Re: [dm-crypt] different default key sizes for CREATE and LUKSFORMAT

[Arno Wagner <arno@wagner.name>]

On Fri, Nov 20, 2009 at 01:14:56PM +0100, Milan Broz wrote:
> On 11/19/2009 02:00 PM, Heinz Diehl wrote:
> > On 19.11.2009, Arno Wagner wrote: 
> > 
> >> Anybody having real slowdown because they run this 
> >> on a 25 Mhz 386SX CPU can still reduce the count via commandline.
> > 
> > I'm afraid there is no commandline option to change that. "--iter-time" is 
> > related to the iterations of the passphrase, not the MK.
> 
> yes but I do not want add another iteration option, too confusing to users.
> 
> What about derive the MK digest iteration count from calculated keyslot
> iteration time (I suggest 1/5 of calculated time) - so for default 1 second
> it is cca 200ms - not accurate, because benchmark uses different message on input)
> but with minimum 1000 iterations?
> 
> (on my quite slow testing VM it is cca 20000 iterations)
> 
> This should be enough.
> This will add maximum 1.6 second processing time if all 8 slots is used,
> so it is acceptable but also provides needed slow-down for brute force.
> 
> Milan

Good idea, I like it. However use the same time as for the keyslots.
There is really no reason to have one use less iterations than
the other. 

The two iterations are linked security-wise, so treating them the same
makes sense.

Incidentially, it should only add the time once, there is only
one Master Key. 

Arno
-- 
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno@wagner.name 
GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
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