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Date: Wed, 27 Sep 2017 21:41:02 +0200
From: Solar Designer <>
Subject: Re: [OSSN-0081] sha512_crypt is insufficient for password hashing

On Mon, Sep 18, 2017 at 07:32:00PM +0000, Jeremy Stanley wrote:
> On 2017-09-17 15:04:10 +0200 (+0200), Solar Designer wrote:
> [...]
> > the wording of the advisory and in the discussion at
> > is weird.
> > 
> > I assume that sha512_crypt refers to the algorithm introduced in
> > glibc 2.7 and now used by many Linux distros and more. It is
> > typically called sha512crypt without the underscore. I also assume
> > that pbkdf2_sha512 refers to PBKDF2-HMAC-SHA512.
> Yes, or more specifically these:

These say that sha512_crypt defaults to rounds=656000, which is very
high, whereas pbkdf2_sha512 defaults to rounds=29000, which is also high
but is relatively lower.

Now, we can't directly compare these: even an optimal implementation of
PBKDF2-HMAC needs two computations of the hash per iteration, but OTOH
sha512crypt may call SHA-512's compression function more times when the
password is long (this dependency on length is an issue on its own, but
that's separate).  That said, overall your pbkdf2_sha512 is likely ~10x
quicker to crack than your sha512_crypt with these default settings.

Yet the advisory continues to recommend pbkdf2_sha512 over sha512_crypt.
I understand that maybe you just haven't gotten around to correcting it
yet.  Also, Morgan has since posted another confused comment to 1668503,
saying "we should be using bcrypt, scrypt, or at *least* pbkdf2 instead
of sha512_crypt", which continues to imply that "pbkdf2" is necessarily
a better choice.

The advisory says that "sha512_crypt algorithm has a low computational
cost factor", but your 656000 is actually very high.  (I did not verify
that you actually use a value this high, though.)  So the problem, if
any, is really not what the advisory says.  Rather, it can be that
modern hashes starting with scrypt also use memory, and there's bcrypt,
which is inefficient on GPUs - these are good things that you were
missing on with sha512_crypt (and would also miss with pbkdf2_sha512).

Per this page:

your bcrypt defaults to cost factor 12.  It is non-obvious whether this
is higher or lower than sha512crypt's 656000.  I just ran some tests
with John the Ripper -jumbo on a 2x E5-2670 v1 machine, and sha512crypt
with 656000 is crackable at ~85 c/s (at candidate password length 8)
whereas bcrypt with 12 is crackable at ~130 c/s.  So for cracking on
these CPUs supporting AVX but not yet AVX2, the bcrypt is slightly
weaker.  However, for cracking on newer CPUs supporting AVX2, it'd be
the other way around (I'd expect ~165 vs. ~130).  For cracking on GPUs,
sha512crypt would be many times faster than bcrypt.

So you can't just say that you're addressing "a low computational cost
factor".  You're addressing other issues.

Also, for your defensive use (that is, non-parallelized computation of
one password hash) bcrypt cost factor 12 is probably way faster than
sha512crypt's 656000.  You could want to bring the default for bcrypt on
par with sha512crypt's 656000 if you really could afford a value this
high before (double-check it first).  My estimate is that bcrypt cost 14
or 15 will be it.

And at cost factors this high (yes, all of these are unusually high) you
really should consider scrypt and on, where a lot of memory could be
filled (if affordable) in that time, resulting in a quadratic growth of
cost of some kinds of attacks.

I hope this helps.

And while I am at it:

On Mon, Sep 18, 2017 at 02:00:09PM -0400, Jordan Glover wrote:
> What number of iterations is considered secure for sha512crypt/pbkdf2 these days?

You'd use as many iterations as you can afford without running into
other issues.  It's not like one number is secure and another is not.

I suppose an advisory could be issued if the number of iterations is
many times lower than what's affordable for the given use case.


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