Re: [Cfrg] RGLC on draft-irtf-cfrg-kangarootwelve-01

["Stanislav V. Smyshlyaev" <[e-mail]>]

Dear CFRG,

With my chair's hat off, I support moving forward with the document after
taking into account the considerations provided by Jean-Philippe Aumasson
in his review on behalf of the Crypto Review Panel.

Regards,
Stanislav

On Thu, 20 Feb 2020 at 16:27, Stephen Farrell <[e-mail]>
wrote:

>
> Hiya,
>
> On 20/02/2020 11:37, Benoît Viguier wrote:
> > Hi Stephen,
> >
> > To answer your question:
> >
> > Yep, I had a quick look and didn't find what I thought
> > I was remembering;-) It may be that what I was thinking
> > about was HKDF (RFC 5869), so let me ask it that way:
> > is there a clear benefit in using k12 instead of using
> > HKDF to get the output length wanted? If there is, and
> > it's possible to state that in a short paragraph, that
> > might be a useful addition. (While HKDF doesn't have
> > arbitrary output length, it can produce long enough
> > outputs for many uses I guess.)
> >
> > -------------------------------------------------------------------------
> >
> > K12 has the merit of being a simpler construction and can be used in
> > place of HKDF.
> >
> > In the scope of IoT or embedded devices, another advantage of K12 when
> > its input contains a key is that it is easier to protect against DPA
> > than SHA-1 or SHA-2.
>
> Fair enough. Doesn't seem like a compelling addition to
> the document in the end, but happy to leave it to you
> and others to figure if adding that is useful or not.
>
> Cheers,
> S.
>
> PS: Just to be clear, the discussion has moved me from
> an "unsure" state to where I think this is ok to publish
> in an RFC.
>
>
> >
> > Kind Regards
> >
> > On 2/18/20 4:18 PM, Stephen Farrell wrote:
> >> Hiya,
> >>
> >> Coupla follow ups below...
> >>
> >> On 18/02/2020 14:45, Benoît Viguier wrote:
> >>> Dear Stephen,
> >>>
> >>> Thank you for your comments on our draft!
> >>>
> >>> Please find below some answers and comments.
> >>>
> >>> On 2/16/20 3:53 PM, Stephen Farrell wrote:
> >>>> Hiya,
> >>>>
> >>>> On 16/02/2020 11:16, Alexey Melnikov wrote:
> >>>>> Dear CFRG participants,
> >>>>>
> >>>>> This message is starting 2 weeks RGLC on
> >>>>> draft-irtf-cfrg-kangarootwelve-01 ("KangarooTwelve"), that will end
> >>>>> on March 1st 2020. If you've read the document and think that it is
> >>>>> ready (or not ready) for publication as an RFC, please send a message
> >>>>> in reply to this email or directly to CFRG chairs
> >>>>> ([e-mail]). If you have detailed comments, these would
> >>>>> also be very helpful at this point.
> >>>> I had my 1st read of this and think it needs a bit of
> >>>> work, but am overall unclear if it ought be published
> >>>> now. (I also looked back at the list archive messages
> >>>> referring to k12.)
> >>>>
> >>>> I don't think CFRG ought be publishing very novel
> >>>> algorithm RFCs and am unclear how much study k12 has
> >>>> gotten outside the author team. The main reference
> >>>> given [1] has pointers to lots of work with titles that
> >>>> mention reduced-round keccak but it's unclear (to me,
> >>>> not having read 'em;-) how  relevant those are to k12.
> >>> Actually, all cryptanalysis of Keccak/SHA-3 is relevant to K12.
> >>> Cryptanalysis resources are scarce, we chose as an explicit design goal
> >>> to make K12 rely on cryptanalysis of Keccak/SHA-3.
> >>>
> >>> To be more precise, K12 is made of two layers:
> >>>
> >>> 1) The inner function F. This layer relies on cryptanalysis. K12's F
> >>> function is exactly Keccak[r=1344, c=256] (as in SHAKE128) reduced to
> 12
> >>> rounds (no tweaks!). Hence, any reduced-round cryptanalysis on Keccak
> is
> >>> also a reduced round cryptanalysis of K12's F (provided the number of
> >>> rounds attacked is not higher than 12 of course).
> >>>
> >>> 2) The tree hashing over F. This layer is a mode on top of F that does
> >>> not introduce any vulnerability thanks to the use of Sakura coding
> >>> proven secure in the paper [Bertoni et al., ACNS 2014].
> >>>
> >>> This reasoning is detailed and formalized in the paper [Bertoni et al.,
> >>> ACNS 2018], which is peer-reviewed.
> >> Thanks. It sounds like referring to that section
> >> of that paper would help so. (Your explanation there
> >> helped me fwiw.)
> >>
> >>>> [1] is also used as [KECCAK_CRYPTANALYSIS] in the draft
> >>>> and is where the authors are pointing us to find
> >>>> security analysis of k12. I don't think an author-
> >>>> maintained web page like that is really good enough
> >>>> as the key reference for an RFC like this.
> >>> Good point indeed!
> >>>
> >>> One option would be that we put all the references in the RFC. We are
> >>> not sure this is the right place for it, as we see an RFC more as
> >>> implementation guide rather than an analysis paper. For example:
> >>> https://tools.ietf.org/html/rfc7748 does not mention anything with
> >>> respect to the security of X25519 which is detailed in section 3 of
> >>> https://www.iacr.org/cryptodb/archive/2006/PKC/3351/3351.pdf.
> >>> Nevertheless, we are open to this option.
> >>>
> >>> Another option would be to refer to [K12, Section 5]. In that section
> >>> attacks with the highest number of rounds are discussed and
> >>> corresponding paper are referred to.
> >> Yep, that sounds good to me.
> >>
> >>> However with the possible
> >>> appearance of new cryptanalysis results, this section would become
> outdated.
> >> Just to be clear - including the URL as an additional
> >> reference as well seems like a good idea. It's only
> >> depending on that as the main source that seems a bit
> >> iffy.
> >>
> >>>> I totally get why the authors would find that
> >>>> easier/better, but that won't be true for a reader in
> >>>> 20 years time, so better to put in the precise
> >>>> references now. That should also clarify the extent to
> >>>> which those are about k12 (as defined here) and not
> >>>> about something else.
> >>> We agree with this, however Keccak cryptanalysis directly applies to
> K12.
> >>>> I think the references really need fixing before this
> >>>> ought be published. I'm not sure where the right line
> >>>> ought be drawn in terms of maturity of an algorithm
> >>>> before CFRG blesses it with an RFC. <4 years does seem
> >>>> short, even if this is strongly based on keccak. (Hence
> >>>> me being unsure overall.)
> >>> For us the maturity amounts to more than 4 years. Being SHA-3, Keccak
> is
> >>> a high-profile cryptanalysis target. As we did not tweak the round
> >>> function, K12 relies on sustained cryptanalysis since 2008.
> >> Ack. If nobody's gotten near breaking 12 rounds and
> >> since you didn't tweak it, that does sound convincing
> >> to me. Thanks.
> >>
> >>>> Separately, the draft could benefit from some guidance
> >>>> as to when this is thought to be useful - presumably
> >>>> that's when one wants a hash output that's >512 bits.
> >>>> IIRC, there are other RFCs (forget numbers, sorry),
> >>>> that describe how to get such outputs using standard
> >>>> hash functions. If there are such RFCs, it'd be good
> >>>> to reference those. Either way saying why and when this
> >>>> is preferable to use of standard hash functions would
> >>>> be good.
> >>> The standard hash functions (SHA-1 and SHA-2) are subject to length
> >>> extension attacks.
> >> Yep, I had a quick look and didn't find what I thought
> >> I was remembering;-) It may be that what I was thinking
> >> about was HKDF (RFC 5869), so let me ask it that way:
> >> is there a clear benefit in using k12 instead of using
> >> HKDF to get the output length wanted? If there is, and
> >> it's possible to state that in a short paragraph, that
> >> might be a useful addition. (While HKDF doesn't have
> >> arbitrary output length, it can produce long enough
> >> outputs for many uses I guess.)
> >>
> >> Cheers,
> >> S.
> >>
> >>
> >>> And in all cases, K12 will be faster than SHA-3. If
> >>> that makes it clearer, we could add a short guidance as to when the
> user
> >>> can benefit the most from K12. We could simply reuse the arguments from
> >>> these slides: https://benoit.viguier.nl/files/K12atMontreal.pdf
> >>>> Other than the above, the draft seems clear (though
> >>>> I didn't try implement) and many thanks for not
> >>>> defining the usual pile of pointless variants/options:-)
> >>> Thanks, yes, that was another design goal for K12.
> >>>
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