----- Forwarded message from Zooko Wilcox-OHearn ----- Date: Tue, 15 Oct 2013 18:42:51 +0000 From: Zooko Wilcox-OHearn To: cryptography@metzdowd.com Subject: [Cryptography] my comment to NIST about reducing capacity in SHA-3 Message-ID: Date: Tue, 1 Oct 2013 15:45:27 -0400 From: zooko To: Multiple recipients of list Subject: Re: On 128-bit security Folks: Here are my personal opinions about these issues. I'm not expert at cryptanalysis. Disclosure: I'm one of the authors of BLAKE2 (but not one of the authors of BLAKE). I personally do not believe that there is any secret agenda behind this proposal, even though I believe that there was a secret agenda behind Dual EC DRBG. One reason that I believe that the motivation behind this proposal is the stated motivation of improving performance, is that Joan Daemen told me in person in January of 2013 that the Keccak team had considered defining a reduced Keccak to compete with BLAKE2, but had decided against it because they didn't want to disrupt the SHA-3 standardization process. Apparently they changed their minds, and apparently their fears of disruption turned out to be prescient! I also do not think that a "security level" of 2^256 is necessarily better than a "security level" of 2^128. *Maybe* it is better, but I'm not aware of any examples where that sort of distinction has turned out to matter in practice, and I can't really judge if it is likely to matter in the future (except, of course, if you forget to take into account multi-target issues…). I suspect nobody else can, either. However, even though I *personally* would have confidence that a Keccak with a 256-bit capacity would be safe and would be free of maliciously induced weakness, I want a standard to be widely accepted in addition to being safe. This is the "Caesar's wife must be above suspicion" argument. It isn't enough to make a secure standard, but also we need other people to have confidence in it. And, I don't know if we can persuade people that "no it isn't actually backdoored/weakened". It may be the kind of thing where if that's the conversation we're having then we've already lost. Would it make sense to go ahead and standardize SHA3-as-a-replacement-for-SHA2 by standardizing the form of Keccak which is most widely accepted by cryptographers and which is closest to what was studied during the contest, and then separately offer SHAKE and reduced-for-speed-Keccak as additional new things? A lot of uses of secure hash functions don't need to be particularly efficient. In my slides about BLAKE2 (https://blake2.net/acns/slides.html) I argue that there are use-cases where efficiency is critical, but it is equally true that there are common and important use cases where a 576-bit capacity Keccak would be fine, e.g. public key certificates. ------- Joan Daemen, one of inventors of AES and one of the inventors of Keccak (SHA-3), replied to my mailing list post as follows: Date: Fri, 4 Oct 2013 05:08:07 -0400 From: Joan DAEMEN To: Multiple recipients of list Subject: RE: On 128-bit security Hello all, Zooko wrote:

I personally do not believe that there is any secret agenda behind this proposal, even though I believe that there was a secret agenda behind Dual EC DRBG.

One reason that I believe that the motivation behind this proposal is the stated motivation of improving performance, is that Joan Daemen told me in person in January of 2013 that the Keccak team had considered defining a reduced Keccak to compete with BLAKE2, but had decided against it because they didn't want to disrupt the SHA-3 standardization process.

Apparently they changed their minds, and apparently their fears of disruption turned out to be prescient!

Yes, Zooko and I met at the end-of-Ecrypt II event on Tenerife early 2013 (24° C in January!). I don't remember our conversation in detail, but I I'm sure Zooko is citing me correctly because that is what we were thinking about at the time. Actually, what we had in mind was to propose something like "Keccak2" to compete with BLAKE2 by drastically cutting the number of rounds, e.g., down to 12 rounds for Keccak-f[1600], but otherwise keeping the algorithm as it is. That might have sent the wrong message indeed, but we just didn't do it. In contrast, the capacity is an integral parameter of the Keccak family that we even proposed as user-tunable in our SHA-3 submission. Matching the capacity to the security strength levels of [NIST SP 800-57] is simply exploiting that flexibility. Kind regards, Joan, also on behalf of my Keccak companions ------- Regards, Zooko Wilcox-O'Hearn Founder, CEO, and Customer Support Rep https://LeastAuthority.com Freedom matters. _______________________________________________ The cryptography mailing list cryptography@metzdowd.com http://www.metzdowd.com/mailman/listinfo/cryptography ----- End forwarded message ----- -- Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org ______________________________________________________________ ICBM: 48.07100, 11.36820 http://ativel.com http://postbiota.org AC894EC5: 38A5 5F46 A4FF 59B8 336B 47EE F46E 3489 AC89 4EC5