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I recently installed Ubuntu onto an encrypted partition using the LUKS+LVM combination that the 'alternate' installer offers. Now I want to create backups of my HD.

From the LUKS FAQ:

6.15 Can I clone a LUKS container?

You can, but it breaks security, because the cloned container has the same header and hence the same master key. You cannot change the master key on a LUKS container, even if you change the passphrase(s), the master key stays the same. That means whoever has access to one of the clones can decrypt them all, completely bypassing the passphrases.

How can the existence of a perfect clone of my encrypted HD break security if it contains the exact same information as my live encrypted HD?

In which sense does keeping updated clones of my LUKS-encrypted HD around "break security"?

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1 Answer 1

up vote 4 down vote accepted

As long as you keep the clone identical to the original, there is no security breach. But if you add data to either the original or the clone, or change the passphrase, the existence of the clone adds security issues.

The problem with cloning is being able to access the clone will forever be equivalent to being able to access the original. First, if you use the clone in an insecure environment where someone can obtain your passphrase (with a keylogger, shoulder surfing, …), then they get your passphrase for the original. Furthermore, if someone is ever able to obtain the master key on the clone, then they have the master key on the original (i.e. they can access the original, whether they have a passphrase for it or not). So if

  • you mount the clone on a compromised machine; or
  • an attacker can obtain your passphrase and a copy of the clone encrypted with this passphrase; or
  • an attacker has access to the encrypted clone and is able to crack your passphrase by a dictionary search;

then the attacker gains access to the clone and can decrypt the original. Since the master key is fixed in stone, this is true even if you changed the passphrase after cloning.

To clone a LUKS volume securely, you should create a new volume, apply the same passphrase (unless you want to change it), and copy the data.

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thank you for your answer. That clarifies things. Your first sentence doesn't really agree with all the info on "the problem with cloning" you provide below, though. –  laramichaels Jun 10 '12 at 1:38

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