phc-discussions - Re: [PHC] Re: Mechanical tests

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Date: Sat, 5 Apr 2014 19:29:21 +0100
From: Peter Maxwell <peter@...icient.co.uk>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Re: Mechanical tests

On 5 April 2014 19:12, Daniel Franke <dfoxfranke@...il.com> wrote:

> "Poul-Henning Kamp" <phk@....freebsd.dk> writes:
>
> > This is not a KDF-contest.
> >
> > This is a password-scrambler contest.
>
> This message's thread-grandparent was a reply to Peter Maxwell, who wrote:
>
> > No, PHK's definition was, probably provably, correct (for password
> > hash or key derivation), assuming the *full* output is being used.
>
> I'm not disputing your (PHK's) claim that a function outputing 1000 bits
> with only 100 bits of entropy can be an acceptable password
> scrambler. I'm disputing Peter's claim that it can also be an acceptable
> KDF, and well as any claim (which I'm not attributing to anyone) that it
> can be an acceptable collision-resistant hash function.
>

Ok, to avoid this being an intractably long evening, please define
"entropy".

If we choose a password from a space of, say, 2^64 -- just for argument's
sake -- then if your output space is >2^64 in size your function obviously
cannot be subjective, i.e. there are lots and lots of elements in the
codomain, y, say for which there isn't an x s.t. y = f(x).

In other words, it doesn't matter what we choose the output length to be,
2^128 or 2^1024, if the function is injective then you don't lose "entropy"
but you also don't fill the output space.

It doesn't mean it makes a good KDF but if the function is injective and
the codomain is larger than the domain, it does mean you do not lose this
very woolly definition of information.

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