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If I can change the key for my encrypted storage without taking the system offline to re-encrypt the storage or seeing any sign of a background rebuild occurring, it rather implies that what I described as the key is actually used to retrieve the clear text of the real key used to encrypt the storage.

If this is the case, then as long as the key I know is as strong as the real encrpytion key, is there any benefit to rotating the key I know?

Is this just a tick box exercise?

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I'll use password to refer to what you know, and key to the random key used to actually encrypt the data.

A password has a lot of exposure. No matter how strong it is, someone might:

  • use a keylogger
  • watch over your shoulder as you type
  • it might be captured on a "security" camera
  • you might be coerced into giving it up
  • ...

These are all reasons to change the password, even if the key is left unchanged.

An attacker can try to crack the key directly, but a key is generally something like 128 or 256 bits of random data. In order to brute force that, you'd use an amount of energy somewhere between boiling the earth's oceans and all of the Sun's output. So that's not really a risk at all.

The only reason to change the key, is when someone obtained the password and managed to decrypt the key before you could change the password. At that point, they know the key and changing the password without the changing the key is not going to help anymore.


By the way, good question! It seems like a simple question with a simple answer, but by reasoning about it in this answer, I actually learned a few things. Like that changing the key might be necessary after a password compromised. I hadn't given that thought before and assumed that the data encryption key could always be static.

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Usually full disk encryption is a two step process:

  • A symmetric key is generated, and used to encrypt the data.
  • The symmetric key is encrypted using a passphrase or similar mechanism, and stored in a predictable place on disk.
  • When you change your passphrase, you change the encryption of the key, but not the key itself.

The key can additionally be stored inside the TPM (bitlocker), or you can have multiple ways to decrypt the key. It can also be stored on a separate drive, or on the network.

You may notice that the key actually used to encrypt the data does not change. If that is to change, all the data has to be read, decrypted, encrypted with the new key and written to disk. It also means that if an attacker gets access to the actual key, and not merely your passphrase, changing your passphrase will not matter.

  • 1
    Thank you for confirming the scenario I described, but I was looking for an explanation of why it works this way. – symcbean Sep 14 '17 at 18:42
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  • There is no reason to rotate keys. Maybe you'd like to rotate passwords instead. This should be doable and could have some logic.

  • The key itself is not the problem. The problem may be the actual security of your password that gets to access your key.

  • Altering the key will require for you to re-encrypt all the data protected by that key, there's no way around this. New key....new data blocks.

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Two situations I can think of:

  • If your key is compromised, but the attacker only 'reads along' (and does not change anything, so you won't notice), you will lock him out again by changing your key.
  • If an old key would be compromised.

Edit: Considering 'your' key would be easier to compromise (by e.g. using a key logger on your computer) than the 'real' key, it would still be wise to rotate your keys.

  • Not really - this is just the argument about whether passwords should be rotated. – symcbean Sep 14 '17 at 11:35
  • So you mean the risk of the 'real' key being compromised is as big as the risk that 'your' key would be compromised? Because this is only the case for brute force attacks, right? – aaphond Sep 14 '17 at 11:59
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    'Your' key could be compromised by e.g. a keylogger on your computer, which is not the case for the 'real' key. – aaphond Sep 14 '17 at 12:07

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