Re: length of keyfiles

Hi Felix,

I assume we are talking LUKS here, plain mode is different.

The longer length is both convenience and helps if you 
use low-entropy input. The keyfile does not actually 
hold a key (LUKS mode), but a passphrase. Passphrases
get hashed, and once you have maximum entropy, you 
cannot get more. I would need to look up what length
is actually used, but it does not depend on the lenght
of the encryption key. That one is stored in the anti-forensic
stripes, protected with the hash from that passphrase.

So, to make this short, if you use LUKS with a keyfile,
putting in more entropy than used is meaningless.
If your random data is from /dev/random or (properly 
initialized) /dev/urandom, 64 bytes are more than enough.

Also, the differences between an 8kB passphrase and a 
64B one in execution time should not be noticeable at all.
Unless you read it from floppy disk ;-)

Regards,
Arno


On Wed, Jan 06, 2021 at 10:17:22 CET, Felix Rubio wrote:
> Hi everybody,
> 
> I have seen that keyfiles can be used in cryptsetup up to 8 kB, but
> internally the master key is 512 bits at max.  Is there any recommendation
> / increased security by using a random sequence of 8 kB w.r.t., let's say,
> one of just 64 bytes?
> 
> I understand that using one of 8kB will require more time than one of 64 B
> when unlocking the volume, but...  is the former really that much more
> secure than the latter?
> 
> Regards!
> Felix
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-- 
Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: [email protected]
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