Re: Hash algorithm analysis

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

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Hi Eric,

On Sun, 22 Jul 2018, Eric Deplagne wrote:

> On Sun, 22 Jul 2018 14:21:48 +0000, brian m. carlson wrote:
> > On Sun, Jul 22, 2018 at 11:34:42AM +0200, Eric Deplagne wrote:
> > > On Sat, 21 Jul 2018 23:59:41 +0000, brian m. carlson wrote:
> > > > I don't know your colleagues, and they haven't commented here.  One
> > > > person that has commented here is Adam Langley.  It is my impression
> > > > (and anyone is free to correct me if I'm incorrect) that he is indeed a
> > > > cryptographer.  To quote him[0]:
> > > > 
> > > >   I think this group can safely assume that SHA-256, SHA-512, BLAKE2,
> > > >   K12, etc are all secure to the extent that I don't believe that making
> > > >   comparisons between them on that axis is meaningful. Thus I think the
> > > >   question is primarily concerned with performance and implementation
> > > >   availability.
> > > > 
> > > >   […]
> > > > 
> > > >   So, overall, none of these choices should obviously be excluded. The
> > > >   considerations at this point are not cryptographic and the tradeoff
> > > >   between implementation ease and performance is one that the git
> > > >   community would have to make.
> > > 
> > >   Am I completely out of the game, or the statement that
> > >     "the considerations at this point are not cryptographic"
> > >   is just the wrongest ?
> > > 
> > >   I mean, if that was true, would we not be sticking to SHA1 ?
> > 
> > I snipped a portion of the context, but AGL was referring to the
> > considerations involved in choosing from the proposed ones for NewHash.
> > In context, he meant that the candidates for NewHash “are all secure”
> > and are therefore a better choice than SHA-1.
> 
>   Maybe a little bit sensitive, but I really did read
>     "we don't care if it's weak or strong, that's not the matter".

Thank you for your concern. I agree that we need to be careful in
considering the security implications. We made that mistake before (IIRC
there was a cryptographer who was essentially shouted off the list when he
suggested *not* to hard-code SHA-1), and we should absolutely refrain from
making that same mistake again.

> > I think we can all agree that SHA-1 is weak and should be replaced.

Indeed.

So at this point, we already excluded pretty much all the unsafe options
(although it does concern me that BLAKE2b has been weakened purposefully,
I understand the reasoning, but still).

Which means that by now, considering the security implications of the
cipher is no longer a criterion that helps us whittle down the candidates
further.

So from my point of view, there are two criterions that can help us
further:

- Which cipher is the least likely to be broken (or just weakened by new
  attacks)?

- As energy considerations not only ecologically inspired, but also in
  terms of money for elecricity: which cipher is most likely to get decent
  hardware support any time soon?

Even if my original degree (prime number theory) is closer to
cryptanalysis than pretty much all other prolific core Git contributors, I
do not want you to trust *my* word on answering those questions.

Therefore, I will ask my colleagues to enter the hornet's nest that is
this mailing list.

Ciao,
Dscho