Re: reencrypt: how to specify old and new key-files?

[Ondrej Kozina <okozina@redhat.com>]

Hi,

On 08. 11. 22 23:52, Philippe Cerfon wrote:
> Hey.
> 
> Sorry for having to ask again.
> 
> When I'm reencrypting a device in order to change the volume key and
> also the keyslot (say from PBKDF2 to Argon2) wouldn't there need to be
> some way to specify two different --key-file?

It's not necessary. With LUKS2 you may change keyslot parameters without 
need for full device reencryption. You may split your goal into two 
individual steps. Reencrypt the device to refresh volume key and later 
change keyslot parameters as you see fit.

> 
> One for the old keyslot with the old VK, and one for the new keyslot
> to be generated?
> 
> Especially if with --key-slot <old slot ID>, the old slot is
> overwritten and all others removed, as explained by the manpage:
>> For reencryption mode it selects specific keyslot (and passphrase)
>> that can be used to unlock new volume key. If used all other keyslots
>> get removed after reencryption operation is finished.
> 
> But it seems there is only the --key-file option, for which it's even
> unclear to me, whether it's for the old or new keyslot/VK?

It's for both, old and new keyslot. We do not want to support 
reencryption where old and new keyslot is unlocked by different 
passphrase for compatibility reasons. If LUKS2 reencryption gets 
interrupted (no matter the reason) it would be difficult to re-activate 
the device again unless it's fully reencrypted. Most libcryptsetup 
applications (including e.g. systemd) does not expect two different 
passphrase prompts while unlocking the device. So two passphrases prompt 
would break system boot, cryptsetup open scripts and so on.

The cryptsetup utility (reencrypt action) therefore supports only two 
setups. Either you provide all passphrases for all currently active 
keyslots (and all keyslots get recreated storing new/future volume key), 
or you choose single keyslot and provide passphrase for it stored in 
--key-file.

Again it's not big deal. You may change the old passphrase(s) before or 
after reencryption operation completes.

We do plan to add support for initializing reencryption using different 
unlock methods (e.g. LUKS2 tokens) but it will be added in future 
version. For passphrase based activation the precondition for old and 
new keyslot both having same passphrase will remain

O.