[dm-crypt] Using plain64/plain IV (initialisation vector) in dm-crypt

Arno Wagner arno at wagner.name
Tue Jul 27 12:47:35 CEST 2010

On Tue, Jul 27, 2010 at 10:46:44AM +0200, Milan Broz wrote:
> This thread is going crazy... :)

Slow day here ;-)

> 1) Facts about using plin IV generator:
> - "plain" IV is 32bit only, supported by all kernels
> - you should avoid using it for >2TB devices
> - it will remain this way because of backward compatibility (howgh:-)
> - "plain64" is fully 64bit, available since kernel 2.6.33
> - for device < 2TB it produces the same output as "plain"
> => use plain64 for new devices if you want to use tweakable encryption
> mode like XTS (or LRW), e.g. cryptsetup -c aes-xts-plain64

Added to the FAQ yesterday. Updated just now that plain64 is
backwards compatible below 2TB.
> p.s.
> Never use plain* IV for CBC mode, use ESSIV there.
> <joke>If you are using ECB mode, you are lost anyway.</joke>
> 2) crypsetup should have always safe defaults.
> It is aes-cbc-essiv:sha256 with 256bit key currently.
> 3) For the resize - we cannot catch all situations, someone can
> dd LUKS disk to another bigger volume without "resize" command.

Yes. My FAQ recomendation is to make a backup, create a new
container in the target size and then restore the data. I 
think resizing the filesystem is just too risky otherwise.

> Tools will suggest using plain64 but it cannot force it.
> > So you guess the the 1TB limit could be actually ...
> Forgot about 1TB limit, it is different XTS only problem.
> We mixed up two unrelated problems here.


Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno at wagner.name 
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