[dm-crypt] length of keyfiles

[dm-crypt] length of keyfiles

[Felix Rubio]

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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Re: [dm-crypt] length of keyfiles

[Arno Wagner]

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
> https://www.saout.de/mailman/listinfo/dm-crypt

-- 
Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno@wagner.name
GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D 9718
----
A good decision is based on knowledge and not on numbers. -- Plato

If it's in the news, don't worry about it.  The very definition of 
"news" is "something that hardly ever happens." -- Bruce Schneier
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Re: [dm-crypt] length of keyfiles

[Felix Rubio]

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

---
Felix Rubio
"Don't believe what you're told. Double check."

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
>> https://www.saout.de/mailman/listinfo/dm-crypt
_______________________________________________
dm-crypt mailing list
dm-crypt@saout.de
https://www.saout.de/mailman/listinfo/dm-crypt

Re: [dm-crypt] length of keyfiles

[Felix Rubio]

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
>>> https://www.saout.de/mailman/listinfo/dm-crypt
> _______________________________________________
> dm-crypt mailing list
> dm-crypt@saout.de
> https://www.saout.de/mailman/listinfo/dm-crypt
_______________________________________________
dm-crypt mailing list
dm-crypt@saout.de
https://www.saout.de/mailman/listinfo/dm-crypt

Re: [dm-crypt] length of keyfiles

[Arno Wagner]

I have not done an in-dept investigation on what openssl uses,
but some people claim that it probably uses /dev/random
as source. So basically the same if these claims are correct.

And yes, those DD floppies are a bit outdated now ;-)

Regards,
Arno

On Wed, Jan 06, 2021 at 12:17:30 CET, Felix Rubio wrote:
> 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
> > > > https://www.saout.de/mailman/listinfo/dm-crypt
> > _______________________________________________
> > dm-crypt mailing list
> > dm-crypt@saout.de
> > https://www.saout.de/mailman/listinfo/dm-crypt
> _______________________________________________
> dm-crypt mailing list
> dm-crypt@saout.de
> https://www.saout.de/mailman/listinfo/dm-crypt

-- 
Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno@wagner.name
GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D 9718
----
A good decision is based on knowledge and not on numbers. -- Plato

If it's in the news, don't worry about it.  The very definition of 
"news" is "something that hardly ever happens." -- Bruce Schneier
_______________________________________________
dm-crypt mailing list
dm-crypt@saout.de
https://www.saout.de/mailman/listinfo/dm-crypt