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

Arno Wagner arno at wagner.name
Tue Jul 27 18:08:10 CEST 2010

On Tue, Jul 27, 2010 at 04:17:12PM +0200, Christoph Anton Mitterer wrote:
> On Tue, 2010-07-27 at 12:47 +0200, Arno Wagner wrote: 
> > 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.
> Why exactly do you mean? Any other pitfalls than that a user continues
> to use plain instead of plain64?
> I thought everything else in the LUKS header would be independent of the
> amount of storage after it.

Yes, but you need to 

  a) resize the raw container (partition, file, disk)
  b) reboot to make the kernel see the changed size
  c) decrypt the container 
  d) wipe the additional space from the decrypted side
  e) resize the filesystem in the container

d) is optional but highly recommended. 

If you get anything wrong here, the risk of wiping your 
LUKS header (with complete data loss) or damaging the filesystem
for dm-crypt (with at least partial data loss) are high enough to
justify doing a backup. Also, power loss or a system crash during
the filesystem resizing can also result in arbitrary bad data loss.

But if you have a backup, recreating the filesystem in the new
size is the easiest option. You do not strictly need to recreate 
the LUKS header, but if you are prepared to do it, you are not 
screwed if it turns out that it suffered damage. 

So, yes, you can do this without backup and just keeping the
original LUKS header. But I will continue to recomend a full
data backup and recreating the container after resizing it. 
If you are proficient enough to not need that, be my guest,
but if you mess up and lose all your data, I will be entitled
to make fun of you ;-)

Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno at wagner.name 
GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
Cuddly UI's are the manifestation of wishful thinking. -- Dylan Evans

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"news" is "something that hardly ever happens." -- Bruce Schneier 

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