Re: [dm-crypt] length of keyfiles

[Felix Rubio <felix@kngnt.org>]

Follow up question: right now I am generating the 4 kB keyfile using 
openssl rand 4096. Should the quality of the key, then, be the same if 
doing dd if=/dev/random bs=64 count=1?

Thank you!
Felix

On 2021-01-06 12:08, Felix Rubio wrote:
> Thank you for your answer Arno... and for confirming that I should
> finally get rid of those double density floppy disks and reader :-P
> 
> Regards!
> Felix
> 
> On 2021-01-06 11:47, Arno Wagner wrote:
>> 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
>>> _______________________________________________
>>> dm-crypt mailing list
>>> dm-crypt@saout.de
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