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I am wondering about LUKS nuke patch efficiency. In fact, I think I don't really understand it.

From what I know of LUKS, a passphrase is used to encrypt a random key (the master key), which is used to encrypt the data. Both the encrypted master key and the encrypted data are stored on the partition. LUKS nuke equals erasing the encrypted master key.

Let's say my passphrase contains uppercase, lowercase, symbols, number, and is 30 characters long.

Now, is the master key encrypted with the same algorithm as the data? If so, how is it safe to erase the encrypted master key? I just have to try to decrypt the data.

Where am I wrong? Does it take much longer? Are the algorithms different? Do I miss something?

What is the security betterment?

Thank you for your explanations.

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  • What difference does it make if the algorithms are different? They are known anyways.
    – SPRBRN
    Dec 22, 2014 at 11:20
  • Maybe a secret encrypted with algorithm A takes longer to try to decrypt than a secret encrypted with algorithm B.
    – bh42
    Dec 22, 2014 at 11:21

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The LUKS master key is generated in a truly random manner. As a result, there is no way to guess it faster than simply trying every possible key, and it provides the full potential strength of the underlying encryption algorithm.

Your passphrase is probably not random (and even if it is, it only provides 197 bits of security, while some encryption algorithms provide as much as 256 bits). Because of this, trying to guess the passphrase is probably faster than trying to guess the key derived from it.

Note that LUKS Nuke provides no additional security over plain LUKS when faced with an intelligent attacker. A smart attacker will make a copy of the encrypted volume before trying to beat your password out of you, and giving them the "erase" password will only wipe the copy, not the original.

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  • OK, so if I use a really strong passphrase, erasing the master key would only protect my data from a guessing, right?
    – bh42
    Dec 22, 2014 at 11:58
  • Or they'd just use their own copy of cryptsetup. No reason for them to use yours.
    – forest
    Apr 11, 2018 at 23:26
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All the LUKS nuke patch does (upon entry of the key) is delete the cryptographic key used to encrypt your hard drive thus making it impossible to decrypt your hard drive, however as Mark said there are way's to circumvent this such as cloning the drive (which is usually the first step in a forensic analysis anyway) However if you were able to anticipate an attack or raid, then it would probably be wise to enter the nuke key in before they have a chance to make backups of your drive, which is not the case for most people. This countermeasure would be much more effective if they were a way to prevent cloning of the drive, someday we'll have this, maybe sooner than later.

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Just to add to CPagan's answer (I'm not allowed to comment)...

Before nuking your key (when a raid is expected or you're about to travel with sensitive data), there is no need to back up the entire drive. Only back up the header of the encrypted volume - it is tiny. For example:
cryptsetup luksHeaderBackup --header-backup-file luksheader.bak /dev/somedevice0

Now encrypt the header file (using openssl or such) and store separately (online or with someone else).

Nuke your key.

To restore header:
Decrypt (using what ever method you used to encrypt) and then restore with something like:
cryptsetup luksHeaderRestore --header-backup-file luksheader.bak /dev/somedevice0

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