[dm-crypt] request for zulucrypt to be mentioned in cryptsetup main page.

Sven Eschenberg sven at whgl.uni-frankfurt.de
Thu Sep 15 18:08:46 CEST 2011

Hi Arno,

On Thu, September 15, 2011 11:00, Arno Wagner wrote:
> On Thu, Sep 15, 2011 at 08:20:38AM +0200, Sven Eschenberg wrote:
>> Well, it would not make to much sense to have more entropy in your
>> keyfile
>> than your MK is long. As such, as little as MK-bits of entropy in the
>> keyfile are sufficient.
> True, but keep in mind that you do not necessarily have 1 bit/bit
> of entropy in the input. In fact you basically never have that.
> So what you do is read more to create a safety margin.

That's true, if you have access to a good HW-RNG that does provide a good
quality, you should be save though (If you can trust that Entropy source
of course). I personally tend to use 64 bytes from /dev/random for
passphrases when MK is 256 bit, hoping the safety margin is big enough.

>> On the other hand there are no contraints of
>> minimum key length, that's all up to the user, afaik.
>> You should consider though increasing the iteration time, when the
>> passphrase is short. The shorter the phrase (the less entropy) the more
>> iterations in Key Stretching should be done, otherwise you could aswell
>> save the computational power wasted in the encryption.
> Unfortunately, passphrase length is only very weakly connected to
> entropy contents. This approach would tehrefore be dangerous.
> What you do is to always iterate like you have a low-entropy
> passphrase, no matter what the passphrase looks like.
> Example:
> "Researcher Builds Life-Like Cells Made of Metal"

That is true, I had something in mind like: Passphrase should be at least
128 chars (or maybe 256) and add to the iteration time for every char
less, if you add 100ms for every char missing to fulfill 256 chars as
passphrase length ...

But then again, the usability worsens, imagine users using 8-10 chars for
their passphrase only *g*.

> would typically be seen as having about 120 bits of entropy
> (2/char). However this is a slashdot headline ans has more
> realistically abouy 15 bits of entropy (my WAG) as a realistic
> measure...
>> On a sidenote: As far as I know cryptsetup will read no more than
>> MK-Bits
>> from keyfiles, but Milan should be able to tell you for sure. This would
>> mean though, that a keyfile is expected to have good entropy.
> THe keyfile for a master key is the master key verbatim, i.e.
> no hasing, iteration, salting. A keyfile containing a passphrase
> is different and goes though the normal process. As such it
> can have arbtrary length. Ther is a aprameter to constrain
> maximum lenght read. This is useful when reading, e.g. from
> /dev/urandom and to cut off a lne end.

Thanks for clarifying this.

>> Best approach of course would be to determine the entropy of the
>> keyfile/passphrase, compare it to the requested keylength (and mode) and
> In practice this is infeasible, see example above.

Coming back to the example, I suggested *calculating* the entropy instead
of a char count. I was thinking of good old Mr. Shannon there. That should
give a feasible measurement of the passphrase quality. Of course this
cannot take into account attacks based on dictionaries...

>> then decide what to do: Reject, compensate by key stretching, Accept.
> You basically can only accept and hope the user knows what they do.

Of course in a perfect world, we could hope for users knowing what they
are doing ;-).

> Arno


>> Regards
>> -Sven
>> On Thu, September 15, 2011 02:41, .. ink .. wrote:
>> > just committed support for opening both mass storage devices and files
>> > using
>> >  either a pass phrase or a key-file both in the command line and GUI.
>> Both
>> > will be officially supported when i make a new release sometime before
>> the
>> > wee is over.
>> >
>> > What feature(s) must the project gain to be mentioned in cryptsetup
>> main
>> > page?
>> > Who must i contant to request the project be mentioned like
>> > "FreeOTFE<http://freeotfe.org/>
>> > "?
>> >
>> > Is there a limit of how small or big a key-file is supposed to be?
>> what
>> > about passphrases?
>> >
>> > The project i am talking about is att:
>> http://code.google.com/p/zulucrypt/
>> > _______________________________________________
>> > dm-crypt mailing list
>> > dm-crypt at saout.de
>> > http://www.saout.de/mailman/listinfo/dm-crypt
>> >
>> _______________________________________________
>> dm-crypt mailing list
>> dm-crypt at saout.de
>> http://www.saout.de/mailman/listinfo/dm-crypt
> --
> 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
> 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 at saout.de
> http://www.saout.de/mailman/listinfo/dm-crypt

More information about the dm-crypt mailing list