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Date: Wed, 3 Oct 2012 16:01:35 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: crypt() non-DES support

* Gregor Pintar <grpintar@...il.com> [2012-10-03 13:56:37 +0200]:
> 2012/10/3, Szabolcs Nagy <nsz@...t70.net>:
> > * Daniel Cegie?ka <daniel.cegielka@...il.com> [2012-10-03 09:16:02 +0200]:
> >> We already have SHA3 finalist:
> >>
> >> http://csrc.nist.gov/groups/ST/hash/sha-3/winner_sha-3.html
> >> http://keccak.noekeon.org/
> >> http://keccak.noekeon.org/KeccakReferenceAndOptimized-3.2.zip
> >>
> >
> > note that crypt is a password based key derivation function
> > that is supposed to be (configurably) slow, protected against
> > dictionary attacks and collision resistant (used for password
> > verification mostly), while sha* is a secure hash that is
> > supposed to be fast and collision resistant (used for integrity
> > protection and digital signatures mostly)
> >
> 
> You can increase number of rounds.

well

the main selling point of keccak is that it
has very simple and efficient hw implementation

which means hw implementation can be a lot
faster than any software implementation
on a generic cpu

this is good for a cryptographic hash, because
you want it to be fast, but it's bad for a passwd
hash, because you want it to have stable run time,
otherwise you cannot set a reasonable iteration
count
(eg for this reason the scrypt passwd hash
was designed so that hw implementation would
use lots of transistors and cannot gain much
performance)

(the nist pbkdf2 often uses hmac-sha hash internally
but it's main application is creating a secure
symmetric encryption key from a simple password,
the stored passwd hash application is more sensitive
to the speed variation of the hash function)

tl;dr: don't use sha3 for passwd hash

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