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Date: Sat, 05 Apr 2014 16:21:14 -0400
From: Daniel Franke <dfoxfranke@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Re: Mechanical tests

Peter Maxwell <peter@...icient.co.uk> writes:

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

(This email will probably be a lot easier to read if you TeX it)

Let $x$ be a random variable ranging over a subset of $\{0,1\}^*$. I
haven't said anything yet about how $x$ is distributed.

By the entropy of $x$ (for which I'll use the usual notation $H(x)$,
but don't confuse $H$ for a hash function!), I mean
$$\sum_{y \in \{0,1\}^*} -Pr[x = y] \cdot \log_2(Pr[x = y]).$$

For some $n$, Let $$F : \{0,1\}^* \rightarrow \{0,1\}^n.$$

By the entropy of $F$'s output, I mean
$$\sum_{y \in \{0,1\}^n} -Pr[F(x) = y] \cdot \log_2{Pr[F(x) = y]}.$$

Obviously, it's the case that
$$H(F(x)) \leq \max(n,H(x)),$$ with equality holding iff $F$ is
injective over the support of $x$.

Now, for the remainder of this discussion, let's suppose that $x$'s
probability distribution is the one from which users choose their
passwords. For the sake of arugment, let's say that $H(x) = 64$.

For $F$ to be a good password hash, it's necessary (but not
sufficient) that $n \geq H(x)$ and that $H(F(x)) \approx H(x)$.

Now suppose that $n=640$, and that each bit of $F$'s output is
i.i.d. such that $$\forall b \in[0..640),\ Pr[F(x)_b = 1] =
0.0129869.$$ So, each bit $F(x)$ carries one tenth of a bit of
entropy, because $$\log_2(0.0129869) + log_2(0.987013) = -0.1.$$

In this case $H(F(x)) = 64$, so $F$ might be an acceptable password
hash. However, $F$ is a terrible KDF, because if I truncate $F$'s output
in order to obtain a 128-bit key, what I really have is a 12.8 bit key,
even if I choose an exceedingly strong password as input. $F$ fails the
mathematical definition of a weakly-secure KDF because an adversary can
with high probability distinguish $F(x)$ from a uniformly-random 640-bit
string by counting its Hamming weight.

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