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How does having an image of the encrypted drive allow them to determine whether the system has 'significantly' changed? Just generating a login in the event log

How does having an image of the encrypted drive allow them to determine whether the system has 'significantly' changed? Just generating a login in the event log should make the image completely different.

kkarakk 8y ago
i think the idea is that their(govt/shady entity - what's the difference?) forensic scientists would be able to tell whether the very act of entering a password started changing a system significantly which would count as refusing to comply
- eeZah7Ux 8y ago
  > entering a password started changing a system significantly When different passwords (for the same user) simply decrypt and access different parts of the filesystem it's not the case.
- ethagnawl 8y ago
  > changing a system significantly I'm rehashing your parent's comment, but: Unless the command wiped the entire disk or the entity gained access to the unencrypted versions of the disk before/after the duress command, an outsider wouldn't be able to tell if a log had been updated or if an entire partition had been wiped.
  - yorwba 8y ago
    Disk encryption prevents revealing the plaintext of files on disk, but you can still observe their ciphertext changing when the file changes. If the duress command causes different files to be modified compared to a normal login, then that can be detected by comparing to the original disk image, even though the actual modifications performed are hidden by the encryption. On the other hand, it's possible to delete an encrypted partition by only overwriting the encryption key, which might be a small-enough change to go undetected.
    - lev99 8y ago
      If at least 98% of your drive isn't partitioned out its going to cause some questions by a curious observer.
    - vageli 8y ago
      > Disk encryption prevents revealing the plaintext of files on disk, but you can still observe their ciphertext changing when the file changes. If the duress command causes different files to be modified compared to a normal login, then that can be detected by comparing to the original disk image, even though the actual modifications performed are hidden by the encryption. How would they even be able to determine which files are modified? If we're talking full disk encryption here, you can't tell which files are being accessed/modified, just locations on disk. Without metadata to map blocks to objects they're flying blind.
      - loeg 8y ago
        E.g., If every block on the device changes, that's a pretty big flag. If only a few change, that's expected. So the magnitude of change is both observable and conveys information to the attacker.
    - zbentley 8y ago
      There's a little bit of subtlety here, but in general GP was correct in that it can be made hard for authorities two tell exactly what changed. Disk encryption, unlike other forms, does not have a terribly high avalanche factor when small changes are made--because it's expensive to write lots of things to disk. However, it is possible to make a small change (as small as, say, writing the audit log file on a real successful login) that renders data completely inaccessible. Consider an encrypted disk on which you can tell the magnitude of changes on the filesystem, but not which data has changed. Let's say you have a lot (many gigabytes) of sensitive data on that disk. If a successful login triggers the encrypted filesystem to decrypt the contents of the disk using an encryption key (of, say, a 4kb length) that is stored only on that disk, then a duress code could simply destroy (or corrupt by randomizing a few bytes) that key, rendering the contents of the disk inaccessible, without writing more than a very small amount of data. This fundamentally trades off deniability for data security: the disk would still contain all of the encrypted data and could be brute-forced, but that would be the case anyway if an image had been taken previously. Of course, situations in which that deniability would be legally well-received are, as others here have pointed out, vanishingly rare.
dTal 8y ago
If the root drive is encrypted then what good does this software do? By the time login_duress has a chance to run, you've already had to type in a decryption password.
loeg 8y ago
That isn't how any full-disk encryption scheme works. A login event entry probably changes a single block.

kkarakk 8y ago

i think the idea is that their(govt/shady entity - what's the difference?) forensic scientists would be able to tell whether the very act of entering a password started changing a system significantly which would count as refusing to comply

eeZah7Ux 8y ago
> entering a password started changing a system significantly When different passwords (for the same user) simply decrypt and access different parts of the filesystem it's not the case.
ethagnawl 8y ago
> changing a system significantly I'm rehashing your parent's comment, but: Unless the command wiped the entire disk or the entity gained access to the unencrypted versions of the disk before/after the duress command, an outsider wouldn't be able to tell if a log had been updated or if an entire partition had been wiped.
- yorwba 8y ago
  Disk encryption prevents revealing the plaintext of files on disk, but you can still observe their ciphertext changing when the file changes. If the duress command causes different files to be modified compared to a normal login, then that can be detected by comparing to the original disk image, even though the actual modifications performed are hidden by the encryption. On the other hand, it's possible to delete an encrypted partition by only overwriting the encryption key, which might be a small-enough change to go undetected.
  - lev99 8y ago
    If at least 98% of your drive isn't partitioned out its going to cause some questions by a curious observer.
  - vageli 8y ago
    > Disk encryption prevents revealing the plaintext of files on disk, but you can still observe their ciphertext changing when the file changes. If the duress command causes different files to be modified compared to a normal login, then that can be detected by comparing to the original disk image, even though the actual modifications performed are hidden by the encryption. How would they even be able to determine which files are modified? If we're talking full disk encryption here, you can't tell which files are being accessed/modified, just locations on disk. Without metadata to map blocks to objects they're flying blind.
    - loeg 8y ago
      E.g., If every block on the device changes, that's a pretty big flag. If only a few change, that's expected. So the magnitude of change is both observable and conveys information to the attacker.
  - zbentley 8y ago
    There's a little bit of subtlety here, but in general GP was correct in that it can be made hard for authorities two tell exactly what changed. Disk encryption, unlike other forms, does not have a terribly high avalanche factor when small changes are made--because it's expensive to write lots of things to disk. However, it is possible to make a small change (as small as, say, writing the audit log file on a real successful login) that renders data completely inaccessible. Consider an encrypted disk on which you can tell the magnitude of changes on the filesystem, but not which data has changed. Let's say you have a lot (many gigabytes) of sensitive data on that disk. If a successful login triggers the encrypted filesystem to decrypt the contents of the disk using an encryption key (of, say, a 4kb length) that is stored only on that disk, then a duress code could simply destroy (or corrupt by randomizing a few bytes) that key, rendering the contents of the disk inaccessible, without writing more than a very small amount of data. This fundamentally trades off deniability for data security: the disk would still contain all of the encrypted data and could be brute-forced, but that would be the case anyway if an image had been taken previously. Of course, situations in which that deniability would be legally well-received are, as others here have pointed out, vanishingly rare.

dTal 8y ago

If the root drive is encrypted then what good does this software do? By the time login_duress has a chance to run, you've already had to type in a decryption password.

loeg 8y ago

That isn't how any full-disk encryption scheme works. A login event entry probably changes a single block.