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

Stephen Farrell <stephen.farrell@cs.tcd.ie> Tue, 18 February 2020 15:19 UTC

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Date: Tue, 18 Feb 2020 15:18:58 +0000
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Subject: Re: [Cfrg] RGLC on draft-irtf-cfrg-kangarootwelve-01
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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
>>> (cfrg-chairs@ietf.org). 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.
>