access to the root keys- find them and "undelete" and re-derive the keys and recover the file contents. Having to wipe an entire filesystem to securely delete a file is not useful.

Or you encrypt the entire drive. Then the secure erasure is a matter of throwing away the keys, and you don't have to bother with thinking through your threat model.

wyager 4y ago

Two issues with that - you have to make sure you dispose of the key(s), and this doesn't let you securely erase just part of the drive.

buildbot 4y ago
3. Technically someone could uncover a flaw in whatever encryption you used.
- rqtwteye 4y ago
  Once you have an attacker with that kind of technical skill, you are in a totally different world.
  - JosephRedfern 4y ago
    Not necessarily -- if the drive was found and decrypted after the exploit was leaked and made public (perhaps YEARS after it being a 0-day), it wouldn't necessarily be a skilled attack.

deleted 4y ago

[deleted]

cryptonector 4y ago

Throwing away the keys is surprisingly hard to do. Say you have one root key and you derive per-file or per-block keys using various metadata (inode numbers, generation numbers, block addresses, whatever). You can't throw away derived keys though! Or say you generate random keys per-file and you store those somewhere (it's annoying to have to store them). Now to throw those keys away you need to re-write the blocks containing them. If there's any behind-the-scenes copy-on-write going on then re-writing a block doesn't necessarily lose its old contents. The only thing that's easy to throw away -maybe- is the top-level / root keys. But that means deleting the entire filesystem.

generalizations 4y ago
> The only thing that's easy to throw away -maybe- is the top-level / root keys. But that means deleting the entire filesystem. Well, yeah, that's why I said you encrypt the entire drive. If you do things that make it hard to throw your keys away, then it's gonna be hard to throw your keys away.
Nursie 4y ago
> You can't throw away derived keys though! You can, AES-XTS does just this. Per-block keys can be derived on the fly from the root key and the block address. When the root key is gone, everything is gone.
- cryptonector 4y ago
  Right, but clearly I meant that you can't arrange to delete individual files and lose their derived keys. The reason is that the bits you need to derive the keys may still be somewhere on disk, and one could -having access to the root keys- find them and "undelete" and re-derive the keys and recover the file contents. Having to wipe an entire filesystem to securely delete a file is not useful.